MX5 NC Fuel Gauge

Hi everyone,
I bought the subject car almost two years ago and love it. The other
day, I went on a short trip of about 200 miles. Normally, the car is
driven around town. Along the way, I started to pay attention to the
fuel gauge and realized it is very non-linear. It is not really a
problem since it is very consistent, but it puzzles me why they
designed it the way they did. In the case of my car, if the markings
on the guage were evenly spaced instead of the way they are, it would
be a lot better. (Here's a link I found using Google to a picture of
the instruments www.pbase.com/slowpokebill/image/43510422.)

Notice the 3/4 and 1/4 marks are closer to the full and empty marks
respectively than they are to the center mark. In my car, however, it
would make more sense if the opposite were true. I can drive about 40
miles (at 70 mph) before the guage gets down to the F line. It only
takes another 40 miles to reach 3/4. However, the next quarter takes
95 more miles. The third quarter takes about 80 miles. The final
quarter goes by so fast, you can actually see the needle moving. it
only takes about 25 miles to get from 1/4 to just above the E mark.
(The light comes on about 8 miles before that). The needle then stays
just above E until I get gas. If I get gas when the needle first gets
to that point, it takes about 9.8 gallons to fill it up, so there is
almost 3 gallons left according to the manual.

Like I said, I am not really complaining, but I am curious as to why
they would have placed the markings the way they did. Here are some
possibilities:

1. The marking placement is left over from an older fuel tank that
was shaped differently.
2. The engineers told the artists to place the 3/4 and 1/4 marks
closer to the center, but they misunderstood and did the opposite.
3. The marks were placed to compensate for an oddly shaped fuel tank,
but later, they also compensated electronically (via the computer?).
4. My car is the only one that does this and is therefore
malfunctioning.

Any thoughts?

Pat

On 2009-02-20, > wrote:

> I bought the subject car almost two years ago and love it.
> The other day, I went on a short trip of about 200 miles.
> Normally, the car is driven around town. Along the way, I
> started to pay attention to the fuel gauge and realized it is
> very non-linear.

IMO, Miata fuel gauges have always been pretty bogus (though
not as bogus as oil-pressure "gauges"). When my NB fuel gauge
is on "E" I've got about 1/4 of a tank left. Quite a ways
below "E" the needle pegs. At that point I've got about 1/6 of
a tank left.

The speedometer is crap also: with the stock wheels/tires it's
off by almost 10% at freeway speeds. The odometer is only off
by about 4% which (judging my my sample of two cars) is quite
the feat of precision instrumentation by Miata standards.

Embarassingly, my parent's GM has a speedometer that is
accurate to a fraction of a MPH at freeway speeds and an
odometer that's dead-on.

I still wouldn't trade my Miata for a GM...

--
Grant Edwards grante Yow! I invented skydiving
at in 1989!
visi.com

I guess the way the fuel-gauge is designed is due to the fact of the
irregular shapes of the tank. It is probably quite square in the middle with
odd shapes on the top and bottom. The fuel gauge sender still is a floating
device connected to a variable resistor. The gauge lets you see the
resistance. With the odd shapes on top and bottom the fuel-level drops
relatively faster creating the fuel-economy-curve you describe...

I'm not sure if the computer does compensate for this kind of behaviour...

Greetings,

Jeroen
Technical Commissioner
Mazda MX-5 Club of the Netherlands

---

> schreef in bericht
...
> Hi everyone,
> I bought the subject car almost two years ago and love it. The other
> day, I went on a short trip of about 200 miles. Normally, the car is
> driven around town. Along the way, I started to pay attention to the
> fuel gauge and realized it is very non-linear. It is not really a
> problem since it is very consistent, but it puzzles me why they
> designed it the way they did. In the case of my car, if the markings
> on the guage were evenly spaced instead of the way they are, it would
> be a lot better. (Here's a link I found using Google to a picture of
> the instruments www.pbase.com/slowpokebill/image/43510422.)
>
> Notice the 3/4 and 1/4 marks are closer to the full and empty marks
> respectively than they are to the center mark. In my car, however, it
> would make more sense if the opposite were true. I can drive about 40
> miles (at 70 mph) before the guage gets down to the F line. It only
> takes another 40 miles to reach 3/4. However, the next quarter takes
> 95 more miles. The third quarter takes about 80 miles. The final
> quarter goes by so fast, you can actually see the needle moving. it
> only takes about 25 miles to get from 1/4 to just above the E mark.
> (The light comes on about 8 miles before that). The needle then stays
> just above E until I get gas. If I get gas when the needle first gets
> to that point, it takes about 9.8 gallons to fill it up, so there is
> almost 3 gallons left according to the manual.
>
> Like I said, I am not really complaining, but I am curious as to why
> they would have placed the markings the way they did. Here are some
> possibilities:
>
> 1. The marking placement is left over from an older fuel tank that
> was shaped differently.
> 2. The engineers told the artists to place the 3/4 and 1/4 marks
> closer to the center, but they misunderstood and did the opposite.
> 3. The marks were placed to compensate for an oddly shaped fuel tank,
> but later, they also compensated electronically (via the computer?).
> 4. My car is the only one that does this and is therefore
> malfunctioning.
>
> Any thoughts?
>
> Pat
>
>
>
>

Complaining about the accuracy of fuel gauges is older than I am, and I'm an
"old geezer".
Complaining about the accuracy of Mazdas and Fords Speedometers is also
nothing new.
Perhaps the newest thing is that GM did improve the accuracy of it's
speedometers on at least some of it's makes and models.

The govt standards for Speedos used to list two different accuracy specs.
Plus & minus so much, and plus with no minus.
Ford & Mazda chooses to use the plus no minus standard. On the Miata, if
you wish to disassemble things a bit, you can set the speedo needle to give
a more accurate reading. (At least on the NA &NB's.)

> wrote in message
...
> Hi everyone,
> I bought the subject car almost two years ago and love it. The other
> day, I went on a short trip of about 200 miles. Normally, the car is
> driven around town. Along the way, I started to pay attention to the
> fuel gauge and realized it is very non-linear. It is not really a
> problem since it is very consistent, but it puzzles me why they
> designed it the way they did. In the case of my car, if the markings
> on the guage were evenly spaced instead of the way they are, it would
> be a lot better. (Here's a link I found using Google to a picture of
> the instruments www.pbase.com/slowpokebill/image/43510422.)
>
> Notice the 3/4 and 1/4 marks are closer to the full and empty marks
> respectively than they are to the center mark. In my car, however, it
> would make more sense if the opposite were true. I can drive about 40
> miles (at 70 mph) before the guage gets down to the F line. It only
> takes another 40 miles to reach 3/4. However, the next quarter takes
> 95 more miles. The third quarter takes about 80 miles. The final
> quarter goes by so fast, you can actually see the needle moving. it
> only takes about 25 miles to get from 1/4 to just above the E mark.
> (The light comes on about 8 miles before that). The needle then stays
> just above E until I get gas. If I get gas when the needle first gets
> to that point, it takes about 9.8 gallons to fill it up, so there is
> almost 3 gallons left according to the manual.
>
> Like I said, I am not really complaining, but I am curious as to why
> they would have placed the markings the way they did. Here are some
> possibilities:
>
> 1. The marking placement is left over from an older fuel tank that
> was shaped differently.
> 2. The engineers told the artists to place the 3/4 and 1/4 marks
> closer to the center, but they misunderstood and did the opposite.
> 3. The marks were placed to compensate for an oddly shaped fuel tank,
> but later, they also compensated electronically (via the computer?).
> 4. My car is the only one that does this and is therefore
> malfunctioning.
>
> Any thoughts?
>
> Pat
>
>
>
>

Chuck wrote:
> Complaining about the accuracy of fuel gauges is older than I am, and I'm an
> "old geezer".
> Complaining about the accuracy of Mazdas and Fords Speedometers is also
> nothing new.
> Perhaps the newest thing is that GM did improve the accuracy of it's
> speedometers on at least some of it's makes and models.
>
> The govt standards for Speedos used to list two different accuracy specs.
> Plus & minus so much, and plus with no minus.
> Ford & Mazda chooses to use the plus no minus standard. On the Miata, if
> you wish to disassemble things a bit, you can set the speedo needle to give
> a more accurate reading. (At least on the NA &NB's.)
>
> > wrote in message
> ...
>> Hi everyone,
>> I bought the subject car almost two years ago and love it. The other
>> day, I went on a short trip of about 200 miles. Normally, the car is
>> driven around town. Along the way, I started to pay attention to the
>> fuel gauge and realized it is very non-linear. It is not really a
>> problem since it is very consistent, but it puzzles me why they
>> designed it the way they did. In the case of my car, if the markings
>> on the guage were evenly spaced instead of the way they are, it would
>> be a lot better. (Here's a link I found using Google to a picture of
>> the instruments www.pbase.com/slowpokebill/image/43510422.)
>>
>> Notice the 3/4 and 1/4 marks are closer to the full and empty marks
>> respectively than they are to the center mark. In my car, however, it
>> would make more sense if the opposite were true. I can drive about 40
>> miles (at 70 mph) before the guage gets down to the F line. It only
>> takes another 40 miles to reach 3/4. However, the next quarter takes
>> 95 more miles. The third quarter takes about 80 miles. The final
>> quarter goes by so fast, you can actually see the needle moving. it
>> only takes about 25 miles to get from 1/4 to just above the E mark.
>> (The light comes on about 8 miles before that). The needle then stays
>> just above E until I get gas. If I get gas when the needle first gets
>> to that point, it takes about 9.8 gallons to fill it up, so there is
>> almost 3 gallons left according to the manual.
>>
>> Like I said, I am not really complaining, but I am curious as to why
>> they would have placed the markings the way they did. Here are some
>> possibilities:
>>
>> 1. The marking placement is left over from an older fuel tank that
>> was shaped differently.
>> 2. The engineers told the artists to place the 3/4 and 1/4 marks
>> closer to the center, but they misunderstood and did the opposite.
>> 3. The marks were placed to compensate for an oddly shaped fuel tank,
>> but later, they also compensated electronically (via the computer?).
>> 4. My car is the only one that does this and is therefore
>> malfunctioning.
>>
>> Any thoughts?
>>
>> Pat
>>
>>
>>
>>
>
>
The speedo on our NC is spot-on at 30 according to the speed indicator
device mounted on a lamp post in our road. Haven't tested it any higher
speed because the neighbours might get upset (and the car might end up
in the village duck pond if I don't make the bend (curve))

Really?! What do you mean 'set the needle'? Is it as simple as removing it,
turning the needle a certain # of 'degrees' and sticking it back on? In
other words, it's not that it's generally out of calibration, it's just set
to show you going slower than true speed all of the time?

Chris
99BBB


"Chuck" > wrote in message
...
> Complaining about the accuracy of fuel gauges is older than I am, and I'm
> an "old geezer".
> Complaining about the accuracy of Mazdas and Fords Speedometers is also
> nothing new.
> Perhaps the newest thing is that GM did improve the accuracy of it's
> speedometers on at least some of it's makes and models.
>
> The govt standards for Speedos used to list two different accuracy specs.
> Plus & minus so much, and plus with no minus.
> Ford & Mazda chooses to use the plus no minus standard. On the Miata, if
> you wish to disassemble things a bit, you can set the speedo needle to
> give a more accurate reading. (At least on the NA &NB's.)
>
> > wrote in message
> ...
>> Hi everyone,
>> I bought the subject car almost two years ago and love it. The other
>> day, I went on a short trip of about 200 miles. Normally, the car is
>> driven around town. Along the way, I started to pay attention to the
>> fuel gauge and realized it is very non-linear. It is not really a
>> problem since it is very consistent, but it puzzles me why they
>> designed it the way they did. In the case of my car, if the markings
>> on the guage were evenly spaced instead of the way they are, it would
>> be a lot better. (Here's a link I found using Google to a picture of
>> the instruments www.pbase.com/slowpokebill/image/43510422.)
>>
>> Notice the 3/4 and 1/4 marks are closer to the full and empty marks
>> respectively than they are to the center mark. In my car, however, it
>> would make more sense if the opposite were true. I can drive about 40
>> miles (at 70 mph) before the guage gets down to the F line. It only
>> takes another 40 miles to reach 3/4. However, the next quarter takes
>> 95 more miles. The third quarter takes about 80 miles. The final
>> quarter goes by so fast, you can actually see the needle moving. it
>> only takes about 25 miles to get from 1/4 to just above the E mark.
>> (The light comes on about 8 miles before that). The needle then stays
>> just above E until I get gas. If I get gas when the needle first gets
>> to that point, it takes about 9.8 gallons to fill it up, so there is
>> almost 3 gallons left according to the manual.
>>
>> Like I said, I am not really complaining, but I am curious as to why
>> they would have placed the markings the way they did. Here are some
>> possibilities:
>>
>> 1. The marking placement is left over from an older fuel tank that
>> was shaped differently.
>> 2. The engineers told the artists to place the 3/4 and 1/4 marks
>> closer to the center, but they misunderstood and did the opposite.
>> 3. The marks were placed to compensate for an oddly shaped fuel tank,
>> but later, they also compensated electronically (via the computer?).
>> 4. My car is the only one that does this and is therefore
>> malfunctioning.
>>
>> Any thoughts?
>>
>> Pat
>>
>>
>>
>>
>
>

Well now, that sounds like a fun challenge; see how high you can get the
readout and still make it around the corner ;-) All you need is a closed
circuit and a professional driver and you have all the makings of a
performance car commercial (including the fine print disclaimer)!

Chris
99BBB

"Invisible Man" > wrote in message
...
> Chuck wrote:
>> Complaining about the accuracy of fuel gauges is older than I am, and I'm
>> an "old geezer".
>> Complaining about the accuracy of Mazdas and Fords Speedometers is also
>> nothing new.
>> Perhaps the newest thing is that GM did improve the accuracy of it's
>> speedometers on at least some of it's makes and models.
>>
>> The govt standards for Speedos used to list two different accuracy specs.
>> Plus & minus so much, and plus with no minus.
>> Ford & Mazda chooses to use the plus no minus standard. On the Miata, if
>> you wish to disassemble things a bit, you can set the speedo needle to
>> give a more accurate reading. (At least on the NA &NB's.)
>>
>> > wrote in message
>> ...
>>> Hi everyone,
>>> I bought the subject car almost two years ago and love it. The other
>>> day, I went on a short trip of about 200 miles. Normally, the car is
>>> driven around town. Along the way, I started to pay attention to the
>>> fuel gauge and realized it is very non-linear. It is not really a
>>> problem since it is very consistent, but it puzzles me why they
>>> designed it the way they did. In the case of my car, if the markings
>>> on the guage were evenly spaced instead of the way they are, it would
>>> be a lot better. (Here's a link I found using Google to a picture of
>>> the instruments www.pbase.com/slowpokebill/image/43510422.)
>>>
>>> Notice the 3/4 and 1/4 marks are closer to the full and empty marks
>>> respectively than they are to the center mark. In my car, however, it
>>> would make more sense if the opposite were true. I can drive about 40
>>> miles (at 70 mph) before the guage gets down to the F line. It only
>>> takes another 40 miles to reach 3/4. However, the next quarter takes
>>> 95 more miles. The third quarter takes about 80 miles. The final
>>> quarter goes by so fast, you can actually see the needle moving. it
>>> only takes about 25 miles to get from 1/4 to just above the E mark.
>>> (The light comes on about 8 miles before that). The needle then stays
>>> just above E until I get gas. If I get gas when the needle first gets
>>> to that point, it takes about 9.8 gallons to fill it up, so there is
>>> almost 3 gallons left according to the manual.
>>>
>>> Like I said, I am not really complaining, but I am curious as to why
>>> they would have placed the markings the way they did. Here are some
>>> possibilities:
>>>
>>> 1. The marking placement is left over from an older fuel tank that
>>> was shaped differently.
>>> 2. The engineers told the artists to place the 3/4 and 1/4 marks
>>> closer to the center, but they misunderstood and did the opposite.
>>> 3. The marks were placed to compensate for an oddly shaped fuel tank,
>>> but later, they also compensated electronically (via the computer?).
>>> 4. My car is the only one that does this and is therefore
>>> malfunctioning.
>>>
>>> Any thoughts?
>>>
>>> Pat
>>>
>>>
>>>
>>>
>>
>>
> The speedo on our NC is spot-on at 30 according to the speed indicator
> device mounted on a lamp post in our road. Haven't tested it any higher
> speed because the neighbours might get upset (and the car might end up in
> the village duck pond if I don't make the bend (curve))

Chris D'Agnolo > wrote:

> Well now, that sounds like a fun challenge; see how high you can get the
> readout and still make it around the corner ;-) All you need is a closed
> circuit and a professional driver and you have all the makings of a
> performance car commercial (including the fine print disclaimer)!

These signs are quite common here in the UK. Some studies have indicated
that they are more effective at reducing speeds than Gatso speed
cameras.

Our local authority have one that they move around the area, attaching
the device to a lamp post in various areas. As you approach it, if you
are exceeding the speed limit, it lights up and tells you what speed
you're doing. There is a fascination amongst the local boy racers when
it appears on a straight bit of road to see how fast they can go through
it, but (un)fortunately the LED display only has 2 digits :-(
--
Rob - Shropshire, UK
So many cats,
So few recipes...

Basically, you have two instruments that measure speed. The electric
speedometer, and the tachometer(indirectly)
We used a GPS that also gave a speed readout as a reference. You determine
what the error is in the speedometer with the GPS or other means, and also
note the tach reading and gear.
Take the instrument hood and the faceplate(gauge cover) off. The best speed
to use for cal& set is one that matches a normal highway speed and, on most
models, is at the top of the speedometer, or about 1/2 the total range of
the speedometer. You may see a white bar that was hidden by the faceplate
and bezel. This white bar represents the range allowed when the factory put
the speedometer on some sort of a test set.

Anyway the speedometer needle is removed gently with something like a fork.
If you have a small needlenose pliers, it can be used to prevent the shaft
from moving while you remove the needle. Using the GPS or another
instrument such as a ScanGaugeII, you set the car to the desired speed.
Cruise control helps. The Speedometer needle is gently pushed on the shaft
so that it is the same distance from the speedometer face as it was prior to
removal, and reads the correct speed. It's a good idea to set it to read
just a bit high, since it's difficult to get it to be exactly correct.
For safety you should have a knowledgeable passenger, and little or no
traffic on the road.
The OBDII digital outputs can be your friend, in that there is a speed and
RPM output that can be read with the aforementioned scangauge II or similar
OBDII devices.
If the tires size is incorrect, the OBDII digital output will not be right.
This forces the use of a GPS or highway calibration posts (if present), and
a stopwatch.
Requirements
More or less straight and flat road that allows you to maintain a steady
speed.

Several years ago, we wrote the general proceedure up, and posted it on
miata.net. I don't know if it ever was placed in the garage section. As I
remember, the NAs & NBs were allowed by law to be off (high) by as much as
5% (of reading?) The "white bar" at 60 or 70 represents this 5%

The total responsibility/liability falls upon the driver/user. We disclaim
any responsibility and or liability.

At first we thought the odometers were also set to read slightly high.
however after several 500 mile runs, they were as accurate as anything else
we used to measure the distance.

If you find that the Speedometer is not linear, in that the readings are way
off in portions of the range, the meter movement is likely bad. Even with an
exactly correct reading at 1/2 scale, the end of range readings (high and
low) may still be off, but should be within 5% of the total range of the
meter, and hopefully within 5% of the reading. There is even a way to
correct this, but it can also easily turn the meter into scrap. The process
uses a test set that produces 5 different readings, and involves cleaning
the bearings lubricating them with a special lubricant. (Jewlers used to
have the lubricant) and carefully setting the springs. It's been more that
thirty years since I occasionally did this adjustment proceedure with lab
"reference standard" meters. Even then we only went to the trouble when
there was no reasonable replacement.
When the last jug of real whale oil ran out, that would be the end of the
effort. The synthetics were not as good, and dried out or gummed up.

As an example, after going through the adjustment process, my 99's
speedometer at 70MPH, reads 1/2 way between 70 and the next small mark.
(About 1 mile high) This is as close as I could set while driving at the
same time.

Some have used another method that I do not recommend--
It's safer in that the car is stationary.
You have established how far off the speedometer is at a convenient speed.
With the instrument hood and clear faceplate removed, you gently push the
speedometer needle to the desired speed.
Clamp the shaft behind the speedometer needle hub with needle nose pliers.
You can then gently push the needle in the desired direction.

A similar process can be used to set the Tachometer to match the OBDII
digital RPM output with the ScanGaugeII.
The 99s tachs usually read one to two hundred RPM high, somewhat masking
early activation of the RPM limiter.

The NA & NB speedometer factory set was generally for a reading higher than
the actual speed.
My 99 and otheres read about 3 to 4 MPH high at 70MPH measured. The
difference was between the OBDII digital output and the meter reading. The
digital output agreed with GPS readings using original OEM Pilot tires.
There can be about 1 MPH reading/speed difference due to variations in exact
tire size with different brands of say 195/50/15 tires.

"Chris D'Agnolo" > wrote in message
...
> Really?! What do you mean 'set the needle'? Is it as simple as removing
> it, turning the needle a certain # of 'degrees' and sticking it back on?
> In other words, it's not that it's generally out of calibration, it's just
> set to show you going slower than true speed all of the time?
>
> Chris
> 99BBB
>
>
> "Chuck" > wrote in message
> ...
>> Complaining about the accuracy of fuel gauges is older than I am, and I'm
>> an "old geezer".
>> Complaining about the accuracy of Mazdas and Fords Speedometers is also
>> nothing new.
>> Perhaps the newest thing is that GM did improve the accuracy of it's
>> speedometers on at least some of it's makes and models.
>>
>> The govt standards for Speedos used to list two different accuracy specs.
>> Plus & minus so much, and plus with no minus.
>> Ford & Mazda chooses to use the plus no minus standard. On the Miata, if
>> you wish to disassemble things a bit, you can set the speedo needle to
>> give a more accurate reading. (At least on the NA &NB's.)
>>
>> > wrote in message
>> ...
>>> Hi everyone,
>>> I bought the subject car almost two years ago and love it. The other
>>> day, I went on a short trip of about 200 miles. Normally, the car is
>>> driven around town. Along the way, I started to pay attention to the
>>> fuel gauge and realized it is very non-linear. It is not really a
>>> problem since it is very consistent, but it puzzles me why they
>>> designed it the way they did. In the case of my car, if the markings
>>> on the guage were evenly spaced instead of the way they are, it would
>>> be a lot better. (Here's a link I found using Google to a picture of
>>> the instruments www.pbase.com/slowpokebill/image/43510422.)
>>>
>>> Notice the 3/4 and 1/4 marks are closer to the full and empty marks
>>> respectively than they are to the center mark. In my car, however, it
>>> would make more sense if the opposite were true. I can drive about 40
>>> miles (at 70 mph) before the guage gets down to the F line. It only
>>> takes another 40 miles to reach 3/4. However, the next quarter takes
>>> 95 more miles. The third quarter takes about 80 miles. The final
>>> quarter goes by so fast, you can actually see the needle moving. it
>>> only takes about 25 miles to get from 1/4 to just above the E mark.
>>> (The light comes on about 8 miles before that). The needle then stays
>>> just above E until I get gas. If I get gas when the needle first gets
>>> to that point, it takes about 9.8 gallons to fill it up, so there is
>>> almost 3 gallons left according to the manual.
>>>
>>> Like I said, I am not really complaining, but I am curious as to why
>>> they would have placed the markings the way they did. Here are some
>>> possibilities:
>>>
>>> 1. The marking placement is left over from an older fuel tank that
>>> was shaped differently.
>>> 2. The engineers told the artists to place the 3/4 and 1/4 marks
>>> closer to the center, but they misunderstood and did the opposite.
>>> 3. The marks were placed to compensate for an oddly shaped fuel tank,
>>> but later, they also compensated electronically (via the computer?).
>>> 4. My car is the only one that does this and is therefore
>>> malfunctioning.
>>>
>>> Any thoughts?
>>>
>>> Pat
>>>
>>>
>>>
>>>
>>
>>
>

In article >,
(Rob) wrote:

> Our local authority have one that they move around the area, attaching
> the device to a lamp post in various areas. As you approach it, if you
> are exceeding the speed limit, it lights up and tells you what speed
> you're doing.

The ones here are on trailers. They are uncalibrated and thus
notoriously inaccurate, but since their displays are not used as
evidence, it doesn't matter. They set off radar detectors up to a mile
away, and certainly make a lot of folks slow down.

--
Lanny Chambers
St. Louis, MO
'94C