Why you should never buy a car without a tachometer

>> Over the years I've owned my current ride, I have observed that it is
>> most
>> fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
>> observed that, at 3000 RPM or less, the mileage on my vehicle drops to
>> 35MPG
>> or lower. My experience with my current vehicle is similar to several
>> other
>> vehicles I've owned. In every case, I've found that the best fuel
>> economy
>> is achieved somewhere above 70MPH. In the case of my current vehicle,
>> 3500RPM in top gear equates to about 78MPH. Now I'm sure someone will
>> state
>> the obvious, which is how do I know that my tachometer and speedometer
>> are
>> accurate? I don't. But the bottom line is, in top gear, I can achieve
>> BETTER fuel economy at slightly higher than normal highway speeds.
>> Actually, the better fuel economy is due to slightly higher engine RPM,
>> and
>> the higher ground speed is COINCIDENTAL.
>
> Your experience seems to be the opposite of most advice given to
> maximize milage. I would like to find out more about how you conducted
> your test.
>
> If I understand you correctly you are saying that your car gets better
> milage at 78mph (3500rpm) in high gear than it got at lower increments
> say: 70mph, 60mph and 50mph?
>

YES. And that is EXACTLY how my particular Otto Cycle internal combustion
engine SHOULD perform. All cars are different obviously, but if they use an
Otto Cycle internal combustion engine, maximum fuel economy will be achieved
at close to 40% of the engine's redline. The only "difference" from one car
to the next (assuming they both use an Otto Cycle engine) will be what
ground speed that most fuel-efficient RPM equates to. In many cars, that
most fuel-efficient ground speed happens to be above 70MPH. My car is one
of them. IF the engine RPM drops below 3000, the engine itself is less
fuel-efficient. This means it takes more fuel to travel a certain distance,
and COINCIDENTALLY, this means that it is less fuel-efficient at a lower
ground speed. In your terms, this means that at 60MPH (for example), fuel
efficiency is LOWER.

For an automobile to be most fuel-efficient at say, 50MPH, the Otto Cycle
engine would need to be running near 40% of redline at 50MPH. IF that
theoretical Otto Cycle engine was running at 40% of redline at a ground
speed of 50MPH, then you would see a DECREASE in fuel economy at either
60MPH or 40MPH. Not because your ground speed changed up or down, but
because the Otto Cycle engine was operating outside of it's most
fuel-efficient RPM. (The ground speed change is coincidental)

I don't understand why you are questioning my test methods, when my results
exactly agree with how Otto Cycle engines are supposed to work. If I said
that I've found that water freezes at Zero Celsius, would you question how I
measured that? Measuring MPG at certain speeds is so simple the average
2nd-grader could do the math accurately. Some of them could even do it in
their head. I've performed the same measurements many times, with similar
results. All of this was done before I even knew what results I should
EXPECT to see. The fact is, my Otto Cycle engine should be most
fuel-efficient at around 3200RPM. BEFORE I KNEW THAT, I'd noticed, through
several years of driving experience, that the most fuel-efficient speed for
the engine was 3500RPM. That means my particular Otto Cycle engine is
performing EXACTLY as it should. If my tachometer is dead-on balls
accurate, my Otto Cycle engine is most fuel efficient at about 40% of
redline, EXACTLY AS IT SHOULD BE.

Yes, this means 78MPH is most fuel-efficient, and 70, 60 and 50 are less
fuel-efficient. -Dave

How did you calculate the 40% bit and so forth in your original post
using the Otto cycle? I'm not disagreeing, just curious how you went
about it :-)

Ted B. wrote:
> >> Over the years I've owned my current ride, I have observed that it is
> >> most
> >> fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
> >> observed that, at 3000 RPM or less, the mileage on my vehicle drops to
> >> 35MPG
> >> or lower. My experience with my current vehicle is similar to several
> >> other
> >> vehicles I've owned. In every case, I've found that the best fuel
> >> economy
> >> is achieved somewhere above 70MPH. In the case of my current vehicle,
> >> 3500RPM in top gear equates to about 78MPH. Now I'm sure someone will
> >> state
> >> the obvious, which is how do I know that my tachometer and speedometer
> >> are
> >> accurate? I don't. But the bottom line is, in top gear, I can achieve
> >> BETTER fuel economy at slightly higher than normal highway speeds.
> >> Actually, the better fuel economy is due to slightly higher engine RPM,
> >> and
> >> the higher ground speed is COINCIDENTAL.
> >
> > Your experience seems to be the opposite of most advice given to
> > maximize milage. I would like to find out more about how you conducted
> > your test.
> >
> > If I understand you correctly you are saying that your car gets better
> > milage at 78mph (3500rpm) in high gear than it got at lower increments
> > say: 70mph, 60mph and 50mph?
> >
>
> YES. And that is EXACTLY how my particular Otto Cycle internal combustion
> engine SHOULD perform. All cars are different obviously, but if they use an
> Otto Cycle internal combustion engine, maximum fuel economy will be achieved
> at close to 40% of the engine's redline. The only "difference" from one car
> to the next (assuming they both use an Otto Cycle engine) will be what
> ground speed that most fuel-efficient RPM equates to. In many cars, that
> most fuel-efficient ground speed happens to be above 70MPH. My car is one
> of them. IF the engine RPM drops below 3000, the engine itself is less
> fuel-efficient. This means it takes more fuel to travel a certain distance,
> and COINCIDENTALLY, this means that it is less fuel-efficient at a lower
> ground speed. In your terms, this means that at 60MPH (for example), fuel
> efficiency is LOWER.
>
> For an automobile to be most fuel-efficient at say, 50MPH, the Otto Cycle
> engine would need to be running near 40% of redline at 50MPH. IF that
> theoretical Otto Cycle engine was running at 40% of redline at a ground
> speed of 50MPH, then you would see a DECREASE in fuel economy at either
> 60MPH or 40MPH. Not because your ground speed changed up or down, but
> because the Otto Cycle engine was operating outside of it's most
> fuel-efficient RPM. (The ground speed change is coincidental)
>
> I don't understand why you are questioning my test methods, when my results
> exactly agree with how Otto Cycle engines are supposed to work. If I said
> that I've found that water freezes at Zero Celsius, would you question how I
> measured that? Measuring MPG at certain speeds is so simple the average
> 2nd-grader could do the math accurately. Some of them could even do it in
> their head. I've performed the same measurements many times, with similar
> results. All of this was done before I even knew what results I should
> EXPECT to see. The fact is, my Otto Cycle engine should be most
> fuel-efficient at around 3200RPM. BEFORE I KNEW THAT, I'd noticed, through
> several years of driving experience, that the most fuel-efficient speed for
> the engine was 3500RPM. That means my particular Otto Cycle engine is
> performing EXACTLY as it should. If my tachometer is dead-on balls
> accurate, my Otto Cycle engine is most fuel efficient at about 40% of
> redline, EXACTLY AS IT SHOULD BE.
>
> Yes, this means 78MPH is most fuel-efficient, and 70, 60 and 50 are less
> fuel-efficient. -Dave

Interesting...apparently Dave is signing for Ted B. or vice versa.
Anyway...

Why don't you tell us in detail how you measured the mileage at various
increments.

A meaningful test would be one that does the following for each speed
increment:

1. Uses the same car.
2. Drive the same route.
3. The car starts with a full tank of gas.
4. Covers a meaningful distance, at least 100 miles but preferably a
full tank.
5. The car is held at a constant speed as much as possible.
6. The milage is computed the same way each time, either with the
car's averaging computer or by a complete fillup at the end.

The reason I'm questioning your statement is that the effect of wind
resistance on milage increases faster than speed. (It isn't arithmetic)
I can see this effect on my cars during a 375 mile trip that is
repeated several times a year to North Carolina. Driving at 60mph
results in 33mpg while driving at 80mph results in 26mpg for the same
distance. I'm puzzled about that you can have such different results.

> wrote in message
ups.com...
> How did you calculate the 40% bit and so forth in your original post
> using the Otto cycle? I'm not disagreeing, just curious how you went
> about it :-)
>

I didn't calculate it. Through research, I learned that the Otto Cycle
engine is most fuel-efficient at about 40% of redline RPM. That's not my
calculation, it's just how the Otto Cycle engine works. -Dave

Dave C. wrote:
> > wrote in message
> ups.com...
> > How did you calculate the 40% bit and so forth in your original post
> > using the Otto cycle? I'm not disagreeing, just curious how you went
> > about it :-)
> >
>
> I didn't calculate it. Through research, I learned that the Otto Cycle
> engine is most fuel-efficient at about 40% of redline RPM. That's not my
> calculation, it's just how the Otto Cycle engine works. -Dave


>From your original post:

"During research on various engine types, I happened to discover
that Otto Cycle engines are most fuel-efficient at RPMs near (not
necessarily at, but near) 40% of redline. I had no reason to
disbelieve
that particular scientific fact. I did the math for my own car, and
found
that (surprise, surprise) I SHOULD achieve maximum fuel economy at
ABOUT
3200RPM, with my own Otto Cycle engine. "

Was curious what math you said you did there for your own car. No big
deal really, was just curious. I've written a couple of engine
simulations and it hasn't yet dawned on me how one would use the Otto
Cycle to derive any conclusions like this. I'm not arguing or saying
it's wrong, after all, the best fuel efficiency isn't going to be at 1
mph, nor 300 mph, so it'd have to be somewhere in between :P

Todd Wasson
Performance Simulations
http://www.PerformanceSimulations.com
http://www.PerformanceSimulations.co.../ToddSim9a.wmv

> The reason I'm questioning your statement is that the effect of wind
> resistance on milage increases faster than speed. (It isn't arithmetic)
> I can see this effect on my cars during a 375 mile trip that is
> repeated several times a year to North Carolina. Driving at 60mph
> results in 33mpg while driving at 80mph results in 26mpg for the same
> distance. I'm puzzled about that you can have such different results.
>

You shouldn't be puzzled at all. You are missing certain facts that would
clear things up quickly for you. Does your car have a tachometer? If so,
what is the highest number that the tachometer reads? Multiply that times
..4. For example, if your tach goes up to 9000, multiply by .4 to get 3600.
NOW, assuming your tach goes up to 9 grand, and your 40% level is 3600, you
will probably find that 3600RPM is somewhere below 80MPH in your car. 33MPG
at 60MPH is not bad, as far as fuel economy goes, and I don't even KNOW what
kind of car you drive. BUT, based on the fact that you get 33MPG at 60 but
only 26MPG at 80, it's easy to (LOGICALLY) conclude that, YOUR engine is
running closer to 40% at 60MPH than it is at 80MPH. This is just a total
guess, but 80MPH for your engine might be 50% or higher, but definitely
higher than the optimum 40% RPM speed. -Dave

How do the manufacturers calculate redline anyway? I suspect that it
can be limited by a variety of factors, such as valve float, piston
speed, vibrations, or a business manager thinking about warranty claims
on those two-sigma outliers on the left side of the quality
distribution. Only some of these factors would directly relate to
efficiency aspects of engine theory. I've got a further hunch that the
lowest limiting factor gets rounded down to the nearest couple hundred
rpm.

Thoughts?
--Joe

Valve float is typically the limiting factor. To keep friction low you
want as light a spring as you can, but that lowers the red line.
Piston speed relates more to connecting rod strength, but they can take
considerable rpm, well above the redline or the typical spot where
you'll want to put the horsepower peak. Basically the power game is to
get the horsepower peak where you want it, then use as light a valve
spring as you can. The result is you usually end up with the redline
not too far over the horsepower peak. Indeed, there's bound to be a
safety factor built in. They'll lower the redline a bit from what will
probably be fine. I ran my engine a good 500-600 rpm past the redline
and the whole engine sound changed. Most likely valve float there.
:-) I don't recommend trying that though of course..

Todd Wasson
Performance Simulations
http://www.PerformanceSimulations.com
http://www.PerformanceSimulations.co.../ToddSim9a.wmv

wrote:
> Valve float is typically the limiting factor. To keep friction low you
> want as light a spring as you can, but that lowers the red line.
> Piston speed relates more to connecting rod strength, but they can take
> considerable rpm, well above the redline or the typical spot where
> you'll want to put the horsepower peak. Basically the power game is to
> get the horsepower peak where you want it, then use as light a valve
> spring as you can. The result is you usually end up with the redline
> not too far over the horsepower peak. Indeed, there's bound to be a
> safety factor built in. They'll lower the redline a bit from what will
> probably be fine. I ran my engine a good 500-600 rpm past the redline
> and the whole engine sound changed. Most likely valve float there.
> :-) I don't recommend trying that though of course..
>
> Todd Wasson
> Performance Simulations
> http://www.PerformanceSimulations.com
> http://www.PerformanceSimulations.co.../ToddSim9a.wmv

Sometimes, sometimes not. I once had to over-rev an old Rabbit GTI,
the valves did indeed float about 700 RPM over redline (was turning
left, oncoming driver ran a red light and was headed right toward me,
didn't feel like taking the time to shift.) I've repeatedly
over-revved my GTI 1.8T however, as the only indication that you've
exceeded the redline is the actual tachometer. Power doesn't fall off,
valves don't float, it just pulls like a freight train. I miss that
car (sold it to my mom, if you can believe that. She loves it.) I
have no idea what consideration the engineers used to set the redline;
the earlier 16V NA engines (same basic block) had a much higher redline
so it couldn't have been piston speed or bearing issues. Maybe just a
little CYA?

Now my old cars - all Studebakers - yeah, the valvetrain is the
limiting factor. 5200 RPM - float city, it's all over. I'm working on
rectifying that situation

Haven't blown up an engine yet (knock on wood) save for an old Bimmer
that I didn't abuse but I suspect that the PO did...

nate

> >
> > I didn't calculate it. Through research, I learned that the Otto Cycle
> > engine is most fuel-efficient at about 40% of redline RPM. That's not
my
> > calculation, it's just how the Otto Cycle engine works. -Dave
>
>
> >From your original post:
>
> "During research on various engine types, I happened to discover
> that Otto Cycle engines are most fuel-efficient at RPMs near (not
> necessarily at, but near) 40% of redline. I had no reason to
> disbelieve
> that particular scientific fact. I did the math for my own car, and
> found
> that (surprise, surprise) I SHOULD achieve maximum fuel economy at
> ABOUT
> 3200RPM, with my own Otto Cycle engine. "
>
> Was curious what math you said you did there for your own car. No big
> deal really, was just curious. I've written a couple of engine
> simulations and it hasn't yet dawned on me how one would use the Otto
> Cycle to derive any conclusions like this. I'm not arguing or saying
> it's wrong, after all, the best fuel efficiency isn't going to be at 1
> mph, nor 300 mph, so it'd have to be somewhere in between :P
>
> Todd Wasson
> Performance Simulations

OH! I guess I misunderstood the question. My own car tops out at 8000 RPM,
so if the maximum fuel efficiency RPM is ABOUT 40% (for any Otto Cycle
engine, not just mine) of that, then 8000 X .4 equals 3200. So, 3200RPM is
ABOUT where my own engine should be most fuel efficient. I already knew
that 3500RPM was the most fuel-efficient speed for my engine, so when I
learned that it should be about 3200RPM, I wasn't surprised, at all. Yes,
that computes. -Dave