What is the realistic accuracy & precision of typical consumerMPG calculations (tripmeter miles/pump gallons)

rickman > wrote:
>
> Air resistance rises as the square of the speed. So faster is worse by more
> than the linear proportion. I find I notice the difference when I drive
> over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
> to overcome air resistance. I don't know how tires impact the equation and
> of course since all these speeds are in top gear the entire drive train is
> turning 33% faster as well.
>

It is true that air resistance goes up a square of the speed, but
the power requirement, and the corresponding rate of fuel consumption,
goes up as the cube. Work=force*distance, Power=force*speed.

root wrote on 7/24/2017 1:00 PM:
> rickman > wrote:
>>
>> Air resistance rises as the square of the speed. So faster is worse by more
>> than the linear proportion. I find I notice the difference when I drive
>> over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
>> to overcome air resistance. I don't know how tires impact the equation and
>> of course since all these speeds are in top gear the entire drive train is
>> turning 33% faster as well.
>>
>
> It is true that air resistance goes up a square of the speed, but
> the power requirement, and the corresponding rate of fuel consumption,
> goes up as the cube. Work=force*distance, Power=force*speed.

You are right that the horsepower requirement goes with the cube. But, that
doesn't impact the gas mileage. Since you are traveling faster you drive
for a shorter time, so that extra factor in power cancels out. No?

--

Rick C

On Mon, 24 Jul 2017 19:23:40 -0400, rickman > wrote:

>root wrote on 7/24/2017 1:00 PM:
>> rickman > wrote:
>>>
>>> Air resistance rises as the square of the speed. So faster is worse by more
>>> than the linear proportion. I find I notice the difference when I drive
>>> over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
>>> to overcome air resistance. I don't know how tires impact the equation and
>>> of course since all these speeds are in top gear the entire drive train is
>>> turning 33% faster as well.
>>>
>>
>> It is true that air resistance goes up a square of the speed, but
>> the power requirement, and the corresponding rate of fuel consumption,
>> goes up as the cube. Work=force*distance, Power=force*speed.
>
>You are right that the horsepower requirement goes with the cube. But, that
>doesn't impact the gas mileage. Since you are traveling faster you drive
>for a shorter time, so that extra factor in power cancels out. No?
No, because the speed doubling takes only half the time, but 4 time
the power. Not necessarilly 4 times the fuel, because the engine may
be "on the cam" at the higher speed, running more efficiently.

An example of this was the 1975 Toyota Celica GT. With the 1975
gearing, it was actually most efficient at 80MPH in 5th, as long as
you didn't have to change speed or pass anyone. (I got 52MPG at just
over 80mph from Waterloo to Kingston Ontario at 2am on a Sunday
morning back in 1979-ish.

Didn't work on the 1976 model - same body (and engine) but different
gearing

wrote on 7/24/2017 9:46 PM:
> On Mon, 24 Jul 2017 19:23:40 -0400, rickman > wrote:
>
>> root wrote on 7/24/2017 1:00 PM:
>>> rickman > wrote:
>>>>
>>>> Air resistance rises as the square of the speed. So faster is worse by more
>>>> than the linear proportion. I find I notice the difference when I drive
>>>> over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
>>>> to overcome air resistance. I don't know how tires impact the equation and
>>>> of course since all these speeds are in top gear the entire drive train is
>>>> turning 33% faster as well.
>>>>
>>>
>>> It is true that air resistance goes up a square of the speed, but
>>> the power requirement, and the corresponding rate of fuel consumption,
>>> goes up as the cube. Work=force*distance, Power=force*speed.
>>
>> You are right that the horsepower requirement goes with the cube. But, that
>> doesn't impact the gas mileage. Since you are traveling faster you drive
>> for a shorter time, so that extra factor in power cancels out. No?
> No, because the speed doubling takes only half the time, but 4 time
> the power. Not necessarilly 4 times the fuel, because the engine may
> be "on the cam" at the higher speed, running more efficiently.
>
> An example of this was the 1975 Toyota Celica GT. With the 1975
> gearing, it was actually most efficient at 80MPH in 5th, as long as
> you didn't have to change speed or pass anyone. (I got 52MPG at just
> over 80mph from Waterloo to Kingston Ontario at 2am on a Sunday
> morning back in 1979-ish.
>
> Didn't work on the 1976 model - same body (and engine) but different
> gearing

What was the lowest speed you could use 5th gear in the 75 car?

--

Rick C

On Mon, 24 Jul 2017 22:32:54 -0400, rickman > wrote:

wrote on 7/24/2017 9:46 PM:
>> On Mon, 24 Jul 2017 19:23:40 -0400, rickman > wrote:
>>
>>> root wrote on 7/24/2017 1:00 PM:
>>>> rickman > wrote:
>>>>>
>>>>> Air resistance rises as the square of the speed. So faster is worse by more
>>>>> than the linear proportion. I find I notice the difference when I drive
>>>>> over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
>>>>> to overcome air resistance. I don't know how tires impact the equation and
>>>>> of course since all these speeds are in top gear the entire drive train is
>>>>> turning 33% faster as well.
>>>>>
>>>>
>>>> It is true that air resistance goes up a square of the speed, but
>>>> the power requirement, and the corresponding rate of fuel consumption,
>>>> goes up as the cube. Work=force*distance, Power=force*speed.
>>>
>>> You are right that the horsepower requirement goes with the cube. But, that
>>> doesn't impact the gas mileage. Since you are traveling faster you drive
>>> for a shorter time, so that extra factor in power cancels out. No?
>> No, because the speed doubling takes only half the time, but 4 time
>> the power. Not necessarilly 4 times the fuel, because the engine may
>> be "on the cam" at the higher speed, running more efficiently.
>>
>> An example of this was the 1975 Toyota Celica GT. With the 1975
>> gearing, it was actually most efficient at 80MPH in 5th, as long as
>> you didn't have to change speed or pass anyone. (I got 52MPG at just
>> over 80mph from Waterloo to Kingston Ontario at 2am on a Sunday
>> morning back in 1979-ish.
>>
>> Didn't work on the 1976 model - same body (and engine) but different
>> gearing
>
>What was the lowest speed you could use 5th gear in the 75 car?
Can't remember for sure, but it was a DOG at 60mph - requiresd a
downshift to get anywhere. I think hey geared the 75 GT the same as
the 4 speed. I know I was shocked by the mileage on that trip - going
out to Kingston to pit crew for Taisto Heinonnen, "The Flying Fynn"
and Tom Burgess on the Twin Lakes Rally. Crewsd for him on the Tall
Pines and the Blossom too.

I was offered his backup Celica Team car in 1980 when we finished
rallying in the navigational rallye series (After finishing 1st,
second and third in 3 years we were no longer elligible) and our R12
was not adequate to run competetively in the performance series but I
decided to quit while I was ahead, since I was getting married.

On 07/24/2017 07:46 PM, wrote:
> An example of this was the 1975 Toyota Celica GT. With the 1975
> gearing, it was actually most efficient at 80MPH in 5th, as long as
> you didn't have to change speed or pass anyone. (I got 52MPG at just
> over 80mph from Waterloo to Kingston Ontario at 2am on a Sunday
> morning back in 1979-ish.

I had the misfortune to own a '71 Audi when the 55 mph national speed
limit went into effect. The German engineers thought 55 was a very brief
period on your way to cruising speed not a speed you'd try to drive.

On Monday, July 24, 2017 at 4:32:58 PM UTC-10, rickman wrote:
> wrote on 7/24/2017 9:46 PM:
> > On Mon, 24 Jul 2017 19:23:40 -0400, rickman > wrote:
> >
> >> root wrote on 7/24/2017 1:00 PM:
> >>> rickman > wrote:
> >>>>
> >>>> Air resistance rises as the square of the speed. So faster is worse by more
> >>>> than the linear proportion. I find I notice the difference when I drive
> >>>> over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
> >>>> to overcome air resistance. I don't know how tires impact the equation and
> >>>> of course since all these speeds are in top gear the entire drive train is
> >>>> turning 33% faster as well.
> >>>>
> >>>
> >>> It is true that air resistance goes up a square of the speed, but
> >>> the power requirement, and the corresponding rate of fuel consumption,
> >>> goes up as the cube. Work=force*distance, Power=force*speed.
> >>
> >> You are right that the horsepower requirement goes with the cube. But, that
> >> doesn't impact the gas mileage. Since you are traveling faster you drive
> >> for a shorter time, so that extra factor in power cancels out. No?
> > No, because the speed doubling takes only half the time, but 4 time
> > the power. Not necessarilly 4 times the fuel, because the engine may
> > be "on the cam" at the higher speed, running more efficiently.
> >
> > An example of this was the 1975 Toyota Celica GT. With the 1975
> > gearing, it was actually most efficient at 80MPH in 5th, as long as
> > you didn't have to change speed or pass anyone. (I got 52MPG at just
> > over 80mph from Waterloo to Kingston Ontario at 2am on a Sunday
> > morning back in 1979-ish.
> >
> > Didn't work on the 1976 model - same body (and engine) but different
> > gearing
>
> What was the lowest speed you could use 5th gear in the 75 car?
>
> --
>
> Rick C

As it goes, if you want the best gas milage, you should always get into the highest gear as early as possible. You can shift into top gear at almost any speed. Well, I wouldn't do it below 25 MPH.