High Tire Pressure = false MPG?

[Mormegil]

I just got my Prius last Sunday, and after reading posts here on better fuel economy, I decided I would try upping the pressure on the tires a bit.

I wonder if some of the fuel economy could now be off.

So we know that odometer readings are based on tire rotations. But if the tires are over inflated, could that throw off the circumferance of the tire, leading to less tire rotations covering the same distance. So, even though you cover say, the 50 miles, the car's computer / odometer thinks you did less miles (since less tire rotations are required). So could that give a false low MPG rating?

[efusco]

No, the tire circumfrence is not affected...at least not enough to have any measurable impact. If you somehow put enough in to make it buldge out in the center then maybe it would be a slight difference.

[bwilson4web]

<div class='quotetop'>QUOTE(Mormegil @ Oct 1 2007, 10:18 PM) [snapback]520127[/snapback]</div>

. . .
So we know that odometer readings are based on tire rotations. But if the tires are over inflated, could that throw off the circumferance of the tire, leading to less tire rotations covering the same distance. So, even though you cover say, the 50 miles, the car's computer / odometer thinks you did less miles (since less tire rotations are required). So could that give a false low MPG rating?
[/b]

Our radial tires are especially strong and the circumference does change with pressure. However, when changing tires, it is worth while to check your mileage with the new tires.

Find a road with mile markers and take a 50 mile trip noting the trip meter, fraction of a mile as you pass the markers. Over a 50 mile interval, you'll be able to calculate the percentage difference, a 'correction factor.'

In an ideal world, you calibrate the old tires and put the new ones on and calibrate the new ones.

Bob Wilson

[patsparks]

Hey, where is the problem if it did give a false reading? The reality would be even better than you think you are getting and what you think you are getting would be better so hey, it's all good.
Tyre age and wear would have a bigger affect.

[alexstarfire]

I'm sorry, but I just don't see how air pressure can affect your tire circumference. Perhaps I'm just a noob, but someone should try to explain that. The volume of the tire would be different, that's for sure, but the surface area, and the circumference, aren't going to change. The best way to demonstrate this would be for you to take a tire, with no rim or anything, just the tire, and run it over a set distance. All you'd have to do is mark the tire, so you can count rotations are such, and then run it over like 100 feet, or more, without putting pressure on the tire and then do it again but put a lot of pressure on it so that it looks like a flat tire or something. I'd bet money that you'd get the same amount of rotations.

If you do over inflate the tire too much you could cause the tire to expand which would give you false readings, but it'd be like 1%-2% at worst. I'd be more worried about the tire blowing out at that pressure than false mileage readings.

[apriusfan]

<div class='quotetop'>QUOTE(Alexstarfire @ Oct 4 2007, 10:39 PM) [snapback]521602[/snapback]</div>

I'm sorry, but I just don't see how air pressure can affect your tire circumference. Perhaps I'm just a noob, but someone should try to explain that. The volume of the tire would be different, that's for sure, but the surface area, and the circumference, aren't going to change. The best way to demonstrate this would be for you to take a tire, with no rim or anything, just the tire, and run it over a set distance. All you'd have to do is mark the tire, so you can count rotations are such, and then run it over like 100 feet, or more, without putting pressure on the tire and then do it again but put a lot of pressure on it so that it looks like a flat tire or something. I'd bet money that you'd get the same amount of rotations.

If you do over inflate the tire too much you could cause the tire to expand which would give you false readings, but it'd be like 1%-2% at worst. I'd be more worried about the tire blowing out at that pressure than false mileage readings.
[/b]

If you over inflate the tires to the point of failure of the tire, erroneous mpg will be the least of your worries.

If you are concerned about whether you are truly getting improved mpgs from adding more air pressure, try this test:

Run a calculated test with the factory recommended (35F/33/R?) air pressure. By calculated, I mean fill up the tank until the auto shut-off engages. Note your odometer reading at the fill-up. Then drive your normal trips until it is time to refill. Refill until the auto shut-off engages. Note gallons put in the tank and the current reading on the odometer. Divide miles driven by gallons filled for a baseline mpg number. Then bump the air pressure to something like 40F/38R and repeat the test. I think you will notice an improvement in the mpg number with higher air pressure in your tires. How high to raise the pressure is a personal decision (up to the maximum the tires are rated for). You will be juggling increased ride harshness (from the higher air pressure) against improved mpgs.

[donee]

Hi Apriusfan,

Your fuel volume measurement technique is flawed. Because the Prius has a variable sized gas tank. The way to get an accurate fuel volume would be to divide the odometer miles driven by the mfd mileage (mpg). As miles (and the effect on the measured miles due to tire variation) are in the numerator and denominator, the effect of the tire variation cancells out, and you get accurate gallons.

[bestmapman]

<div class='quotetop'>QUOTE(Mormegil @ Oct 1 2007, 11:18 PM) [snapback]520127[/snapback]</div>

I just got my Prius last Sunday, and after reading posts here on better fuel economy, I decided I would try upping the pressure on the tires a bit.

I wonder if some of the fuel economy could now be off.

So we know that odometer readings are based on tire rotations. But if the tires are over inflated, could that throw off the circumferance of the tire, leading to less tire rotations covering the same distance. So, even though you cover say, the 50 miles, the car's computer / odometer thinks you did less miles (since less tire rotations are required). So could that give a false low MPG rating?
[/b]

For that tire there should be no change in distance covered for each rotation regardless of tire pressure. Here is why. The tire does not slide on the rim. The tire tread on the outside of the tire is a fixed length. The steal belts in the tire do not let it expand. The circumfrence of the tire does not change. As you put more air in the tire the footprint or "flat" area where the tire meets the ground decrases thus the car raises and apears to get bigger.

The bottom line is the tread is what matters. If when you put more air in the tire the tread would stretch, then the tread would get longer. That would affect rotations per mile. The steel belts and the banding prevent the tire tread from stretching. If the tire tread does stretch it is so small as not to be measurable.

When you change tires that will change rotaions per mile. Also as the tire wears that changes the circumfrence and thus rotations per mile.

[kram]

<div class='quotetop'>QUOTE(bestmapman @ Oct 5 2007, 09:08 AM) [snapback]521668[/snapback]</div>

For that tire there should be no change in distance covered for each rotation regardless of tire pressure. Here is why. The tire does not slide on the rim. The tire tread on the outside of the tire is a fixed length. The steal belts in the tire do not let it expand. The circumfrence of the tire does not change. As you put more air in the tire the footprint or "flat" area where the tire meets the ground decrases thus the car raises and apears to get bigger.

The bottom line is the tread is what matters. If when you put more air in the tire the tread would stretch, then the tread would get longer. That would affect rotations per mile. The steel belts and the banding prevent the tire tread from stretching. If the tire tread does stretch it is so small as not to be measurable.

When you change tires that will change rotaions per mile. Also as the tire wears that changes the circumfrence and thus rotations per mile.
[/b]

Are you sure about that? Perhaps the steel bands in the radial tires do prevent the tread from expanding but I'm not sure. Also, I think it's likely that a flatter tire on a vehicle makes fewer revolutions per a given distance. The reason I believe this is based on experiments I did with my bike. I was setting up an odometer for my bike and since there are a myriad of different wheel sizes/tire sizes available for bikes the instructions call for a means to determine what the effective circumferance of one's tire is so as to properly calibrate one's odometer to one's bike. I performed the calibration procedure under three different scenarios:
1) Filled tire pressure to max allowed value. Marked tire and ground; walked bike for two tire revolutions and measured the distance traveled.
2) Same as above except I used about 20 psi less (about 20%)
3) Same as 1 except I was on the bike while traversing the 2 revolutions

Results:
Longest distance traveled was with 1). Both 2) and 3) gave measurably less distance.
Conclusions:
At least for bike tires, degree of inflation directly affects the distance traveled per revolution and therefore affects the odometer reading. Same is true for the weight of the "vehicle". The difference between 1) and 2)-3) was about 0.5%.


Actions:
I used the distance in 3) to calibrate my bike's odometer.
As yet I have not tried this with my Prius.

[apriusfan]

<div class='quotetop'>QUOTE(donee @ Oct 5 2007, 03:54 AM) [snapback]521637[/snapback]</div>

Hi Apriusfan,

Your fuel volume measurement technique is flawed. Because the Prius has a variable sized gas tank. The way to get an accurate fuel volume would be to divide the odometer miles driven by the mfd mileage (mpg). As miles (and the effect on the measured miles due to tire variation) are in the numerator and denominator, the effect of the tire variation cancells out, and you get accurate gallons.
[/b]

I must be missing something. How does the Prius have a variable sized gas tank? If you are referring to the bladder inside the tank, that is part of the evaporative emission control system, not a variable sized gas tank. When the tank is empty, it will hold something like 13 gallons of gas (every time the tank is empty).