Secondary Effect of Tire Pressure on MPG

[OldYogi]

Increasing tire pressure not only decreases rolling resistance, it also increases the diameter of the tire. Here is my analysis of how that affects mpg.

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OldYogi

[john1701a]

Tire wear must be taken into account too.

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[DaveinOlyWA]

ya right... interesting math you have there...but all based on the assumption that the difference in PSI is enough to enlarge the tire...

its my thought that the difference is not enough to significant or insignificantly increase the size of the tire so your difference in mileage i suspect is actually much smaller if there is a difference at all.

i would look at tire expansion due to temperature differences making a bigger difference

[Rokeby]

Originally Posted by OldYogi

Increasing tire pressure not only decreases rolling
resistance, it also increases the diameter of the tire...

OldYogi,

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[OldYogi]

Originally Posted by DaveinOlyWA

ya right... interesting math you have there...but all based on the assumption that the difference in PSI is enough to enlarge the tire...

its my thought that the difference is not enough to significant or insignificantly increase the size of the tire so your difference in mileage i suspect is actually much smaller if there is a difference at all.

i would look at tire expansion due to temperature differences making a bigger difference

Increasing the PSI doesn't "enlarge" the tire, but it changes the area of the tire in direct contact with the road. If you let air out of the tire it goes flat, and more tire surface contacts the road -- similarly, put a heavy load in the car and the tire flattens a bit. Conversely, lighten the load or pump up the tire, and less tire surface contacts the road. What remains constant is that the area contacting the road measured in sq.inches, multiplied by lbs/sq. inch equals the weight in lbs that the tire is carrying.

[DaveinOlyWA]

ok. i understand that the less out of round the tire is while rolling down the road, the lower the rolling resistance will be and the better the mileage. but i dont imagine that that increases the circumference of the tire.

[donee]

Hi OldYogi,

Are you sure about that? Maybe you mean the ride height radius increases? I think we had a thread about pressure and rolling diameter here on PC. No significant change in rolling diameter (revs/mile) with reasonable pressures (25 to 45 psi). As the belt length does not change, and the wheel pushes the belt around one revolution per wheel revolution.

[OldYogi]

Originally Posted by donee

Hi OldYogi,

Are you sure about that? Maybe you mean the ride height radius increases? I think we had a thread about pressure and rolling diameter here on PC. No significant change in rolling diameter (revs/mile) with reasonable pressures (25 to 45 psi). As the belt length does not change, and the wheel pushes the belt around one revolution per wheel revolution.

Yes, I suppose it is the ride height radius I am thinking of. And yes, you are right that the belt length doesn't change. So you may be right that the belt is working like a tank tread, and regardless of how flat the tire may be, and thus how much the ride height radius is shortened, the car moves one belt length per wheel revolution. Hmm! I'd better think again. Thanks.

[patsparks]

Look at a bulldozer with a track. If the track has a circumference of 10 metres and 4 metres of the track is in contact with the ground the bulldozer will move forward 10 metres for each full revolution of the track.

Now with the same length track raise the drive wheel up to make the tracks form a triangle, and shorten the length of track in contact with the ground by half a metre to 3.5 metres remembering we maintained the length of the track at 10 metres.
When the track turns a full revolution the bulldozer still moves forward 10 metres.

Tyres are no different to the tracks on a bulldozer, the contact patch length or the height of the centre of the wheel above the surface has nothing to do with it, the circumference of the tyre is what determines the rolling distance per revolution.

No wonder no one bothers to discuss this effect.