High Tire Pressure = false MPG?

Thats what i was sayin & Jimbo summarized the error clearly ... whether the effective radius is negligibly larger or not, its bigger and that makes the error such that you are actually at least getting better mpg not worse than indicated MFD. The main reason high tire pressure's gives u better mpg is there is less tire flex as it rotates ... more pressure is less contact patch, less contact patch is less flex ... tire flex is heat and heat is wasted energy.

EDIT: the link below is pretty technical but is explains what Kram & i were trying to say in layman's terms about the larger radius:

Raw materials

The Article ends with:
"The effective rolling radius turns out to increase with increasing speed and increasing inflation pressure. The variation with speed is strongly dependent on the tyre carcass structure.

A radial-ply tyre rolling radius appears to be almost constant for varying speed in contrast with the diagonal-ply (bias-ply) tyre. This phenomenon has to do with the radial response of the tyre to higher circumferential speeds."

The speed increase is due to centripetal force working against tire load. The pressure effects are what Kram and i described. Again whether the increased rolling radius is 1.0000001% or 1.02% its an error in our favor and increased tire psi is a true Fuel savings, not a fictitious thing on the gauge.

Its all so easy to check if someones curious, go on the interstate and go at least 10 to 100 miles between mile markers for the two tire psi's and write down how far ur odmeter said u went to the 1/10 mile. 10 miles would show differences as low as 1% and 100 miles would show differences as low as 0.1%.

My GUESS is the error is much much less than 1%, which is at MOST 1/2 mile per gallon in ur favor and probably much closer to 1/20th gallon in our favor.

 

In previous threads here, a consensus developed that rolling or effective radius isn't particularly relevant to radial-ply tires.

Instead, they have a rolling circumference which is determined primarily by the length of the steel belts beneath the tread, and changes very little with tread depth, inflation pressure, and speed. Higher pressure or speed will stretch the steel slightly, but very little compared to ordinary rubber. Tread depth changes will shift the neutral axis very slightly, but rubber stretches and compresses much easier than steel so that neutral axis remains anchored very close to those steel belts.

 

i think pressure still effects effective radius on both bias ply and radial tires. yes the steel belt in radial tires will not stretch significantly so that's why speed variations which causes centripetal forces outward dont effect radial tires much, but speed does effect older bias ply tires which the article stated ..... but as for pressure changes doesn't air in the tire act as spring which compress and uncompressed less with higher pressures. Just because the steel belts don't stretch doesn't mean they form a perfect circle when the weight of the vehicle is on the tire. My thoughts are the steel belts don't need to stretch (or compress) for the tire to change shape with variations in load and pressure ... with more air pressure it approaches a perfect circle, with less it is further away from a perfect circle (more deflected ).

Just my thoughts and its really kinda theoretical at this point in terms of fuel mileage but i think some people think tires don't deflect and they do. Thats the only reason i am stating this. If tires didn't deflect there would be no latitudinal slip angles when cornering and no longitudinal slip ratios accel/braking and there would no such thing as an effective loaded tire radius, we would just use the actual tire diameter. All those things exist as part of vehicle dynamics and race tuning.

Below is an article in TireRack.com on pressure, high speeds and tire deflections. It explaining how the rapid tire deflections at speed create heat and destroy the tire but it explains basic deflections n laymens in the second paragraph:

Because of the weight they bear, pneumatic tires' sidewalls bulge and their treads flatten as they roll into contact with the road. This results in dimensional difference between the tire's "unloaded" radius (i.e., between the center of the axle and the top of the tire) and its "loaded" radius (between the center of the axle and the road). The engineer's call the difference between the two radii "deflection." Increasing vehicle speed will cause the tires to deflect quicker and increasing vehicle load will cause the tires to deflect farther (if tire pressure isn't increased).

Tire Tech Information - Air Pressure/Load Adjustment for High Speed Driving

 

When figuring tire revolutions per mile, the radius change or deflection, loaded or unloaded, seems to mean little.

Instead, look at just the contact patch. It is a section of flat tread with virtually fixed length longitudinal steel cords running through it. The tread is laid flat on the ground, the vehicle rolls over it, then the tread is lifted at the back, looped over the top, and laid down again in front.

Think in terms of the linked metal treads on a caterpillar tractor or a tank. The shape of the return loop -- nearly circular on a tire, just a flat loop on a tank tread -- doesn't matter. Over a fixed distance, the number of times the loop goes around doesn't change unless the length of the loop itself is changed.

 

They dont act like tank treads, but with radials i will grant there is less change with pressure. my belief is above normal inflation it is going to be small change ... tank tread dont change at all and a small change is not none so i fill its a bad analogy... but it will change and if u find the your odometer correction factor accurately enough (figure out ur odometer error between fixed highway marker on the interstate and actual odermeter readings, 10miles will show 1% error, 100miles 0.1% error) you will see it. Again the deflections probally aren't large enough to notice. Distance = angular rotation x radius. On a tank tread you use the radius of the sprocket to turn the tread and the tread doesn't change length. On Cars its close the effective radius at speed.

i got really interested in this and researched some. i have book beside me "Race Car Vehicle Dynamics" that tells what effects tire rollout and pressure are among them. Anyway, i found this discussion has come lots on the internet. Rarely is any real test done just people stating opinions but i found one thread where this exact same discussion came up in thread and some people tested it. One person believed they acted like tank treads another person said from his real life experience calibrating his bicycle odometer they did not act like tank treads ...... someone else had a 3 wheel motor bike with radials in the front and bia plys in the back and tested ground rollout He did mention the front is lightly loaded so that would make the effective pressure in front higher but no one mentioned that. Anyway, They under inflated the tires & tested the distance of few tire revolutions and measured. Then over inflated and tested the rollout and measured and compared the differences. The bias plys had nearly linear distance changes with inflation pressures. the radials changed with different pressures but not as much. they concluded radials act in-between the two because rollout changed but not as much.

Again the radials did show roll out changes with pressure just the changes where not as pronounced but definitely enough to be significant. The test was repeat multiple times. it was static test, not at speed but i would expect to see similar results at speed. i read it few nights ago but found it again and here is the link, just gloss over the part about how odometers and speedometers work:

CR4 - Thread: How accurate are vehicle odometers?

 

Small, yes, but I've been understanding that it is small enough that a constant circumference model is closer than an 'effective radius' model.

None of my bicycle tires have been steel belted radials, so this model is not relevant to them.

'Lightly loaded' is a very key point here. Unloaded, with no to little deflection, I'll not be surprised that the effective radius model works well, and even tread wear effects are quite noticeable measurable.

I would expect the tire behavior to transition towards a constant circumference model as the load increases to the normal operating level, creating a significant flat spot of a contact patch. Those 'lightly loaded' tires could easily be in between these two models. I'd prefer to see tests in the normal automobile tire loading range.