[MetroMPG]

Hi Jim -

I'll answer your questions as best I can:

Originally Posted by jimepting

What concerns me is that the conclusions run contrary to several world class hypermiles practices. 70-80psi is standard pressures for those trying to wring out the very best fuel economy.

I'd like to see their as-controlled-as-possible tests. I'm open to new evidence!

(Food for thought: lots of "elite" athletes perform strange pre-game rituals they wouldn't dare omit prior to an event. It's easy to see why a "higher must be better" theory might be adopted with respect to tire pressure.)

I studied your chart and I see a couple things which don't look quite right. First, there is some inconsistency at the 60&70 psi level. In both cases, there were two significient outliers.

No argument there. All I can say is I've done this test 4 times now on 3 different vehicles, and each experiment showed very diminished returns (to the point of changes disappearing in the testing variation) above the 50 to 60 PSI level. Could be limitations of the testing. Could be tire make as well. Maybe some tires continue to improve > 60 PSI, and I didn't happen to try those.

Second, not mentioned in the methology was warming up the transmission(I guess it has one) and tires. By starting cold and doing the tests in a reverse order of pressure, there is a bias injected in favor of the low pressures.

That's possible. I have no evidence on temperature changes, but judge for yourself whether transmission/tire warm-up is likely an issue or not based on this:

• The car was driven less than .5 km to the test area in summer temperatures at speeds less than ~30 km/h average (because I was using the hand brake to stop, to avoid engaging the front hydraulic brakes).
• Starting the test at elevated pressure means even less propensity for heat build-up on the way to the test area.
• The transmission was electrically driven, so no heat transfer from an I.C.E.
• The speed during the test was never much beyond about 5 km/h (whether coasting down hill or reversing back to the starting line).

Personally, I wonder if the transmission & tires ever got much above ambient temperatures throughout the experiment.

On the other hand of course, is 80 psi over 60 psi better to any significant degree for most folk. The answer is probably "no."

I'm with you there. That's what I tend to think, based on trying this 4 times in 3 different cars.

I've won a couple of competitive hypermiling events myself (placed 2nd in another... incidentally, the 118 MPG winner of that one ran @ 50 PSI), and don't bother going above 60. I'd wager the outcome of those events (or long-term extreme hypermiling efforts in the real world) depends far more on other factors than 60 vs. 80 PSI.