I expect to be out of town, but I was interested in the MPG competition that will precede or be part of the Green Car and Transportation Festival on July 29 in Ipswich, MA. It looks like it will be a great event. The emphasis on tire pressures in the rules caught my attention. Manufacturers generally specify cold inflation pressures. If a Prius has a tire inflated to 37psi after running the 24 mile course, will the judges disqualify it? The rules say nothing of the tire size. They are using the MFDs to measure tire revolutions rather than miles per gallon. Using a worn tire or smaller size tire should give higher computed MPG even if the true MPG is unchanged. Does anyone plan to use compact spare tires on the front or all four wheels? I don't know the rolling resistance of the temporary spare tires. Toyota does recommend inflating these to 60 psi. More importantly, the odometer and mpg will read 6% higher than with 185/65-15 tires, if the same distance is covered with the same actual mpg. I thought of this after I realized that my larger 195/65-15 tires would have put me at a disadvantage by rotating 2% fewer times per mile traveled.
They'll learn. Several lessons will probably be brought over from Tour de Sol, which has tried to cover a lot more bases with regard to mileage competitions. For one thing, 20 or 25 miles is way too short to eliminate a bunch of factors -- it should be at least 100. The "tires <= 50 psi" rule worked well for TdS -- common SIDEWALL pressure plus a little slop for warming. [See near the end of http://techno-fandom.org/~hobbit/cars/tpms/ for what happens when you run Integrities at what Toyota says...] And as you mention, putting non-stock smaller tires on would yield more revs and show more miles, presumably for the same amount of fuel burnt. [Perhaps measurement of tire circumference will start becoming another criterion...] . Accounting for all these things is NOT an easy problem, if you want to be absolutely fair about it. The gas tank bladder, for example, threw the whole idea of measured refuelings into a cocked hat. And if everyone just trusts the onboard MPG computer they're still not getting an accurate long-term picture since they tend to read a bit optomistically from pumped gallons. . _H*
