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I have been driving my Geo Metro convertible with 40psi in the tires at 70 degrees F. I made 2 recent trips of 730 miles each within 3 weeks taking the same roadways. The first trip netted me 44.1 mpg after setting the tires to 30psi front and rear. I switched back to 40psi and averaged 47.8mpg on the second trip. I held local speed limits on each trip which included stretches of 70mph. The load was myself, my dog, toolbox and overnight bag.
The ride is slightly rougher with the additional air in the tires but it is well worth the added mileage. The car is stock except for the XFI cam and timing gear changes, ATF in the transmission. Driving was with the top up and windows closed on both trips. The weather was similar both times.
I did some maintenance before the first trip and found one tire to be at 30psi, so rather than open the garage again for the compressor I just evened out the tire pressures with the thought of checking mileage changes. This was about as accurate a test as I have done in a long time. I also fill right up to the filler neck to be sure the usage is accurate.
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