i have a rental car, an 05 sentra "s" edition with an automatic trans and bridgestone potenza tyres. the car has about 37,000 on the clock. this car features a digital miles per gallon gauge that calculates my average miles per gallon untill i reset the gauge. for instance, if i reset the gauge and drive for 30 miles, it calculates the average mileage for the 30 miles i drove. i have been doing some testing on tire pressure versus fuel economy tonight on an empty highway road where there is no traffic and i can set the cruise control and watch the mpg gauge. the praticular road i travel on has a 10 mile strip of nothing but straight road. i kept my speed at exactly 40mph with overdrive on and the engine rpm's are just about exactly 1500 rpm's. i had no stops and no traffic to draft behind during my tests. no interference or obstructions at all. i used known land marks to have a rolling start at 40mph and noted the average mpg at another known landmark at the end of the stretch of road. first test was with the tyres at 30psi and a relatively cold car after the 20 mile round trip i averaged 38.6mpg. next i immidiately the same testover again with the exact same conditions, and averaged 40.4mpg. i attribute the increase in mileage due to the overall warming up of the car components like the verious oils greases and tires. finally i did the same test again but this time i inflated the tyres to 44psi, which is the max recomended pressure on the sidewall. other than that 1 change, everything else was identical. i averaged 46.1 mpg. i would like youre opinions, if you will.