[Jim Allen]

When translating the effect of LRR tires into your own mpg readings, it will pay to be up on the topic of speedometer accuracy, or lack thereof, due to tire size differences. Tire Rack (see link below) presented some info that bears consideration, else you make false claims of joyously increased mpg or become suicidal because you lost mpg due to a tire change.

I ran into this some time back checking the FE of my Accord, which, it turned out, has shorter tires than the speedo was calibrated for (namely 205/60R-16). These are factory installed tires ,BTW, but the speedo is calibrated for 205/65R-16 tires. The end result is that I was traveling about 64 mph while indicating 70 and indicating about 110 miles while traveling only 100. Needless to say, I was "Whoo-hooing" my great 31-32 mpg out of a 3.0L V6 while actually getting under 30 mpg.

Well, we're "only" going to get about 70K from the OE Michelins but I will be replacing them with 65 series tires. I contacted Honda, BTW, and they claimed to have no way to recalibrate the car for the shorter tires.. unlike some other mfrs that can reflash the PCM for a variety of tire sizes. The the 65 series tires have a radius about 3/8-in taller and will put the speedo back to a more accurate state, but also drop the rpms by about 75 at 70mph, which could deliver a slight mpg gain.

I have a programmer on my Ford pickup and I measured the tire circumference to the millimeter at my standard pressure, entered that into the programmer and my speedo is deadnutz. As the tires wear, I can remeasure to keep the speedo as accurate as possible. On my other cars, I determine a correction factor as accurately as possible and use that in calculating my true miles traveled.

Tire Tech Information - Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires