2000 Astro Speedo adjustment?

I've got a 2000 Astro AWD and ever since I bought it used 3 years ago the speedo has been running 10% fast - it reads 82 at 75. I've checked it against 4 different cars and trucks on the interstate - cell phones DO have a use. I've also noticed the same amonut of error on the different "your speed is" radar signs and trailers around town - 10% fast.

My tires are P215/70R15's all around instead of the reccomended P215/75R15's (seems the tires shops all agree on this after 3 sets) which accounts for a 3.1% difference in circumference (miata.net). I contacted a local Chevy dealer about adjusting the speedo and they said it's an hour - $95.00. When I asked if this included checking to see if the correct sending gear is in it and they said "why?"

So, can a 10% adjustment be made through the onboard computer, or should I ask a (different) Mr. Goodwrench to check the gear and adjust it?

harryevan

It is corrected by re-programming or "flashing" the PCM. Each tire size generates a certain number of pulses per mile, (PPM) depending on the circumference: bigger tire = less PPM, smaller tire = more PPM. Your 70's have a smaller circumference than the 75's, that's why you're speedometer is indicating fast, (more PPM). The Vehicle speed sensor in the transmission counts pulses, (that's why they asked "why?"). BTW, the incorrect speed data that the PCM is receiving will affect ABS, engine, and transmission performance also. Probably well worth the $95 to have it flashed.