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Re: 2000 Astro Speedo adjustment?
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.
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You have to know how it works,
to figure out why it doesn't.
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