adjust the speedometer

[DrPablo]

Hi,
how can I adjust my speedometer. I have 235/75 R15 tires on my GMC Jimmy 1995. On a 5 miles test at the freeway I saw that my speedometer has a difference about 0.3-0.4 miles per 5miles.
I know that if you have other tires then the original, you have to adjust your speedometer. Does only the dealership can handle this or is there a cheap/easy way to do this?
Thanks a lot

 

[River_Tam]

Many of the aftermarket chips give the option to adjust speeds for oversized tires...to be honest as little as yours is off if all you want out of the chip is to adjust the speeds I wouldn't wast the money. Chips will also give you slightly better power and better mpg but need to be removed during any kind of emisions test where they actuals put a probe in your tailpipe. Depends on where you live for how serious the tests are. Here if its obd2 compliant they check to see the check engine light works, and is not on and that's the whole test. Wouldn't even need to pull the chip for that

 

[River_Tam]

Vin will tell you your stock tire size, sounds like you might need 235/70/15s