I was wondering if there were any aftermarket companies that make a computer reprogrammer that you can change the tire and gear sizes multiple times, not just the one time when you send the company your information and they then burn you a new chip. it is for my 89. The truck only gets around 12 mpg since i put the bigger tires on.
From my understanding they don't reprogram your computer they just make a new chip for your specs and when you want to change it you have to have it done again. They typically reprogram the 0bdII computers. Other than buying a chip you could change your gears to match the tires.
The '89 is going to be something mechanical that needs replaced. It is going to take a calculator and a trip to the dealer to get it fixed. Fastchip cannot take this into account, I have one of their chips.
Should be very inexpensive, but maybe mathematically intense to get the correct gear reading on your speedo. The dealer is where to go.