Pontiac rearend swap, shift problems on computer controlled tranny...?

Question

I got a whole new(newer) rearend for my 97 formula recently and it has a computer controlled transmission and the rearend I put in isnt the same exact gearing and it seems to shift too late now and my speedo is off, I realize there is programmers to change shift points and change ratios, but I am not sure what the new ratio is... and if I did know, how would I go about changing this back to stock shift points(or better) for cheap or free. all I know right now is that when im going 55 mph, my speedo says roughly 68-69 mph, give or take a bit since I was just using a pace car with cruise on. Any help on this would be great!

Answer 1

if you don't want to buy a programmer take the car to a good diagnostic or performance shop or your local dealer and have them reset the computer by changing the gear ratio and tire size options, they can be changed with the factory scan tool.

Answer 2

You just need to change the gear on the speed sensor to adjust for the different axle ratio. Speedometer repair shops can do this for you or see if you can buy a different gear for it at a dealer. Use to be 1 tooth equals 5 mph difference on older speedo gears so if you're reading 15 mph high you need to add 3 teeth to the existing number on the gear...i.e. if the gear on the speed sensor is 12 teeth now, you need one with 15 teeth. That formula was pretty accurate for older GM cars...newer speed sensor gears may require more or less to get the ratio correct. A correct gear may not be available for it in which case, you'd have to get a custom gear reducing adapter to install between the trans and the sensor....again, through a speedo repair shop.