I have 3.73s installed in my 93 Firehawk, my speedo and odo are off by 15%.
I remember SLP used tomake a calibration device for that. I was hoping someone has one they are willing to part with.

You can also just change the plastic speedo gears in the trans. Doing it that way is considerably less money than the SLP/Dakota Digital device.

I found a used SLP unit for $50.