I have a 92 mustang with 3:55 gears. I have the 140 mph speedo, now also I have a 1986 transmission in the car, now my question is how do I get the speed back to normal? Since the transmission in 86' was for an 85 mph speedo would this be a factor in getting the right speedo gear?
Yes, the 86 tranny is a factor, but not because of the speedometer. The pre 90 transmissions have a 7 tooth gear inside the tranny, while the 90 up models have a 8 tooth. According to Steeda, you need a Black 20 tooth gear for the 3.55s if you have a stock 86 T5 tranny in it. The speedos are all calibrated to count the same number of revolutions of the cable, so it should be fine. Right now you probably have the 23 tooth white gear and it is turning the cable slower than it did with the old tranny, causing your speedo to read wrong.