Speedometer off by 20mph

When I first got the inline hypertech I changed both tire and gear size due to just installing 4.88’s. With that change the speedo was waay off. Dropped it back to stock 3.90 and only kept tire size changed, then it read proper speed. No idea why the hyper tech has parameters to correct for aftermarket gears when it clearly does not coincide with the changes that otherwise would seem appropriate.

 

Mostly due to the fact that the Tacoma gets it's ground speed input from the wheel speed sensors, so it wouldn't even matter if you had gears at all (as if you were flat towing it rolling on hubs like a trailer).

Older vehicles had the ground speed input coming off the transmission or transfer case output shaft speed. Change your gears, you changed your ground speed. Change your tires, you change your ground speed. Change them both the right way and you can get your ground speed input pretty close to accurate again.

So, with wheel speed input as your only input, that is what our speedos are looking at on these trucks.