Ok so I have a 99 gt. I pick up a supercharged motor and tranny out of a wrecked 2000 gtp, unfortunately I couldn't get the ecu and wiring harness out of that car. So my stock tranny has been slipping so I decided to put the HD tranny in (I'm doing the rebuild of the l67 block in the mean time) I put it in and everything works fine except I have no 4th gear no overdrive. Then I find out the gear ratio is different from the HD tranny and the stock gt tranny and that's why it won't go into overdrive . It just stays at 3000 rpm at 70 mph I was wondering if they is a way to get my stock ecu to work with the HD tranny.
