I have 4.10's in my 98gt 5speed
guys at cjponyparts did the entire install and gear speedo correction....
EXCEPT
its not perfect, in fact its off.
I believe they used a 21 tooth ill have to check my receipt.
The faster I drive the more the MPH is off.
I got a great deal from cjpony parts ($535"i believe thats what i paid cant remember" including tax for frpp 4.10s speedo correct and labor, install kit, etc.) I didnt have the money, but if i did i would have probably gotten the ford racing speedcal (i think thats what its called)
anyway the guys at cjponyparts told me that going with anything higher than a 21tooth has led to failure. i think i remember them telling me they tried a higher teeth gear 23etc. and they were breaking.(in other cars)
dont quote me on that exact cause but im pretty sure thats what i remember ill have to check my receipt again to make sure the numbers are correct.
its not the worst thing in the world. On the highway i just stay with the flow and i also have a gps that shows an est mph display. I pretty much figure its in between the 2 readings i have. its kinda ghetto but its not the end of the world. Like i said the higher the mph the greater my speedo is off.