speedometer problem
 

i bought my 87 gxl a few months ago and 10 days after i got it, the tranny blew up cuz all the fluid leaked out the screwed up front seal(prolly why the guy wanted to sell it). so i went down the junkyard and found a 89 GTUs stripped to the bone :'-(, except the tranny. so i bought it and swapped it with my blown one. as per my friends instructions i swapped the speedo gears as well. the other day i was following my friend and he infromed me that we were doing 75 mph but my speedometer said i was doing 80. how do i fix this problem, do i need a non-GTUs s5 speedo or is there something i need to do in the dash/cpu/whatever in my car. thanks for any help.

Depending on what rims and such you have on there it could register different. That's all I know, not sure on the rest.

the rims are stock

but are the GXL rims and the GTU rims the same size?
also are the rear ends different?

i know the differentials are different, not sure about the rims, but thats why i used the speedo from my old tranny instead of the GTUs one

bump

up to 5 MPH more than actual speed is within stock variences.

If it is a real concern you need to have the speedo checked at a speedo shop.

I would not be taking a "friends" report of speed as being correct. If his is off 2.5 miles off and yours is also off 2.5... there is your 5 MPH.