I have the 85mph speedo in my 87 GT. I notice my speedo needle goes way above the 85 mark. Could I simply buy an aftermarket kit that sticks over the gauges with the 140mph mark?

 

no, the numbers are NOT in the same spot

 

No, but the solution isn't too hard to accomplish. Just need to get a 140 speedo, then replace the drive/driven gears for the cable.

 

Ok thanks guys.. Why would I need to replace the drive gear though?

 

I might be mistaken but I think the stock gear for 85mph speedo was a 6 tooth, and 7 tooth on the 140's

 

Well I have 3:73s with the purple gear installed and my speedo is 100% accurate now so I guess I will just leave it alone.

 

its an automatic ?

how sure are you its accurate ? because i was pretty sure all fox body aod's are 8t drive, and its part of the output shaft so you can't just swap it without a 7t or 6t output shaft.

purple is AOD only and 21t, it's correct for 3.27 gears and 8t aod

both my aod cars were 8t, thats why i think that

 

Well I read a post somewhere on here about gears and it said with an AOD and 3:73s I would be going 75mph at 2500rpm. My speedo is dead on with that statement and while driving by a speed trap the local PD had set-up I was within a couple MPH of there radar.

 

The current gearset makes no difference if you plan to go with a 140 speedo. The gears must be changed, or it will be way, way off.

 

i dont think you are correct.

how fast were you going ? I'm guessing you are 10% in error. about 5% per gear set is typical. so if you passed the radar at 35...then you'd be off only 3.5

but you might be...im just saying look into it more, if your next to a friend on the freeway see where yours and his are at like 75

thats a lot easier to tell then. or zero out your odometer and drive about 10 miles and see if your still hitting 100% on the mile markers