quick ?? i have a 85mph top speed cluster in my gt (it was swapped in when i bought it) could i just swap in a 140mph one and be good or do i have to do some wiring??
That exact cluster on ebay will fit in an 87-89 car. The 87-89 have a flat bottom on the cluster. 89 only had the 140 speedo. And you are correct, the 90-93 do have different wiring requirements, and an arch cut out on the bottom. They can be made to fit by rewiring the connectors, but it is not an exact drop in fit. See the revision to the ebay ad.
No, if you are not making any other changes. The stock speedos (85 and 140)are calibrated to count the same number of cable turns for any given speed. You only need a new speedo cable gear if you swap out the rear end gears or change the drive gear inside the tranny on the output shaft to a different one (6,7,or 8 tooth gears).