I just did the same thing with my 75, Now my speedo is incorrect. What 5spd did you use? I used a 81. yes I did use the speedo cog from my original tranny. I like the new 5spd other than that. I will change out the diff this summer. At 85mph my rpm's went from 4200 to just a tad over 3600, so I gained about 5mpg. what differances have you seen in yours?
Hey 75 Sizzler
You had better watch your speed, you're not doing 85 you're actually going 99 mph.
According to my calculations, your tire diameter is approximately 24.4 inches. You need the tranny cog with lesser theeth to make your readings correct. But if your going to put in a 3,9 diff, I'd just save the cog off the 81' and use it when you install the diff.
Anyway, my 5spd is off of an 81ZX also, I have'nt had the chance to drive the car yet, but if I find the acceleration to be acceptable I may just leave the 3.545 diff in the car. Better highway gas mileage.
My speedometer is about 7mph off at speedo reading of 80mph with the 24 inch tires ( I'm using the cog off 81 tranny). Speedo would be correct if I had 21 inch tires.
Are you the one with GPS to measure your speed?
Interesting concept, wonder how accurate it is?
Yea i'm the one that has the gps. Yesterday I was going down the interstate in my Z and my wife in her car (95 olds achevia with quad 4) rite next to me. Our speedos were within a a mile of being the same at speeds up to 90mph. Gps said I was doing 83mph in my Z. Got home and took my wifes car out to the interstate, Guess what, The gps matched her speedo to the mile. I can only think that there must be something in my Z that sends the gps out of whack. So when I've been zippin down the road thinking I was doing 85mph (gps reading) I was really going just over 95 (speedo reading).