Sooo, the 700r4 went out in my 84, i replaced with a 87 r4, now i read about 10-11 mph highr at 65 mph, but its more accurate at lower speeds, only off by like 2-3 at 30-35mph. I used the same speedo cable and gears from my 84, is it possible that i installed them wrong? whats up?

 

With a 3.73:1 gear, you need the 15 tooth drive gear, and for 25.7" tires such as 245/50R16s, a 44 tooth driven gear. For tires like 235/60R15, a 43 tooth, and for 245/60R15s, a 42 tooth. I bet you just swapped the driven gear but not the drive gear. There's 2 threads in here with part numbers for every 700R-4 drive and driven gear, I know because I submitted the lists to be made into stickys.