I have a 91 camaro RS 235/55r16 tires (also stock size according to door sticker). I have switched from 2.73 to 3.08 gears from my research I should currently have a 17 tooth drive gear and a 37 tooth driven gear. The problem I am having when I do the calculation 770 revs per mile, with the old 2.73 rear it shows I should have a 17 tooth drive gear and a 35.7 (36) driven gear. Was my spedometer off from new? Will a 17 drive 39 driven gear be close enough instead of the reccomended 17 drive 40 driven gear per the calculator? I just want to be sure so that I get the right stuff the first time.

If you were able to enter your exact tire height in whatever calculator you used, it should be correct. Whenever I get a result like 39.3 or 39.7 for example, I just round up or down to the closest whole number. 39.3=39 tooth, 39.7=40 tooth.