Here is my question. My original tire diameter from 67 was 25.5". Right now the tire I have(215/60R15) equals 25.1". The gear I got from GM Jim is for this tire size(25.1"). How affected would the speedo be if I go to this tire size? 225/60R15 which is 25.6"? Thanks.

THis will help: http://chevelleengineer.home.comcast.net/speedo.htm

Thanks for the link, but unfortunately it doesn't have the TKO, but even if it did, I don't think it would tell me what I need to know. I am trying to find out if you have your speedo set up for a 25.1" tire, how off would it be by going to a 25.6" tire? I don't think the gears are that precise. I think it would still work, but I am not 100% sure.

This helped me when I changed transmission and axle gear:

http://www.novaresource.org/speedo.htm

I spoke to GM Jim and he said the 225 would actually be better for the gear he sold me. The 215 is at the high end of the gear so using a 225 would actually make the speedo that much more acurate. Thanks guys for the help.