I don't even know how to go about asking this if it doesn't make sense I'll ammend it.

I'm getting a different transmission with different gearing, but I'm using the same wheels/tires I had before. Is there some formula I can use to calculate how it will effect my speedometer and from there, what size wheels and tires I need to offset the gearing chage?

It's a ZC transmission replacing a stock 91 Civic Si transmission 185/60/14 tires. Transmission specs:
ZC: Si:
1st: 3.250........ 3.250
2nd: 1.944....... 1.894
3rd: 1.346........ 1.259
4th: 1.033........ .937
5th: .878........ .771
final 3.888........ 4.250

 

I'm figuring a good approximation is to compare the final drives, which is 3.888/4.25=.913. So .913*60=54.7, so maybe if you went to a 55 tire it should be approximately the same. I just did this off my head, I didnt think about it too much.

 

[Modified by Crazydave, 11:05 PM 5/29/2002]

 

http://www.sgmotorsports.com/mph.htm#GearSpeed

 

since the speedo is driven off the differential, no change is need to the speedo or tire size.