If I switch from sotck tires on my 15 inch Rallys to 17 inch wheels with 255/45/R17s, how far off will my speedo be, and what does it take to make it accurate again?

Thanks,
Bryan

 

Check specs for your new tire diameter. Stock is ~27.2". I am guessing yours will be ~26". If you go with 255/50VR17's I am told that you get the diameter back up to ~27" - just about stock. With your current choice, your speedo will read high by the ratio 27.2/26 = 1.05 That means indicated 60 MPH would actually be 57 MPH - assuming that you had the speedo calibrated correctly before you switched tires... Try a speedo driven gear with one more tooth - i.e go from 22 teeth to 23 teeth and see what that does for you. It will drop your indicated speed a bit. Check the data on the forum to learn what 60 MPH equates to in RPM for various rear-end gears. Pick your gear and get the speedo calibrated to give you the specified RPM's at an indicated 60 MPH. Tom's 73 Vette web pages have all this data for the '73 Vettes. For instance, my 4 speed car has stock 3.31 rear gears. RPM's at 70 = 2933 and RPM's at 60 = 2513. I tried the 21-tooth gear and ended up using a 22-tooth gear to get 60 indicated MPH when my tach reads 2500 RPM. From a good book I have:

Gears MPH per 1000 RPM
4.11 18
3.70 21.4
3.55 22.3
3.36 23.55
3.08 25.7

Pick your gears from the above, solve for RPM's at 60 (or any speed you care to use - higher the better for accuracy!) and begin to experiment with speedo gears! Have fun!

 

go to........ http://richmondgear.com/101032.html
RICHMOND Street Performance Calculator

 

Good stuff!