Gear Ratio Calculations

Buckshot · Post by **Buckshot** » Sat Dec 24, 2005 3:30 pm

In case you guys missed it, on Superlifts Offroad Adventures, during the tips section of the show, the host gave a simple equation for calculating RPM's with any given tire size. So I tried it out with my CC, I already knew from the code in the glove box that I have a 3.42 gear ratio. I didn't really know what the stock tire size was but with this equation I can solve for it. The trick is that you cannot be in overdrive to fiqure. The equation that was presented on the show was:
RPM = MPH*G.R.*336/T.S. To solve for T.S.(tire size) use
T.S. = MPH*G.R.*336/RPM. For my test, I put the CC in 3rd gear (1:1 ratio) and drove exactly 55 mph and noted the rpm's and in my case was 2250 rpm's in third gear. When I did the math it came out to 28.0896 inches. I want to go to a lower gear (higher numerically). If I swap my 3.42 for a 3.73, the rpm's with the stock tire size will be 2462 @ 55 mph. The rpm's with a 30" tire will be 2297 with a 3.73 gear ratio @ 55 mph. If you do not know your gear ratio, you can solve for it by re-arranging the equation to : G.R. = T.S.*RPM/MPH*336. All in all a useful tool if you want to see how changes in tire size, gear ratio's, rpm's and mph can affect each other. Just thought I'd share. MERRY CHRISTMAS!!!

Buckshot · Post by **Buckshot** » Sat Dec 24, 2005 7:46 pm

I can't believe I didn't see the calculations sooner on the home page....ooops!!!!

F9K9 · Post by **F9K9** » Sat Dec 24, 2005 7:58 pm

NO problem, we all miss things