Trying to figure the gear ratio in my car. Had it out for a drive today. 60 mph was 2900 rpm. This is with P235/60R15 tires (listed as 26.1 height). I think this means my gear comes in at around 3.73?? I was thinking that I only had 3.36's. How big of effect does the tire height have on the rpm that I am turning. I know that a shorter tire is like increasing the gear ratio.....I think. Or maybe my speedo is off due to changing the tire size. Any ideas about what the height on the stock tires would have been? Any input would be appreciated.
wanarace
Feb 6th, 05, 9:57 PM
This may help.
http://www.miata.net/garage/tirecalc.html
Steve
rancherlee
Feb 8th, 05, 9:57 AM
My thunderbird with 26" tall tires spins 2850rpm in Drive @60mph (1.00:1 ratio) with 3.73's in the back. I thinks its safe to say you have 3.73's
Pat Kelley
Feb 8th, 05, 12:38 PM
Road testing is not the best way to determine the gear ratio if you have an automatic trans. The torque converter is slipping some and the engine will run at a higher rpm leading you to think you have lower gears. With the TCC locked on my 200-4R trans the rpm usually drops some 200-300 rpm at 60 mph.