i just drove my 3800 series II car on the interstate and got it upto 70MPH and noticed it was reading 3200 RPM at 70mph this is a stock transmission from a 86SE v6, a 4 speed, im kinda curious if thats what the RPM's are supposed to be, or if my RPM gauge could just be reading wrong.

like stated my tires are 245/45/17's final drive on my transaxle is 3:65 but i dont know what the 4th gear is, im wondering if anyone can calculate what my RPM gauge should read?

matthew

bump

MPH = (RPM x Tire diameter) / Gear ratio x 336

Tire diameter = (245 x .45) / 25.4) x 2 + 17 = 25.68 inches tall

25.4 mm = ~ 1 inch. 45% of your tire width = your tire side wall height x 2 since this is a circle plus the rim diameter = 25.68

I haven't worked it out to confirm it but I'm sure the overall gear ratio is what's intended for the equation so for 3rd gear which should be 1:1 for the transmission you should be able to enter your final drive ratio and come up with what your odometer should read in third gear. In any other gear I believe you'll have to multiply the final drive by that gear ratio.

[This message has been edited by Joseph Upson (edited 10-01-2006).]

should be about 2850 RPM... more or less depending on the amount of tread on your tires. Fourth gear should be 0.81 on a standard Muncie 4 speed trans with 3.65 final drive.

------------------
FierOmar

[This message has been edited by FierOmar (edited 10-02-2006).]

maybe your speedo is off

whenever changing your tire size if the overall tire size is larger than what your original tire size was you will be increasing the speed at anygiven rpm. and vice versa.....if you can find one of those radars that are on the side of the road. get up to what your car says is X MPH and see what the sign says then add 2mph to whatever the sign says. (cops lie!!!) but that will tell you if your speedo is off.