Thread: RPM's to high?

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Originally Posted by 34_40

I finally got the coupe out and went onto the highway to see how it would do. All went well, nice smooth ride, no shakes or rattles / noises. Even the exhaust was real smooth! I was worried that it would drone and cause me major annoyances.

My only "concern" was the engine rpm at 65 was a touch over 2500 rpm. Do you guys consider this to high? the current combo uses an AOD with 3.90 gears. Tires I think were 38" tall...

Anyway, what do you think?

Mike,
Something's wrong with your numbers. That AOD has a 0.67 to 1 OD ratio, right? If you're running 3.90 rear gears in OD a 28" tall tire would have you at 2,038rpm at 65mph. To get it up to 2500 in OD would be something in the range of 22"diameter on the tire. Were you maybe not in OD? At a 1.0 ratio in third a 33" tall tire would have you cranking 2581rpm. Sure your tach is right? Using a "fudge factor" to get the units right, RPM = (MPH x Gear Ratio x 336 x Final Drive Ratio)/Tire Diameter in inches (65mph x 3.9 x 336 x 0.67)/28" = 2038rpm. If you're in third, (65 x 3.9 x 336 x 1) = 3042rpm. Something's not right here.

Do you maybe have an AODE? That has a 0.7 OD ratio, which would put your numbers a little higher, but still not matching up with your 2500rpm at 65mph intro.