Rear gear calculation

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DartGTDan

'71 Dart GT Fan
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When I bought the Dart GT the previous owner told me it was a 3.23:1 sure-grip (in the 8 3/4" rear-end). The gear set axle tag is long gone.

The car has P215/75R14 tires on the rear. At 70 MPH the engine is turning ~3400 RPM. According to online calculators I come up with 3.73 gears.

Does 3.73:1 sound right? Were 3.73:1 gears available/optional in 1971, or were they swapped in at a later date?
 
I come up with 3370 RPM with 3.73 gears and 26.7" tire. Now could be a 3.23 gear with the incorrect speedo gear in the transmission. Jack the car up, mark the tire and the pinion so you can see both marks. Spin the tire around one complete revolution, count how many times the pinion spins around.
 
How much does tire height influence rpm ? Or does it just change the speedo? I'm asking because I'm going to run a 235 75 15 tire with 3.73 gears will that still be 3370 at 70mph?
 
I'd say the real question is are you really doing 70mph? The rpm seems high, but if the speedo gear is off because the transmission was ordered in a different car, or with a different rear ratio in that car, then the speed will be higher or lower. If the car is a sure grip, jack the back end up by the meatball, mark the driveshaft and tire, and turn the tire, counting the driveshaft rotations.
 
My 80 corvette has the correct tire size and trans, and speedo gear. With a 3.55 gear and a 255/60/15 which is 27" tall. 60 mph is 3000 rpm. 3.08 gears used to be 2600 but the car was a stone, much better now.
 
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