Rear gear calculation

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?