Super-low Gear Ratio in a 7.25” Rear End??

Okay, this is weird. I finally drove my son’s project car (’69 Dart, slant 6, A904, 7.25 rear) on the freeway for the first time, and the engine RPMs were waaaay lower than what we’d expected.

If I calculate the 2.76:1 axle ratio and 1.00:1 3rd gear ratio and 25.7” overall tire diameter (225/55/16), I should be getting 2500 RPM at 70mph. See my chart below.

Then I drove it: At 70 mph in 3rd gear, the tach shows only 2000 RPM. Maybe slightly less.

Dropping the stock A904 into 2nd gear at 70 mph, the tach showed only 2700 RPM. Almost what I was expecting to get in 3rd gear!

What in the heck? Is there a rear axle ratio numerically lower than 2.76:1 for a 7.25” rear end? The differential cover has no identification tag on it.

If I reverse-calculate the rear end gear ratio to match what the real-world RPM was, it’s something like a 2.10:1 ratio!

And to make it weirder, the car accelerates pretty stoutly off the line in first gear, which I would not expect with such a "granny gear" (granted,this motor is built up pretty extensively from the stock configuration. Even at 90 mph this car has plenty of power reserves)