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

I can vouch that the tachometer is perfectly good. In fact, it's part of the Holley Avenger EFI system that the car has, read off of the hand-held computer, so it's gotta be a very robust signal from which to meter the fuel injection (it runs great!). Plus, my ear is calibrated well enough - that motor is simply not revved high at 70 mph.

It is the stock 1969 A904 transmission.

And a locked-up torque converter would only make my chart more accurate. If it's not locked-up, then it'll slip somewhat, right? - making the engine RPM higher than otherwise.

I'm certainly not complaining about this lower-than-expected cruise RPM. In fact, we were researching what it would take to retrofit a A998 4-speed Automatic into the car to lower the cruise RPM. This will be my son's daily driver, to include lots of freeway miles, and fuel economy is a factor. Looks like we don't even need a 4-speed!