Gearing is a fixed reference point. If you're running the factory 3.91:1 differential and the R154 transmission then you should see in 3000RPM at 41.8MPH, 3500RPM at 48.7MPH, 4000RPM at 55.7RPM, 4500RPM at 62.7MPH all in 3rd gear regardless of the load... So whether the car is floored or not those are absolutes. The only thing which will cause those numbers to vary regardless of how hard you're pushing the car is a slipping clutch.
Really any RPM at a given speed should be consistent despite load, those are just a few quick reference points I know.
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