There are two calculation tables, one for current trans/tire/rear axle/desired speed and one for potential trans/tire/rear axle/desired speed allowing you to compare any changes you are thinking of making.

The numbers used as a default for the first set are what I currently have in my 1967 El Camino; a TH350 transmission, 235x60x15 tires, and a 3.31:1 rear end gear ratio. The numbers used as a default for the second set are a 'what-if' I change ONLY the transmission to a 700R4. At 75mph my current engine speed in 3rd gear is approximately* 3200 RPM. Changing to a 700R4 reduces the engine RPM at 75mph to approximately* 2300 RPM due to 4th gear being an overdrive gear with a 30% gear reduction.

Simply input your current trans gear set from the list or if not listed you must know the gear ratios, tire size, and rear axle ratio into the first table then 'play' with the second to see the differences. You don't HAVE to use both calculators but plugging your current setup into the first allows you to play 'what-if' a little more freely with the second calculator.

* Naturally some factors come into play such as (1) not all 235x60x15 tires are EXACTLY the same diameter, and (2) your torque converter (if an automatic transmission) may 'slip' more at a given speed resulting in a higher RPM than calculated.

If you are thinking of changing your rear end gear ratio and nothing else your speedometer reading is going to differ from its current reading.