You are focusing on the wrong side of the equation. In order to go 20 mph, you need X sustainable watt. (Where X depends on your exact bike configuration and setup.) If you have heart & lungs to do X sustainable watt, you can do it at 60 rpm or at 90 rpm, it does not make all that much difference. (Most cyclists would find that it's better to do it at 90, because trying to maintain high power at 60 quickly burns out legs and trying to do that repeatedly can hurt knees.)

I understand that you have equipment limitations (48x14 low gear & 26" wheels) and that is something you want to deal with anyway, sooner or later. This subforum is called "road cycling" and that implies a road bike (700c wheels with thin tires and drop bars). You can get one used for 200-300 bucks in a decent shape.

You need to do something about measuring equipment too. Numbers you're quoting are too vague to be useful. Wired bike computers with speed sensors are dirt cheap. Strava smartphone app + some hills would give you a better feel for your estimated power.