beng1

90rpm is an average pedal rpm for riders who train for speed or who are in a race. I am 60 years old and train at that pedal rpm often, but I am going between 19 and 20mph. If your average speeds are only 12-13 mph then you do not have any gearing problem at all, if anything you are pedaling too slowly in too high a gear. By simply changing gears down so you are pedaling at a little higher rpm you will get a faster top speed once you are used to it. It is the same as a gasoline engine, up to a limit they make more and more horsepower at higher rpm because there are more power-pulses per minute, so even if the engine is not as efficient at a higher speed, the higher number of smaller power pulses per minute give it an overall higher power level at the higher rpm.

13mph is very low speed and low-performance cycling, and the number-one factor in going faster at that speed is the rider, not the equipment. Until you get up to average speeds around 20mph the equipment does not make any difference at all. If you get to the point where you can average 16 to 18mph while sitting up pedaling, then getting a "road bike" with drop bars and the horizontal riding position it allows will get you close to 20mph. Today I was riding my mountain-bike with another rider on a paved flat route and I had no trouble going 20mph for a number of miles on it because I had the bars set very low so I could get down in a road-bike position, and I also put 80psi in the tires, which helps. You may gain a mph just by running high pressure in your tires.

You have a long, long way to go before it is the equipment that is holding you back for sure.