Regarding RPMs, competitive cyclists in general tend to run higher a higher cadence than non-competitive cyclists. I used to do a lot of road racing, mostly criteriums, where the average speed is 25+ mph and can easily exceed 30 mph during attacks, surges etc. Sprints in the higher categories can exceed 40 mph. Most racers maintain over 90 rpms in those races and for trackies like me it's more like 100 rpms. The laws of physics dictate higher rpms in order to achieve higher speeds. Wind resistance increases as the square of the speed, so it takes four times the power to go twice as fast.

Power = Torque X RPM

The human engine is similar to a motor in that power output increases with rpm up to a point, after which it no longer increases or drops off because torque output drops. This point is approximately 120 rpm for a trained human engine. On the road with gears, the athlete can maintain a fairly constant cadence by selecting different gears, but on the track with only one gear a compromise must be made between a gear that is low enough not to bog down at slower speeds and high enough to permit higher speeds such as during sprints. A fairly typical racing gear ratio on the track is 50 x 15, such that assuming 700 x 23c tires, a cadence of 120 rpm equates to about 31.3 mph and 40 mph requires 153 rpm. However, 40 mph can only be achieved if the power output at that cadence equals or exceeds the power required to overcome the wind resistance at that speed.

Riding fixed non-competitively on the road is a different situation than racing on the velodrome. There are other factors involved such as hills and lower average speed. This is why lower gearing is beneficial and the goal is to maintain an efficient cadence between 80 and 120 rpm during most of the ride. So 48 x 20 gearing with 700 x 25c tires results in 15 mph at 80 rpm and 22.6 mph at 120 rpm. Since you require less torque to generate the power necessary to maintain a given speed, you are putting less pressure on your joints while maintaining the same aerobic level. I have several road FG bikes with varying gearing, but never more than 70 gear inches. The main purpose of these bikes is to recover from hard training and racing by exerting relatively little force on the pedals while maintaining a relatively high rpm. They are also far more enjoyable for social rides at a leisurely pace.