Thrust, whether forward or reverse, is not only a function of RPM, air density and temperature, but also speed. Forward thrust decreases with speed and reverse thrust increases. When standing still the maximum reverse thrust can be as low as 5 to 10% of the maximum forward thrust. At 150 knots it can be 20 to 30%, but that number decreases rapidly as the aircraft slows down.
Most aircraft uses tires that have a speed limit of 195.5 knots, so that will be the maximum speed on the ground. This can be limited further by the energy the brakes are able to handle, but this is not a fixed speed as it is of course is depending on weight. It may be higher or lower that the tire limiting speed.