To find what kind of speed you can get from your engine, you have to convert its horsepower to velocity. Velocity measures the amount of time it takes an object to travel a certain distance and is most commonly measured in kilometers or miles per hour. Horsepower measures the amount of effort over a set period of time. The equation for power, which is the amount of thrust, or power exerted on an object, multiplied by its speed or velocity, can be used to convert horsepower to miles per hour.
Multiply the amount of horsepower by 33,000 to convert it into lb.ft./minute, which is a measure of pounds moved a certain number of feet per minute. For example, if your horsepower is 20, then multiplying 20 by 33,000 equals 66,000 lb.ft./minute.
Divide the amount of horsepower by the amount of power or thrust. For the example, the measure of thrust is 6,000 lbs. Dividing 66,000 lb.ft/minute by 6,000 lbs. equals 11,000 feet per minute.
Multiply that number by 0.0113636364, which is the conversion constant from feet per minute to miles per hour. For the example, 0.0113636364 multiplied to 11,000 equals 125 mph.