Since you say you have 8" wheels, I'm going to assume you ment 8" rims and the actual diameter of the tire is closer to 12" at least. If on level ground, you're doing 25 kph (15 mph) at full throttle; which should be 6800 rpm then your drive system is around a 15\1 reduction. Tachometers are fairly inexpensive and easy to install. Then you'll know exactly what your engine is doing.