RPM stands for "revolutions per minute." It's a measure of how fast the engine is spinning. In general, the faster an engine spins, the more power it makes. For any gear given, more RPMs, the faster the car goes. A small gear will have to make more spins around the crankshaft to keep pace! But a larger gear will do the same job with a smaller number of spins, which means you can charge the engine less and maintain the same speed.