Hey mate. Your probably right, motor heat is usually gearing. But the right gearing depends on alot of factors. Where are you running? A track or parking lot or something? How big is the area? What motor is in the car? Is it brushless or brushed and if you know how many windings?
The gears in question, the big one is called a spur gear, the little one is the pinion. Depending on your car, you may need an adapter to change the spur gear, so it's easier just to change the pinion. Most rtr cars will use 48pitch gears, so make sure you get a 48p pinion.
Determining the gear ratio you have now works by dividing the spur by the pinion, then multiplying that by the cars internal ratio. So spur/pinion x internal ratio. So to help we need those numbers. They should be in your manual.