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We're using a 4:1 planetary on our drill motor and feeding that into a 29:1 spur gear box. We have six inch wheels. This gives us a GR of 116:1 from the drill motor output to the wheel axle.
This gives us about a 4.1 feet per second top speed and about 1000 lbs of pushing power (barring slip of course). We have four motors in the drive system, so don't go trying to figure out how to get these numbers with just the two drill motors.
Since our second year (2001), we have used a big, achin' aluminum tube heat sink surrounding the motor with small muffin fans constantly forcing air through and across the motor.
We have not (knock on wood) experienced any over-heating with our system and we have pushed it pretty hard.
We had two years (2000, 2002) where we used a much higher gear ratio. I don't have the numbers in front of me, so I cannot give them to you, but figure at least 2x the above GR. In those years, we popped circuit breakers when we really ran our robot roughly.
There are issues of gear box efficiency that factor into this mix. If your GR is comparable to the one I quoted, but you still have these problems, you might discover that you are losing a lot of energy in your gearbox and therefore increasing the current your motor draws in order to run at the above torque/speed point.
There may also be issues with how your drive system is laid out. If you are using differential rear wheels and casters, you will have some difficulties with maneuvering on the hill. You have to use a lot of torque when you correct to go straight on the hill, because your system will not naturally resist turning.
I don't know what you can do at this point. If you have sprockets driving your output, you might be able to put a big sprocket on your wheels and keep motoring.
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