The motor is almost a dead short across the battery and will draw HUGE currents when heavily loaded eg when your car is at a standstill. Massive gearing is one answer but hard to do -
A better solution is to get hold of an OLD car DYNAMO not a modern alternator but a DC DYNAMO this will run quite happily as a motor.
Most Truck electrics are 24 volt
Ebay will show you a number of 100 to 300 watt DC motors suitable for electric cars at reasonable prices.
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where can you get those for cheap?
Like said above, If you are going to run it for any longer than a minute, it WILL burn up. The time and effort it would take to make this type motor last would not be worth it. There are some pretty decent PM motors sold online that were meant for this application starting at around $20 through All Electronics. Hope that helps.
. I doubt if you can cool a starter motor enough for it to run 100% duty cycle without something like a water jacket. They are designed to run only 10-15 seconds and then go through a cool-down period.
Two large DEEP CYCLE batteries wired in parallel would be better. The batteries made for use in tolling motor etc. will allow you to discharge them very low and often with out damage. Regular starter batteries are not made for this use.