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As in measuring the number of wheel rotations? Is this an electric or nitro? If it's electric, brushed or brushless?
THe most power plant independent way of doing (best simple:reliable ratio) is to sense the number of wheel rotations and take into account the radius of the wheel and assume no slip. At the speeds and accelerations of RC cars though, no slip may not be such a valid assumption.
Since the wheels on hobby grade cars are on suspension, it might not be so easy to mount something stare at the wheels and watch their rotation- dirty and dangerous for whatever is doing it. Your best bet might be to watch the motor's rotations and take into account the gearing and wheel radius. THere will be of course errors in the initial measurements of the gearing and wheel radius as well as changes in those measurements as they wear and deform. This will cause an unbounded error that will go to infinite over time swamping the actual measurements- but that's always the case when you are making relative measurements.
What does your motor look like? It probably has a tiny little shaft coming out the rear end that you could probably mount an encoder disc to, then you mount a sensor to watch it to see how fast it spins.