The problem with calculations are they don't allow for the massive balooning of buggy or MT tires. On-road cars can use calculations, and in fact I did install a Venom Speed meter in my NTC3 where the internal electronics calculate it for you.

The stopwatch is actually quite accurate as the delay in reflex in starting the watch is close to the same as the delay in stopping the watch. It's just that you need a very long straight stretch where you are doing WOT the whole time.

With a RADAR gun or Speed meter, you will record the peak speed which is probably what you want for bragging.....LOL.

Super-Cheap method (and you're wacky if you really try it): You could put sand in a funnel and attach the funnel to the back of the RC vehicle. The faster it goes, the thinner the trail of sand....less grains per inch. So count the number of grains over a 1" space. Compare the grains to those dropped from a full size car at known speed. Maybe the wife and kids can help? Would make quite the Youtube video......LOL.