I wouldn't waste your money on one from the toy store. My brother has one of those hot wheels radar guns and although it does work for slower vehicles it couldn't pick up my 1/6 scale buggy. I believe the problem is with the distance that it works in and the time it takes to register the speed. It only picks up the vehicle when it gets within 5-10 feet. For that split second that it's within range the radar gun just doesn't have time to pick it up. You'd need something that can read speed from a longer distance.

I've looked around and haven't found any radar guns worth buying for less than a few hundred dollars. There is one out there (forgot the brand name) that is roughly $100 but from the reviews I've read it works fine for a little while then starts to get very inaccurate.

You'd probably be better off buying one of those venom speedometer things.
http://www2.towerhobbies.com/cgi-bin...?&I=LXHUH5&P=7