Oh, okay... for a second there you came up with a radar gun. But that was tested using a 1/10 car with normal electronics. So for future reference, remove the speed meter and find Officer Friendly!
let me put the suit back in the closet....... 
You didn't try the AA cell cause you thought that the motro will gobble it up, right? Well, I am looking at the NiMH rather than the standard NiCad cells. Also to need to shave weight by replacing the standard servo for a durable, micro servo. If you are trying to do this on a straightaway, you don't need much steering at all. Maybe changing the batteries to AA MiMH from Sub-Cs will help you lose some weight, i don't know, but it might help. Judging from the type of Sub Cs you have now, The AA NimH may help out in power, I guess.