Micro low voltage relays.
 

Found these micro relays at Newark Electronics. Contacts are rated for 1 amp. Coil voltage is 3 volts, spec sheet says they will make at 75% (2.25 volts). Should work for a dual cell. Max coil voltage is 150% (4.5 volt). This should work for three cells. They are also available with 4.5 and 5 volt coils, for more cells. Newark part number is 29c7707 for the 3 volt model. Newark also has the mosfets for the mosfet mod. I got some but I don't know if I have a steady enough hand to try to solder those little buggers. The relay should be a great drag racer mod, dump the juice from the battery straight to the motor. Results to follow, I have a Zip sized (Zip body fits it) custom four wheel drive with kit racer gears to see just how fast these little cars will go before they get airborne.

Here is a pic of the relay:

Here is a pic of the relay sitting on a bit PCB, fits nice in the little blank spot on the top of the board. A dab of glue, and solder a few wires, then I will be ready to crack my 11.6 mph speed record!

what's newark's website? google gave me dumb links to universities. does it say how much voltage is needed to trip the relay? how much were they?

http://www.newark.com

The spec says they will trip at 75% of rated voltage. They are rated for 3 volts so that would be 2.25 volts. They were $3.68 each, a little spendy, but I wanted to get the mosfets too.

i can see it now, dual button cells to power the pcb and like 6 regular cells to power the motor all in a really stretched chassis :D i'd make one if i had the time

Hey, now you're talkin!