1. What voltage does your car run at?
2. Is it a brushed motor? Please tell me it's a brushed motor. What voltage are the truck batteries?
3. What model is your truck?
4. What are the truck's dimensions?
BASIC STAMP 2 - Board of Education (from the BOE-Bot kit):
USB-
http://www.parallax.com/detail.asp?product_id=28850
Serial-
http://www.parallax.com/detail.asp?product_id=28150
Others- **broken link removed**
OOPic/OOPic-R (this is the one I had offered to you):
http://www.oopic.com/
It can come as a chip but you'd be stupid not to get it as a development board which it almost always is anyways.
PICAXE (I think this is the most inexpensive)-
This seems to be the only US and on Canadian distributor for PICAXE.
US-
http://world-educational-services.net/cart/index.php main_page=index&cPath=7
Canada-
www.hvwtech.com
I think you'll be pleasantly suprised by the cost of the PICAXE starter kits.
EDIT:
**broken link removed**
So that's a mechanical speed control? I always assumed every truck used an electronic speed control, but then I guess "electronic" is there for a advertisement purposes for a reason right?
So yeah, this is the exact same thing then as if it was a nitro truck. Just imagine the mech speed control is the throttle of a Nitro engine. Both are controlled by a servo which you plug into your control board. The more the servo moves a certain way, the faster it goes. I'd really get that replaced with an electronic speed control though...but that's is very obviously biased. Depending on the current you could build your own (now or eventually), but you probably have your hands full right now. You could also buy one if you know the current that the motors can draw (from a RC store if you want to pay big money and interface it just like a servo motor or cheaper from a robot store, as long as the current isn't too high and you can do some neat "computer-things" with it .
WIth the mechanical speed control, you plug all servos into to the controlboard to PWM-capable pins on the uC (microcontroller, u is the latin letter for micro like micrometers). if it's an electronic speed control it may be PWM or serial or something else, either way it all plugs to the right pin on the microcontroller.
Have you seen this thing?
http://www.geology.smu.edu/~dpa-www/robo/jbot/
I wanna build one like it, hehe...after I finish my current horribly overbudget robot.