Stock they do about 7 mph. They have a bunch of different upgrades. You can get new stocks, AWD, body kits, new motors, wheels, tires, lights, metal bearings, and there are like 12 different radio frequenices. I have heard they can get up to 15-20 depending on if you do everything right. Here is a link to the page so you can check out everything.
http://www.xmodsrc.com/car-builder/ 
