Just made my beautiful mini last night to match daddy (above)!

Anyone got any ideas about this? (may not be mini specific)...

When I drive forward the car travels in a forward (good) and right (bad) line. When I reverse it travels in a backward (great) and right (terrible) line.
If I continue driving forward and back a dozen times my car ends up a foot to the right of where it stared off.

Comments suggestions? (be easy on me it's my first self made car).