I just received this information from the Dirty Derby Czar. The rules for Dirty Derby 2010 are as follows
1. 4 inches at its widest, including the wheels and their axel hardware. Six inches high and eighteen inches long.
2. Weight not to exceed 12 pounds.
3. Minimum of four wheels, must touch the racing surface.
4. No fuels or explosives. Only allowable propellant is gravity.
5. All wood exterior required except for the wheels.
6. The Dirty Derby Czar has the final word on what is acceptable during the check in process.
Hope this clarifies things.