There must be some way of faking the GPS out.
There has to be an antenna for the GPS somewhere. Just make a tinfoil hat for the antenna (stick it on with duct tape) ;^)
I'd want to put some unbelieveable mileage on the car so that it looked like the car did something like 6,000 miles in a 24hr period which would be 250 mph. Can you imagine the reaction when their immaculate computer spits out a 250mph average speed?