I think they installed a ScanGage to get readings over 99.9mpg...

 

They used a supermid to get the readings. Just because a car maxes out the FCD doesn't mean thats what its mileage is. For instance on an ideal course built to exploit the advantages of a hybrid car Wayne Gerdes maxed out the insite gauge at 150mpg. It was later reveiled that someone noticed when switched to l/100km it read 1.3 or something. This is about 180mpg or so. So don't think what a gauge reads is what your actual fuel economy is. The gauges can be maxed out.