Truck makers were faced with the choice: We face a penalty if we keep doing what we’re doing. Do we invest heavily in research and development that (we hope) results in fractionally more efficient engines for our trucks, or do we find another way to comply with the new standard? Well as it turns out, if your current engine isn’t fuel efficient enough at its current footprint, all you have to do is move your finger along the curve a bit to the efficiency level you can achieve, and then increase the vehicle footprint to the corresponding value. As CAFE standards gradually increased, so did the truck footprint.