Complex sentence: Starting in 2011 the CAFE standards are newly expressed as mathematical functions depending on vehicle "footprint", a measure of vehicle size determined by multiplying the vehicle's wheelbase by its average track width.
Split into simpler sentences: Starting in 2011 the CAFE standards are expressed as mathematical functions depending on vehicle footprint. Footprint is one measure of vehicle size, and is determined by multiplying the vehicle's wheelbase by the vehicle's average track width.