At highway speeds, weight affects mpgs more through "rolling resistance" than acceleration.

Every kilogram you add to your car increases the friction in the wheel bearings and the tyres on the road, regardless of acceleration or slowing down. Try pulling an office chair with the fattest person in the office sitting on it - I guarantee the friction will be higher!

So in every case, more mass equals worse miles per gallon.

http://mysite.wanadoo-members.co.uk/ecotech/ecot.htm