...size &
weight!
...in a mid-sized (3,000 lb class vehicle), every 100 lbs of weight reduces FE by 1-2 mpg.
...here's a GM-based (and EPA) equation(*) to
estimate the mpg-change versus weight-change:
(mpg2 / mpg1) = (wt1 / wt2)^(0.72)
...where:
mpg1 = original mpg
mpg2 = new mpg
wt1 = original weight
wt2 = new weight
...(*) source: equation #1 (top of 3rd page, pg.226) of article by H.Maclean & L.Lave:
http://pubs.acs.org/doi/pdf/10.1021/es9905290