Before i added my Tonneau cover i got aobut 17.5 mpg on average, no after a soft top tonneau..(TekStyle, form summit Racing) i get a lil over 20 when i try to get good gas mileage, going form NV to california i've gone form 20mpg to 22 mpg, so it does make a differnece, tailgate down i think was a lil under half the gain i got form the tonneau