I can't explain how. It is a factory freak! I don't have a chip either, cause I couldn't get past 100mph (tested on the freeway). The other guys with expeditions were a little pissed cause they were running 15.9 chipped. The f150 guys were also pissed cause they were running high 16's and 17's! When I put my 3rd seat and spare tire in, I run a 16.1.
Oh I forgot I have an airraid intake, but that really creates more sound then power.