mike, i have a calmini exhaust with a jwt pop charger. they give little or no difference in city driving because i have to fill up my tank at the end of the week every week since i bought my vehicle. however, i see some noticeable difference on the highway. on a full tank (~19gal) i can drive from fayetteville, ark. to sherman, tex. with these add-ons, i can make it all the way to dallas, an increase from ~14mpg to ~17mpg. i'm impatient most of the time so i drive with a heavy foot.