I HAVE A 1998 2500 QUADCAB SHORTBED, 4X4, AUTO, W/OD, K-N FILTER, 410 GEARS, 265 TIRES, 3. 5 CLEAR THRU EXHAUST. THE TRUCK CAME WITH 245 TIRES STOCK. I'M CURRENTLY GETTING ABOUT 15 MPG USEING MY SPEEDO TO CALCULATE AT EACH FILL-UP. I WOULD LIKE TO GET SOME WHERE IN THE 19 MPG RANGE WITH OUT...