I have an 03 XLT 4x4, 5.4, 3.73s, 90k miles. In order to improve mileage, I ran Royal Purple synthetic, a Gibson cat-back, and a K&N intake kit. Then I replaced the tires with Wrangler AT/S, which likely killed any gains I might have seen. I get around 13-14 on the road and less in town. The...