I USED to average 20 mpg highway, with 23 being the best I got with a 1981 F150, 4.9L, T-18 trans, 2:75 rear end ratio and 215-75R15 maypop tires. I said used to. Now it's more like 17-18 hiway. That's because I changed the ratio to 3:50, put on a set Michelin tires (9.50 R15) on American Mag 10 spoke Vectors. Also threw in an Advanced Adapters Ranger Torq Splitter (27% o.d.). You can say my truck has 8 foward and 2 reverse speeds.