So I am running a 89 bronc with a 4" lift and 33's, with a 4.9L and a C6. All of this massive power (yeah right) is transfer to the pavement through stock 3:55 gears. I know my speedo is about 10mph slow. At anything above 40mph. My question is how do I get an acurate figure on my gas mileage? I don't want to change the speedo gear because I am going to 4:10's and lockers by the end of April. Anybody have a formula? I searched around and could find what I was looking for.