Just completed my first all highway trip and got 21 MPG handcalculated driving 65 MPH. The funny thing is that the overhead reading is showing the same. It used to be off until I got the spedometer recalibrated for my bigger tires. Even though the spedometer is recalibrated, the overhead ODO reading is not the same and does not seem to be affected by the spedometer recalibration. Oh well! At least it matches my hand calculations. I'm running a mixture of PPO & MMO about 16oz/tank.