@Triedaq: You may be confusing gas mileage with maintaining speed. If a truck’s engine does not have enough power (or maybe that’s torque) to maintain a certain gear, due to heavy headwinds or upgrade or any reason, then he (or she) must downshift. The same applies to riding a bicycle uphill. That is, the rider simply isn’t strong enough to push the bike uphill while in the highest gear. However, in both cases, you are not travelling as far on a fixed amount of energy (that is, you have to expend a lot more energy to move the same distance then you would expend in a higher gear).