In a semi truck are lower gear ratios going to give better highway mileage per gallon?

No, the lower the gear ratio, the higher the rpm of the crankshaft verses the rmp of the drive shaft, thus more torque. In order to obtain higher torque, the higher the engine rpm and thus the more gas needs to be burned. The reason why Nixon instated 55 mph as the national speed limit is because the highest gear ratio of car (except those with overdrive) was still too low and because of air resistance. Today, cars and semis have wider gear ratio ranges and are more aerodynamic.