The gearing of a tractor (drive axle ratio) is based on several factors including the drive tire revolutions per mile, transmission top gear ratio, engine torque rating, GCW, gradeability requirement, tractor-trailer aerodynamics, application, and vehicle speed. Gearing is a compromise between truck performance and fuel economy. Fuel cost is a substantial part of the total owning and operating cost (second to driver's wages) and therefore optimum gearing leans toward the fuel economy side of the equation.

Some operators of trucks geared for best fuel economy can compensate for any reduced performance by down shifting prematurely and more often to keep the engine rpm in the peak horsepower range. Driving in this manner defeats the purpose of "Gear Fast – Run Slow" and can lead to poor fuel economy and subsequent complaints.