The ultimate purpose of a gear fast, run slow combination is to find the best possible fuel economy for an engine that’s running in its optimal cruising range. On today’s electronic engines, this approach keeps the engine at a speed that’s typically between 1,400 and 1,600 rpm by using faster overall drivetrain ratios.

Minor variations in engine rpms can exist between specific makes and models of engines, but this concept maximizes the benefit of high-torque, low-speed engines.

Your overall driveline ratio is calculated by multiplying the axle ratio by your transmission gear ratio at cruise. For example, a direct-drive transmission with a 1:1 ratio in the top gear and an axle ratio of 2.93 results in an overall ratio of 2.93:1. This axle ratio is considered to be a “fast” ratio when it’s compared to a 3.73:1 axle ratio. In an overdrive transmission with a 0.73:1 top gear, using the same axle ratio of 2.93, the overall driveline ratio would be calculated by multiplying the numbers for a resulting ratio of 2.138:1.

Keep in mind that engine horsepower cannot be multiplied, since horsepower is the rate at which the work is done. But you can multiply torque – the twisting and turning force at the crankshaft/flywheel. It is torque that pulls a load up a hill or pulls a dump truck free when it is up to its axles in mud.

That’s why the choice of a “gear fast, run slow” combination means a compromise between fuel economy and pulling power.

Torque can be further multiplied when you run it through the driveline (the combination of transmission and axle ratios) in proportion to the overall gear ratio.

The faster the axle ratio (indicated by a lower number), the higher the driveline torque must be to provide the same torque at the wheels.

You can gear fast and run slow by using either an overdrive or a direct drive configuration, since it’s the overall drivetrain ratio that is the deciding factor.

Your vehicle speed is a combination of the engine speed, the transmission gear ratio, the auxiliary transmissoin ratio, the rear axle ratio and the size of your tires.

About 85 to 90 per cent of linehaul trucks are equipped with gear fast, run slow drivetrain configurations, where they also spend close to 85 to 90 per cent of their time running in top gear.

Direct-drive transmissions, especially the 10-speed variety, offer a fuel efficiency advantage compared to overdrive transmission combinations. In fact, direct-drive transmissions can provide as much as a three-to-five per cent gain in fuel economy over a similarly spec’d truck/tractor using an overdrive transmission.

One reason is that torque in a direct-drive transmission travels straight through the input shaft to the main shaft, and exits directly at the output shaft in your top gear. In an overdrive model, the torque is routed through a couple more gears, resulting in higher parasitic power losses due to friction.

In a twin-countershaft transmission model, the two countershafts can contain between 550 and 600 teeth (counting all of the gears in constant mesh) that are rotating through the lube oil. Therefore, in an overdrive transmission model, the two countershafts tend to rotate between 30 and 35 per cent faster than they would in a direct-drive model. This action creates more oil churning losses in a similar overdrive model, meaning better fuel efficiency in the direct model.

The direct model also tends to be quieter than an overdrive model when cruising in top gear because torque is not being transmitted through any active gears in mesh.

Another thing to consider is, in an overdrive model, the front bearings in the transmission are placed under load in top gear, while in the direct-drive model they are not.

Taking all of these items into consideration reveals that driveline and other parasitic on-the-road losses in a truck with an overdrive transmission are somewhere between 30-35 per cent greater than in a direct-drive model when you’re in cruise mode.

Direct drive transmission models can also provide deep first, second and reverse gear ratios for outstanding startability and reverse maneuverability, even while using fast axle ratios. Fleet tests of identical vehicles with direct and overdrive transmissions indicate that the direct models enjoy a fuel economy gain of between 0.085 to 0.114 km per litre or 0.20 and 0.27 mpg/U.S. gallon.

Keep in mind that when you choose a gear fast, run slow configuration, you need spec’s that allow your engine to run at nearly the same rpm to achieve identical cruising speeds. n

– Bob Brady is the president of HiTech Consulting in Burnaby, B.C.