Understand torque, hp, and axle ratios

The chart was chosen for its clarity. It represents a Detroit Diesel DD15 engine with a rating of 455 hp and 1,550 lb-ft. The engine is available in ratings from 400-505 hp and 1,550-1,750 lb-ft.

TORONTO, Ont. — Horsepower always gets top billing in engine ratings, but it’s torque that does most of the heavy lifting in today’s powertrain. Here’s what the numbers mean, how they relate, and how they affect performance: Torque is a measure of the rotational force applied to the engine’s crankshaft and ultimately to the drive wheels.

Torque is expressed in pound-feet (lb-ft). Horsepower is a measure of the amount of work the engine does over time, expressed as X horsepower at a given engine rpm (445 hp @ 1,600 rpm). In very unscientific terms, torque is “pulling power”, while horsepower is what keeps the truck moving at a given speed. As you can see from the torque and horsepower chart, engine torque is highest at low engine speed (1,550 lb-ft @ 975 rpm), while horsepower is highest at higher engine speed (455 hp @ 1,625 rpm).

The axle ratio defines the number of times the input shaft (driveshaft) will have to to produce one full rotation of the axle shaft. For example, a 4.11:1 ratio means the driveshaft will have to rotate 4.11 times to produce one full turn of the axle. Likewise, a 2.24:1 axle ratio means the driveshaft rotates 2.24 times for every one rotation of the axle.

For fuel efficiency, a low numeric drive axle ratio (2.24:1) gives you a low engine rpm (where all the torque lives) at cruise speed, thus reducing fuel consumption. A higher numeric axle ratio (4.11:1) keeps the engine running in the higher horsepower range, thus improving performance. If fuel efficiency is your goal, you’ll want an engine that produces a lot of torque at low rpm (1,100-1,200 rpm).

Provided the weight and terrain are suitable, the dealer might recommend what is called a “downsped” driveline (gear fast, run slow) with low numeric axle ratio, like 2.47:1 or 2.26:1.

If high performance is your preference (high weight and/or hilly terrain), a higher numeric axle ratio will be more suitable to keep the engine in the higher horsepower range of 1,300-1,400 rpm (3.36:1 or 3:55:1).