From tire size to torque rise, there are a lot of numbers to crunch when you spec a drivetrain. You can leave the math to your truck dealer’s computer, but it’s your priorities that ultimately guide your combination of engine, transmission, clutch, driveline, and axles. Is fuel economy your top concern? A given road speed? Driveability? Startability? Gradeability? Maybe durability? Chances are you want to combine all of these elements into one perfect drivetrain spec.

Your loads and roads will determine how much power you need and the gear and axle ratios necessary for the job, so before you show up at the dealer make an honest assessment of where you’ll be operating and at what gross weights. Think, too, about how you want your truck to be driven. Assuming you’ll run all or most of the time on-highway, choose your top road speed and cruising speed. Leave 8 to 10 kilometres an hour between the two and design the drivetrain to hold that cruise speed on the highway.

“Spec for the job” is a cliché for a reason. Be specific about how and where you want your truck to operate and write it down. Then you can get a jump on some other other issues you’ll confront when you’re putting together the drivetrain.

The Sweet Spot

With any engine, you want to find the “sweet spot” for optimum performance and economy. Cruising speed should occur at the lowest point of the fuel-consumption curve and high on the horsepower curve.

Aim to cruise at 1,550 rpm or so. This now-common “gear fast/run slow” philosophy means the truck’s gearing will carry it faster than you actually want to run, and you cruise at a lower, more efficient rpm.

Torque is really the main reason to choose one engine over another. It also determines the capacity of all other driveline components, so when you’re looking at spec sheets, the minimum torque requirement for the job is your baseline. Most people spec components 10 to 15 per cent heavier than they need.

Ask your dealer for detailed torque, power, and fuel-consumption graphs for each engine you’re considering. You may find they’re more scarce than they used to be — they’re not routinely published — but do press; you need them.

Gradeability

Gradeability measures performance in terms of how steep a hill the loaded truck can climb without losing speed in a given gear. A standard rule of thumb says that a fully loaded vehicle should have 0.3 per cent gradeability at full engine speed or 1 per cent gradeability at peak torque when in cruise gear. It should maintain this gradeability standard at cruising rpm.

Productivity falls when a truck is too slow, and safety suffers — along with the public’s goodwill — when it’s significantly slower than companion traffic, especially on two-lane roads. Of course, drivers don’t like to have to row through the gears. Your truck should be able to keep up with traffic on all but the worst hills.

Startability

This refers to the steepest grade on which a vehicle can be started from rest in first or low gear and accelerated, without beating the clutch and driveline to death. Startability is based on clutch-engagement torque at 800 rpm and it’s usually expressed as a minimum number or percentage. A common recommendation is a minimum startability factor of 11 per cent for on-highway trucks.

Note that if a transmission has an overdrive top gear, its low and first gears will be numerically lower than a transmission with a direct-drive top gear. That’s because the axle ratio in either case will compensate, yielding about the same overall low-gear ratio.

Transmission Steps

Gear steps are important both to the driver and to the truck’s long life. With misplaced gears, the driver shifts constantly. Gears that are too tall strain the entire drivetrain.

Gear steps in most ratios should be close enough so at least 90 per cent of the engine’s rated horsepower is available after each normal upshift. This is determined by following the engine’s power curve downward as revs drop at each upshift, and noting the power available at that point. If it’s at least 90 per cent (or 85 per cent in some less demanding cases) of the engine’s highest published output, then the transmission is okay; if it’s less than 90 per cent or 85 per cent, then you probably need more ratios.

The step percentage is governed by the operating characteristics of the engine. A high-torque-rise engine allows a wider, or taller, step. However, it isn’t unusual to have a really wide step between first (or low) and second gear while the rest are within a few percentage points of each other. Just be careful that a tall first step doesn’t cause problems climbing out of holes.

Your loads and operating conditions will dictate the answer here, but there may be many choices. And they’ll get complicated. This is where you need your truck dealer’s spec’ing program and maybe a chat with transmission reps.

Everything Else

We’ll assume correct spec’ing of a rear axle for its load capacity, but you’ve still got to find the right differential ratio. It’s directly related to several other factors, mainly engine rpm and drive-axle tire size. Smaller tires need a taller diff or a tall overdrive transmission.

For example, with an 0.73 overdrive transmission, 3.90 rears, and an engine turning over at 1,550 rpm, a 22.5-inch tire will deliver 65.5 mph or 105.5 km/h. Change to a 24.5-inch tire and you’ll be at 68.6 mph or 110.4 km/h. That’s not a small difference. You can do the numbers yourself using the road-speed calculator on Eaton’s web site (truck.eaton.com).

Your differential choice will affect your driveshaft choice. If the diff ratio is tall, you’ll find that the driveshaft is under extreme torque stress. The result is a heavy, expensive driveshaft. To balance the drivetrain, you may need a deeper-reduction diff and move up to a taller overdrive transmission. Otherwise, you’ll need a section of sewer pipe for a driveshaft.

Should you view the driveshaft as the weak link, or the “fuse” that protects the expensive bits at either end of it? Most people do, and it’s not bad thinking.

One idea is to spec an easily accessible segment of the driveshaft to be optimal for its anticipated load, while choosing extra strength on the rest of it. Some people go so far as to grind a notch into the outer end of an axle shaft for this purpose, but nobody’s going to recommend it.

One last point: you may save money and spec a lighter driveshaft — and lighter axle as well — by choosing an engine that manages torque output and adds torque in upper gears where it doesn’t stress the rest of the driveline.The so-called ’02 engines are different, for sure, but only in one case do they demand new thinking in terms of designing a drivetrain. That’s at Mack, where the 12-litre Maxidyne “ASET” vocational engine’s torque output has dropped from 1,450 pound-feet to 1,250. Torque rise is still high at 60 per cent. Cruising revs also climb in the new Maxidynes, from 1,500 to 1,700, and the operating range is now 1,300 to 2,100 rpm, up from 1,000 to 1,750 rpm.

So Mack added one more gear in a pair of new 6- and 7-speed gearboxes instead of the current 5- and 6-speed offerings. The new TM308 and TM309 boxes have two additional low-low ratios handy for off-road manoeuvring or starting out on steep grades. The overdrive high gear on the new transmissions stays about the same, so the truck needs a faster (numerically lower) axle ratio.

Torque is an issue, too. Getting out of a mud hole on a construction site, for example, used to mean letting the engine dig into its torque reserves at 1,000 rpm. Now that happens at 1,300, no small change for a driver who’s driven the old way for many years.