by Paul Abelson, technical editor

A few years ago, before today’s big power engines were available, a reader called to ask me to review the spec’s for his new truck. He got a large settlement from an insurance company, and, after good financial planning, he had enough left over to afford to get what he called his dream rig. He wanted a large car with the biggest Cat engine available (then a 475 with 1,750 pounds foot of torque). Behind it would sit a 13-speed “double-over,” tall rubber and a high-speed rear end.
He heard about The Maintenance Council, and read their advice to “Gear Fast, Run Slow.” TMC advocated that if you gear your truck so it would be capable of greater than your desired cruise speed, you would wind up cruising in your engine’s most fuel-efficient range, and still have some speed in reserve if you needed it. The driver took the advice to heart. Unfortunately, he also took it to extremes. Fortunately he checked before writing a check.
With 11R24.5 tires, a 2.93:1 drive axle and a 0.73 overdrive, the truck was capable of well over 100 mph at 2,100 rpm. The problems were that he wouldn’t be able to use his overdrive gears, and his hill climbing ability would have all but disappeared … even with the biggest engine then available. If memory serves me, he would have had to run in 10th (7th OD) to cruise between 55 and 70 mph.
Just a change to 22.5 rubber would have let him cruise in overdrive. The main problem, however, was not with any of the individual components. It was the way they all worked together as a system. When you spec a truck, you have to take a number of factors into account. They all affect performance, and they all interrelate. They are:

• Engine horsepower, torque and rated speed
• Transmission ratios lowest, highest, and percentages between gears
• Tires size, type of tread and revolutions per mile
• Rear axle ratio
• Maximum load
• Aerodynamics frontal area and drag coefficient

If you understand how each of these factors affects your truck’s performance, you’ll be able to make the right trade-offs so you’ll be happy with the end result.

We’ll be looking at gear ratios quite a bit, so let’s make sure we understand the concept, and why we need gears. If you have a shaft, whether it’s an axle or a drive shaft, if you put a turning motion on one end, the other end will turn at the same rate. That’s obvious with a solid shaft. If you want to slow the far end, or speed it up, you can put gears in between the ends. Gears are sets of wheels with teeth that interlock. If the gear you are turning has 20 teeth, and the gear connected to the other end of the shaft has 40 teeth, you’ll have to make two turns of your shaft (2 X 20 = 40) to get the other shaft to turn once (1 X 40 = 40). That is a 2:1 gear ratio (output gear/input gear).
If your output gear had only 10 teeth, it would be driven to make two turns for every one turn of input. Using the same formula (10/20 = 0.5) we wind up with a 0.5, or 50 percent overdrive. That defines overdrive. It is when the output shaft turns faster than the input. Put another way, it lets the engine turn slower while still maintaining a given road speed.
Transmissions, drive shafts, differentials and drive axles all transmit the turning force, or torque, developed by the engine. Horsepower is merely torque delivered over a given period of time. That’s why engineers look at horsepower when working with speed. Speed, whether “feet per second” or “miles per hour,” contains a time factor. Engineers look at torque when considering starting and hill climbing, which are not time-dependent.
When gears multiply (or divide) the number of turns of the input shaft, they change not only the rotation speed of the shaft, they also multiply (or divide) the torque transmitted through the shaft. If you apply one pound of force one foot from the axis of a shaft, you have applied 1 lb.-ft. of torque. If the input gear (20 teeth) turns twice to make the output gear (40 teeth) turn once, the torque on the output shaft will be doubled to 2 lbs.-ft. But the output shaft will turn only half as fast.

Let’s apply this to your rig. You probably have an engine that develops anywhere from 1,450 lbs.-ft. to 2,505 lbs.-ft. of torque at around 1,200 rpm. For ease of calculation, we’ll say the engine is rated at 1,800 rpm. That means it has an effective power band of about one-third of its rated rpm (1,800-1,200=600, 600/1,800=33.3 percent). The engine works well in that range. If you could somehow miraculously start at 40 mph and never wish to exceed 60 mph, you could do all your driving in just one gear. But that, of course, is not the real world. Gears allow your engine to stay in its operating range, while allowing you to determine your driving speed. A 10-1 ratio (40 teeth driven by four teeth) will allow the engine to stay within its range — as low as 4 miles per hour, given the same vehicle. Fifteen to one will get you down to crawl speed. An overdrive gear of 0.75:1 (20 teeth driven by 15), the theoretical truck could go as fast as 80 mph. These speeds are examples only. In practice, the drive axle ratio and tire size affect actual speeds.
We’ve seen how transmissions, and in fact all gears, affect speed and multiply torque. Without the torque multiplication, an engine producing 1,650 lbs.-ft. of torque would never be able to start an 80,000-pound rig, let alone start it going uphill.
The drive axle ratio also enters into the equation, and must be considered when developing truck specs. A direct drive transmission (1:1) may work well with a low-reduction, high-speed axle (3:1). An overdrive transmission (0.75:1) may work equally well with a deeper reduction drive axle (4:1). But put the overdrive transmission (0.75:1) with the high-speed axle (3:1) and the result is only 2.25:1. This is too high a gear to allow any decent hill climbing, especially with larger tires.
Just as “fast” axle ratios can increase speed while they reduce available torque, tire size can do the same. A “tall” 11R24.5 tire may need only 473 revolutions to travel a mile, while a low profile 275/80R22.5 may need 516 revolutions. The bigger tire goes 8.3 percent farther per revolution of the drive axle, but has 8.3 percent less torque multiplication. The difference in distance-traveled-per-revolution has the same effect as varying gear ratios.
These are simplified examples, but they show the relationships between engines, transmissions, tires and drive axles.

Aerodynamics and load
obviously affect performance. The greater the load, the more power is consumed overcoming inertia and gravity. Aerodynamics dictates how much engine power will be used pushing air out of the way, and how much will be left to climb hills and run down the road. Total drag is measured by taking the frontal area, which is pretty well fixed for a truck, times the speed squared (which is why, above 50 mph, speed has such a great effect on fuel economy), times the drag coefficient. Drag coefficient is a measure of how much effort it takes to move the truck through the air. Pointed and rounded shapes move air more easily than flat surfaces, which is why aerodynamic trucks like the Century Class, the T-2000 and the 387 get 20 percent or more better fuel economy than the Classic XL, W-900L and 379.

Calculations
When spec’ing, you need to know ahead of time how the truck will perform, how well it will start uphill, how well it will hold its speed on a long grade, and approximately what kind of fuel economy you can expect. There are formulas that will tell you how much horsepower you will need to maintain speed up a 0.5 percent grade, or how much torque is required to hold a gear going up a 1.5 percent (or 2.0 or 2.5, etc.) grade. Other formulas will tell you what percent grade you can start on. Here’s an example:

For on-highway use, S should be at least 16.

Similar formulas exist for all the other calculations. To calculate fuel mileage, you would need access to each engine’s fuel map, and a set of calculations that tells the fuel management computer how much to deliver at any given set of variables. Engine computers look at throttle position, rate of change of throttle position, engine temperature, intake air temperature and other factors that all affect combustion. Fortunately, these are all in dealers’ or engine distributors’ computers. Programs to calculate startability, gradability and speed in gears are also available to dealers. With them, you can determine ahead of time if you’ll be able to cruise at your desired speed at the best rpm. If not, a slight change of drive axle ratio may make all the difference between a truck you love and one you hate. For example, you may want to hold 1,350 rpm at 60 mph and keep it under 1,600 at 70, for fuel economy. With a 3.73 drive axle ratio, you’ll be doing 1,650 at 70. With a 3.90, you’ll be over 1,700 at 70. Good for performance, but bad for fuel economy. At 60, you’ll be running almost 1,500, but at 50, you’ll still be in the engine’s torque range. With the 3.73 drive ratio, you might have to split a gear to go 50. Little things make big differences.