Ensuring that the equipment you drive is in the best possible condition is critical in order to preserve its quality for as long as possible and bring in extra income through fuel economy. And, the way that you spec your truck makes all the difference in how it drives.
More specifically, your choice of drivetrain gearing from the transmission to the rear axle has a significant impact on your truck's fuel efficiency and drivability. Get it right, and you'll see the benefits through income generated. Over- or under-do it, and unfortunately, you lose out on that income.
Owner-operators pursuing a better fuel economy can run into problems when the work strays from the spec. By design, truck engines operate most efficiently at specific speeds (rpm), so if you spec a truck to run at 70 mph, you'll be at a disadvantage if you spend a lot of time at 55 or 60 mph. Conversely, a truck spec'd to run 60 or 65 mph would be running at a much higher engine speed than originally intended when driving at 70 or 75 mph. Fuel economy suffers either way.
Unfortunately, there's no all-purpose spec that works perfectly in every application. It's always a compromise, but knowing what kind of work you'll be doing, and where you'll be running is helpful. This is something to consider when partnering with a carrier. Your long-legged western highway truck won’t perform well on regional runs in states with lower speed limits or lots of long hills. Mountains are another story.
The essence of the problem is the gearing. Once the truck is built, switching out drive axle gears for lower or higher ratios is expensive. Changing tire sizes can help in a small way‚ more on that later.
The engine and the wheels are mechanically linked through many gears in the transmission and rear-axles. If the engine turns at a certain rpm, the wheels will turn at a speed corresponding to the various gear ratios. That's pretty easy to understand. Typically, 8-, 10-, and 12-speed transmissions have a step between the top two gears of about 35-37%, giving you an rpm range of about 400 to 500 rpm between gears.
That would give you a road-speed range of 10 to 20 mph between the top two gears (depending on the gearing). For a hypothetical example, a truck in 10th gear could operate at 65 mph at 1,400 rpm. If you slowed to 55 mph, the engine rpm would drop to, say, 1,000 rpm. But if you sped up 75 mph, the engine might be turning 1,800 rpm.
With multi-speed transmissions such as 13-or 18-speed models, the step between the top two gears is reduced to about 17% by the direct/overdrive splitter. This yields a range in engine speed between the top to gears of about 5 to 10 mph, or about half that of a full gear change.
In other words, with the extra half-step between the two full gears, you can manage road speed more precisely without running the engine at the extremes of the efficiency range.
We'll return to the transmission shortly, but first, here are a few words about engine rpm, torque, and horsepower.
Engine Power Curves
Modern engines (less than 10 years old) are their most efficient at fairly low rpm. Operating ranges have been trending downward for the past 25 years or so. While running an engine between 1,500 and 2,000 rpm was once common, newer machines are more powerful and much more efficient between 1,000 and 1,400 rpm.
That transition didn't happen overnight, but for owner-operators who might change trucks at 10-year intervals, the change in the operating profile of a decade-old truck compared to a new one could take some adjusting.
In the 1980s and 90s, before the EPA emissions rules came into play, peak torque on a typical diesel was fairly high up in the rpm range. Peak horsepower was up there too, so it was common for those engines to run their best in the 1,400 to 2,000 rpm range. Torque curves also tended to have distinct peaks or humps, meaning the driver had to work with a narrow rpm range (200-300 rpm). The more gears you had, the better your chances of keeping the engine at or just above peak torque where fuel economy was best.
Since then, the trend in torque and horsepower curves has been to lower peak torque to around 1,000 to 1,200 rpm. Rather than a hump, the torque curves have flattened to a plateau. Most engines now offer peak torque ranging from approximately 900 (yes, 900) to 1,400, and it's more or less flat. You have all the torque the engine can produce across a range almost a full gear wide. That has radically changed the shifting strategy. It has also widened the road-speed range where drivers can stay in top gear without downshifting.
The downside of that trend is that many drivetrains are geared for an optimum road speed that's lower in the torque band because engines are more fuel-efficient at lower rpm. You'll notice how torque drops off precipitously at 1,000 rpm on the chart below. For the driver, dipping below 1,000 is like turning off the key. There's still technically quite a bit of torque there, more than enough to get you rolling through the lower gears, but it's not enough to sustain highway speed, especially on hills.
You can see in this Cummins X15 torque chart in two power ratings the blue is 525 hp/1850 lb-ft and the red is 656 hp/2050 lb-ft. Other engine makes and models have similar torque curves, though not identical. It shows that cruising at any speed at about 1,200 rpm gives you 200 rpm to fall back on before you run out of torque and have to downshift. If you gear the truck for the desired cruise speed that is too close to the lower end of the peak torque band, you’ll have nothing in reserve when you hit a grade or get into headwinds. And the heavier you are, the more inadequate your engine will feel because the revs are going to fall off even faster. Driveability suffers because top-gear gradeability is compromised.
The result is you'll probably want to run a gear down or split the top gear with a multi-speed transmission. However, that will increase your engine speed and fuel consumption. And that’s contrary to the primary objective here, which is to conserve fuel.
Gear Fast, Run Slow (DOWN SPEEDING)
In order to take advantage of all the torque available at low engine speeds, rear-axle gear ratios have grown steadily "taller", or numerically lower. While 3:90:1 and 3:73:1 gear ratios were common 15 years ago, today you're more likely to find 3.08, 2.85, or even 2.64:1 gears in a fuel-efficient highway truck. Gears as low as 2.28:1 are now listed as standard ratios in some OEM data books.
Low gear ratios are the domain of the so-called down sped driveline that requires automated transmissions. Because they run so close to the bottom end of the peak-torque band, the transmission does a lot of shifting between the top two gears under many driving scenarios. That kind of shifting would drive most drivers crazy. And besides, it would require such close attention to engine speed, most drivers with manual transmission would give up trying to maintain optimum engine speed.
While it might be tempting to spec the truck for the lowest engine speed possible at highway cruise speed, you'd have to carefully consider what portion of the time you’d be driving at a lower road speed. Because of the drivability considerations, you'd likely be running at least one gear down, maybe two, at 50 or 55 mph if you had spec'd the truck to run 75. And of course, for every gear back you run, the engine will be turning far more revolutions per mile two gears down than it would in top gear, compromising fuel economy.
Suppose you spec for a variety of operating conditions (hilly terrain, lower speed limits, or other). In that case, it's best to choose rear axle ratios that will keep you closer to 65 mph with a 13- or 18-speed transmission where the splitter can help keep engine revs from dropping out of the peak torque band at lower speeds.
With that spec, running at 75 mph would increase your engine speed by a hundred rpm or so, which would also hurt fuel economy, but so would the additional aerodynamic resistance.
It would be simpler if trucks could be spec'd for a single application. Fleets have an advantage when they spec trucks for specific lanes and geographical regions. That's a luxury few owner-operators have.
Adjust your Tire Size
The tires' circumference also factors into determining road speed at a given engine speed. Larger tires, such as 11R24.5 tires, will turn fewer revolutions per mile (470) than a low-profile 275/80R22.5 tire (510). The taller tires have the same impact as going down a notch in rear-axle ratio, or about 100 rpm in engine speed at highway cruise speed.
If you find the truck's application has changed and you're seeing more regional miles at lower speeds, switching to a smaller tire can help get the cruise rpm back up into drivable range. It’s a much less expensive way of solving the problem than re-gearing the truck.
A good sales rep can help you determine the optimum powertrain spec for the application and road conditions, including leaving the proper margin between your chosen cruise speed/engine speed and the downshift point. But if you tell the sales rep you want to run at 65 and end up driving a lot at 55 or 75, the calculations will be off, and you have either an undrivable truck or a truck with poor fuel economy – or maybe both.
You must be honest with the sales rep about your plan with the truck. When buying a used truck, it's in your best interest to do some of these calculations yourself to ensure you get a truck with the right powertrain for the job. When buying a used truck, it's in your best interest to do some of these calculations yourself to ensure you get a truck with the right powertrain for the job.
You can use this calculator to get a rough idea of how to spec your powertrain. Your dealer can help you zero in on the right spec for your job.
As a reminder, spec your truck properly from the get-go, and then drive it the way you spec'd it.
Other relevant articles:
Buying a Used vs. New Semi-Truck