By Paul Abelson, senior technical editor
The first cars were not necessarily that easy to drive. Couple that with their exorbitant price tags, and not many experienced life behind the wheel.
Henry Ford is credited with taking the best ideas of the existing technology, tossing in a few from his team, and building a car that eventually cleared the technology and cost hurdles, evolving into a car for “everyone.”
Today, it most certainly won’t be one man who is credited with bringing together the best ideas and technology in the trucking industry and crafting a 10 mpg truck for every trucker. It will be a collaboration of the truck, engine and component makers, each doing what they have done in the past – only better.
The cornerstone of the 10 mpg truck is engine efficiency. A great deal of the energy provided by fuel is consumed overcoming internal friction losses. That happens everywhere from in the engine to the drivetrain, everything from the back of the engine to the treads of the tires. Friction losses start with gears.
Transmission and drive axle manufacturers have been working to reduce those losses in energy as a result of friction, which in turn will increase the overall fuel economy of the truck.
With so many options, transmissions offer the most choices. To evaluate spec’ing options, you have to understand how the gears work.
The “highs” and “lows”
For starters, there are “low” and “high” gears and gear ratios. The engine cruising speed on many trucks is right around 1,500 rpm, turning tires about 500 revolutions per mile. Since 60 mph is a mile a minute, cruising at 60 mph means turning the tires at 500 rpm.
Say you have an engine operating at 1,500 rpm and you only need the tires turning 500 rpm. Gears in the transmission and drive axle reduce the engine’s rpm down to one-third. That’s called a 3-to-1 reduction.
To drive at 30 mph or 250 tire rpm, a driver would slow the truck down until the engine speed was 750 rpm if possible. But the engine would run poorly, if at all. To keep the engine at 1,500 but keep the tires at only 250 rpm, we now need a 6-to-1 reduction in rpm through the transmission.
For higher road speeds you need less reduction of engine speeds, the 3-to-1 ratio in the first example. For slower road speeds or tire rpm, you need more engine speed reduction, the 6-to-1 ratio in the 30 mph example.
That’s why the low range provides “high” numerical ratios and high range provides “low” numerical ratios.
For good fuel economy, we want the fewest engine revolutions per minute that still provides enough power. That means that for fast speeds and low engine rpm, we drive in “high” gears with “low” numerical ratios. For slower speed and high engine rpm for increased power, we need “low” gears with “high” numerical ratios.
Finding the sweet spot
Transmissions come with a variety of gears.
Years ago, nine speeds were common in fleet trucks. Variations on the nine-speed, which can consist of an extra low gear and four more gears in low and high ranges, include 13-speed with overdrive in high range only and 18-speed that has overdrive in all gears.
Today, 10-speed transmissions with five speeds in both high and low ranges are common. Overdrive is an extra gear set that changes the ratio in a given gear.
Shifting between gears makes similar changes in rpm as you go up or down the gears, usually 34 percent with every gear change. That can be cut in half to 17 percent by “splitting gears,” which is going in and out of overdrive.
You not only want to stay within the engine’s useful rpm range, you want to be as close to the engine’s most economical, fuel-efficient speed as possible. The more gears you have to choose from, the closer to the “sweet spot” you will be for better fuel mileage.
The trade-off for more gears is that the transmissions are more complex, which translates to higher cost.
Another set of options involves selection of drive axle gear ratios. That, in turn, depends on whether you select a direct drive or overdrive transmission.
The drive axle gears and transmission gears must work in harmony. If they’re mismatched, you could have a virtually un-drivable truck.
About 20 years ago, TMC started to promote the gear-fast, run-slow concept. The idea was to select upper gears that would allow a truck to run very fast, but to not use the potential speed. In fact, at normal cruise, the gears would allow trucks to run at the slowest engine speed.
Previously, many trucks had 4.11-to-1 and 4.56-to-1 drive axles. To get engines turning even lower rpm, it’s now common to see 3.36-to-1 gears used with overdrive transmissions. Direct drive trucks have ratios below 3.00, with some now at 2.47-to-1.
It’s important to understand how ratios can affect how a truck runs and how you’ll have to drive it. Take for example a trucker who was going to buy his “dream truck” with the best of everything. It’s a good thing he had his specs reviewed before he signed on the dotted line.
He spec’d his classic long-nose truck with a 550 hp engine, a double-overdrive 18-speed and, in line with gear-fast, run-slow thinking, a 2.64 rear end. The truck would have delivered better than 8 mpg – on paper.
The trouble was that he would have had to come out of overdrive every half a percent grade in the road, and shift a full gear for every 1 percent change grade change. The gearing ratio was so high, he would have had no hill climbing ability. Despite his high horsepower, gears would have slowed his truck to about
40 mph or so up a steep hill on an interstate. He would have been passed by every truck.
To make sure you have specs you can live with, have a dealer run a simulation. The computer programs won’t tell you exact fuel economy, but they can provide comparisons between different sets of specs. They’ll be pretty accurate when comparing acceleration, climbing ability and starting ability on grades. These are the things that determine if a truck will be a pleasure to drive, or a nightmare.
Automated transmissions offer improved fuel economy. The computers compare grade, throttle position and other inputs. They not only select the best possible gears to be in, but also communicate electronically with the engine control module to determine the optimal amount of fuel, multiple times per second.
Automatic transmissions improve fuel economy in urban driving when a great deal of shifting is needed. Manual transmissions, including automated manuals, break torque flow when they shift. During those seconds, the truck loses speed as it coasts. That loss must be made up when the next gear engages. Providing constant torque, automatic transmissions don’t have that pause and resulting coast down and “catch-up” time. The downside is that full automatics are significantly more expensive.
Weight equates to either fuel use or revenue. If you load on a scale until you gross out, every pound translates to more revenue on the load. If you don’t reach maximum weight, every pound saved improves fuel economy. Axles and wheel end assemblies can contribute to excess weight.
Weight often equals strength and durability, but when does enough become too much? Regulations limit axle weights to 34,000 pounds per tandem. Most are rated at 38,000 or 40,000 for a reasonable safety margin. But there are also trucks that run solely on-highway with 44,000 and 46,000 tandems. That is overkill in most cases – unless you also do specialized or heavy-haul loads.
If you’re spec’ing for the 10 mpg truck, don’t forget lighter weight wheel-end components such as aluminum hubs.
Weight and fuel savings can be realized with tires and the wheels they’re mounted to. Wide-based single tires replace a pair of dual tires, mounted on a single wheel. The four wheels replacing eight in a tandem save more than 700 pounds. Since a single tire has only two sidewalls compared with the four per set of duals, there is less energy-absorbing flexing taking place, further improving fuel economy.
Sidewalls absorb energy, but so do treads. In recent years, a great deal of progress has been made in tread designs and compounds, saving several percentage points in fuel economy. Some duals are about as efficient as wide singles. LL
Edited by Jami Jones