Can you get 40 miles per gallon in a two-ton luxury vehicle, just by driving carefully? Almost. As long as that car is the new-for-2019 Mercedes-Benz CLS450.
- The 2019 Mercedes-Benz CLS450 has been completely redesigned.
- A new 3.0-liter, turbocharged inline six-cylinder engine makes 362 horsepower.
- Improved fuel efficiency comes from a 48-volt mild hybrid system that can also add 21 horsepower for short periods.
- EPA fuel economy is rated at 30 miles per gallon, but with some special driving techniques we did much better.
- Price starts at $70,195.
This Mercedes sedan is big and beautiful. Also heavy, with the 4MATIC version tested here weighing 4,255 pounds. Under its hood lies a 3.0-liter inline six-cylinder engine that burns premium fuel. A turbocharger gives it 362 horsepower and 369 pound-feet of torque. None of those specs are a boon to fuel economy.
Mercedes does say the “four-door coupe” shape of the CLS makes it very aerodynamic, and its EPA fuel economy numbers are decent for its size and class at 23 miles-per-gallon city, 30 highway, and 26 combined. Yet at first glance, it hardly seems like the CLS has any business being mentioned among other green cars.
The CLS does have a few tricks hidden within its drivetrain. An integrated starter-generator (ISG), essentially an electric motor, is packaged between its engine and transmission. Also under the hood sits a 48-volt lithium-ion battery, which can store energy recovered while the CLS is decelerating.
Mercedes calls this mild hybrid system “EQ Boost.” EQ Boost makes the CLS quick and smooth, and adds a layer of refinement to an already polished package.
No Belts, More Power
As you might expect, EQ Boost allows the engine to instantly shut down and restart. This stop-start system, a common feature on luxury vehicles, means the car does not waste fuel idling at stop lights.
The 48-volt system in the CLS produces other fuel economy benefits, as well, including eliminating the parasitic drag of belt-driven accessories. The air conditioner and water pump in the CLS are both electrically powered. The 48-volt system also uses a DC-to-DC converter to supply current for the accessories that run on the car’s standard 12-volt electrical system, like the infotainment system. So yes, the CLS450 still has an old-fashioned 12-volt car battery.
It’s the 0.9 kWh Li-ion battery that makes the CLS special, however. Its power can be used to spin the ISG, which adds another 21 horsepower and 184 pound-feet of torque. But that extra power isn’t going to help us get to 40 miles per gallon.
Coast For The Most
What makes the CLS truly different from most mild hybrids is its ability to coast. Shutting down the gasoline engine and relying on the electric drive motor is common practice in full hybrids, yet mild hybrids are different. They generally don’t have powerful enough electric motors to propel the cars by themselves. Nor does the CLS.
So coasting in the CLS is different than coasting in a hybrid. Hybrids tend to allow the electric motor to spin while coasting, increasing deceleration while charging up the battery. But in the CLS, the nine-speed automatic transmission decouples the drive wheels during coasting, which means the ISG doesn’t spin. The CLS only recharges its battery while decelerating with the engine on.
We thought coasting might give the CLS some untapped potential to realize higher fuel economy numbers than its EPA ratings. So we put this theory to a real-world test, driving roundtrip from Vancouver, Washington, to Hood River, Oregon, and back.
To start, we just drove the CLS450 4MATIC around for a few days to get a baseline familiarity with the car. We discovered pretty quickly that unless you set the drive mode to “Eco” or adjust the “Individual” mode appropriately, the car will not coast. Eco certainly deadens the throttle, but this was desirable for what we were trying to accomplish—maximum mileage.
After putting about 100 miles on the car, trying out its four standard drive modes in a combination of city and highway driving, we filled the fuel tank to get a preliminary mileage number. While we weren’t expecting to better the EPA estimate of 26 miles per gallon, recording 18 only made us nervous about the wholesale reversal of driving tactics we were about to attempt.
Departing from Vancouver, we were as gentle with the throttle and brake as traffic permitted. We observed the speed limit at all times, which ranged from as low as 50 mph on I-205 in Portland, to as high as 65 mph on much of I-84 once we had departed the city. Although we took it easy on the speed, we did not drive purposefully under the limit. We used the Distronic adaptive cruise control and the Mercedes’ driving assist systems most of the time (more on that later). The climate control system was left on, but with the air conditioner switched off in the interest of overall energy conservation.
Check The Technique
With an eye pinned to the car’s fuel economy readout, hoping we could keep it above 40, we navigated through Portland’s relatively light midday freeway traffic. With the Mercedes doing most of the work, the drive was relaxing and enjoyable. Except for one thing: The displayed fuel economy was dropping and the car was not coasting.
While we’re unsure whether the CLS will, in fact, coast while using the cruise control—we never witnessed it—the realization rapidly dawned that this test would be rather pointless if we didn’t intervene. Otherwise we would just be gauging its highway fuel economy. Fortunately, as I-84 runs through the Columbia River Gorge it is littered with relatively gradual hills. So we began playing with the cruise control, canceling it and lifting off the throttle to coast down the inclines and then turning it back on while ascending.
The car responded by shutting down the engine on descents and climbing automatically, with what we can only assume was the minimal fuel needed to keep the engine spinning and the transmission in the highest possible gear. This meant seventh, eighth and ninth, with the engine never passing 2,000 rpm. And as we prodded the car to coast, even if just for a few seconds at a time, we watched the log of the numbers of miles traveled without the engine on tick higher and higher.
By the time we exited at Hood River, halfway through the drive, that number had reached eight miles, or 13 percent of the trip thus far. With the onboard display showing fuel economy approaching 40 miles per gallon, we excitedly turned around and headed back to the start.
The return trip proved to be less work for us, but more work for the car, as the route was more uphill than down. This gave us time to investigate some of the other features buried in the car’s infotainment system, including the onboard power display that shows the gasoline engine’s output in real time. While cruising at highway speed the CLS required between 25-35 horsepower to maintain 55-65 miles per hour.
When we reached the same gas pump at which we’d started the journey, the trip odometer displayed 122 miles, 12 of which had been covered with the engine off. Average speed was 57 miles per hour, and the trip had taken two hours and nine minutes. After pumping 3.2 gallons of gasoline, the tank was full again, and our overall fuel economy registered 38 miles per gallon. Not quite 40, but 20 mpg more than we’d seen in our combined test and a full eight miles per gallon better than the EPA highway estimate.
While we aren’t pretending that this was a rigorous scientifically sound experiment, it is reflective of what an individual driver might be able to achieve in the real world. And it serves as a good demonstration of both the effects of the way you drive on fuel economy and the merits of electrification even in non-hybrid vehicles. As the saying goes, “Your mileage may vary.”