We Test the Tips Part II

Save Gas with Smart Driving and Slick Aerodynamics


  • Gas-saving Tips - Fueling up

    Gas-saving Tips - Fueling up

    We ran the tests using a car, an SUV and a pickup truck. After each test run we refueled at the same gas pump in the same way. | March 18, 2010

8 Photos

How the Tests Were Run
Vehicles Used for Fuel Economy Testing
Effects of Driving Style on Fuel Economy
Aerodynamics and Fuel Economy
Tire Pressure and Fuel Economy
Editors' Final Thoughts

With gas prices as high as they are, the media is awash with lists of fuel economy tips. Well how's this for a tip? If you take our advice, you can see huge savings without having to buy a hybrid.

By changing your driving habits and by using and maintaining your car correctly, you can improve fuel economy up to 38 percent (depending on how and what car you currently drive). Combine several tips and perform routine maintenance and you will save real dollars, not just pennies.

A miracle? All we did was take several of the most common tips out there and put them to the test over a 60-mile highway route in central California. Some of them worked like a charm. Some of them didn't work at all. We'll give you the breakdown.

These tests were done under real-world conditions — not in a government lab somewhere. Our results can be matched by anyone — even you.

How the Tests Were Run

We selected a car, an SUV and a pickup truck to perform the tests (more about the vehicles later). A section of straight, flat road in the Central Valley of California was chosen to run 60-mile loops over the same stretch of highway. This "out and back" loop was designed to nullify variations in conditions; if we were traveling with the wind going south, we would be driving into the wind going north, and the effects would be cancelled.

After each test run, we returned to the same gas pump and refilled the tank to measure fuel usage. The tank was refilled on the slow nozzle setting until it automatically clicked off. We then paused and continued filling until the pump clicked off a second time.

In addition, we used a ScanGauge that connected to the OBDII (onboard diagnostics) port. It showed how much fuel was used, the miles per gallon for that trip and many other data points. Two of the vehicles came with onboard fuel mileage meters, and this information was also recorded.

We input the results of the testing into a spreadsheet and reviewed it carefully to look for inaccuracies or data that didn't track. In one case, a wind came up halfway through a loop, giving inaccurate readings for half of the test run. The results of that test were thrown out.

Ultimately, we found that small changes in gas mileage were most accurately measured at the pump, not with the ScanGauge or the onboard fuel economy meter. This was in part because the ScanGauge didn't directly measure fuel flow but instead predicted the amount of gas used based on other data from the engine such as rpm, throttle opening and engine load.

While we were careful to make these tests as accurate as possible given the time and equipment available, we believe that small variations might still occur in other vehicles. In some cases, this is because the vehicle is set up differently by the manufacturer by programming different shift points in the transmission or the aerodynamic shape of the vehicle itself. Ultimately, we offer these tests to help drivers see the big picture on how different driving styles and conditions affect fuel economy.

Vehicles Used for Fuel Economy Testing

We ran these tests with a car, an SUV and a truck.

2007 Lexus ES 350 : We chose this car because it has adaptive cruise control, a radar-based feature that allows the driver to set a following distance behind another vehicle. Our intention was to use this feature to test the aerodynamic effect of drafting behind an 18-wheeler. And since the ES 350's powertrain, tires and body shape are closely related to the top-selling Toyota Camry, we felt the basic results of our other tests would be applicable to the average family sedan.

2008 Buick Enclave CX : To include an SUV, we chose the Buick Enclave from our long-term fleet. It provided the extra benefit of allowing us to put roof rails on it so we could test the aerodynamic effects of carrying luggage in this way. Interestingly, the Enclave boasts surprisingly good fuel economy and is set up to maximize efficiency.

2007 Toyota Tundra SR5 DoubleCab 5.7L V8 : We had to include a pickup truck in our tests, since these are notoriously thirsty vehicles. Furthermore, the aerodynamic qualities of trucks are more similar to a brick wall than a sedan. But since many owners use their pickups for work, fuel economy is a bottom-line issue.

Effects of Driving Style on Fuel Economy There is so much to be said on the topic of how driving style affects gas mileage. However, we decided to break this category into three test runs.

  • Aggressive Highway Driving: This has three components: cruising speeds from 75-80 mph; constantly accelerating around slower vehicles and changing lanes; braking sharply rather than backing off the gas when obstacles appear ahead.
  • Calm Driving: This refers to driving with the cruise control set to 65 mph. For many tests, this became our "base run" for comparison purposes. Since our chosen route was almost entire highway, with little stopping and starting, this was definitely the high end of the mileage that a vehicle was capable of getting.
  • Fast Cruising (or Going With the Flow): This involves driving with the cruise control set to 77 mph and minimizing lane changes and midrange acceleration. Also, we looked farther down the road to eliminate unnecessary braking.

Driving Style Results

Using calm driving as a baseline, we found that calming down and slowing down showed these results:

Lexus ES 350: 33 percent improvement from 24.5 mpg (aggressive) to 32.5 mpg (calm).

Toyota Tundra: 35 percent improvement from 15.5 mpg (aggressive) to 20.8 mpg (calm).

Buick Enclave: 38 percent improvement from 19.7 mpg (aggressive) to 27.2 mpg (calm).

Note: Because the Enclave is set up for maximum fuel economy, it is reluctant to downshift for lane changes. If forced to downshift, it shifts from 6th to 4th gear, forcing higher revs and consuming more gas.

High-Speed Cruising

If abrupt lane changes and sharp braking are eliminated, and the driver sets cruise control to 77 mph, fuel economy is significantly improved. So, using aggressive driving as a baseline, and comparing that to a steady speed of 77 mph, we saw these results.

Lexus ES 350: 15 percent improvement from 24.5 mpg (aggressive) to 28.2 mpg (77 mph).

Toyota Tundra: 9.8 percent improvement from 15.5 mpg (aggressive) to 17 mpg (77 mph).

(This test was not run for the Buick so we could retest other factors.)

Notes on Driving Styles

While we had decided to test the effects of what we called "aggressive driving," we found that we were not, by any means, the most aggressive drivers on the road. And this, by the way, was in the middle of farm country. While in aggressive mode, we were still passed by faster vehicles, and we saw more abrupt lane changing than what we were doing.

Driving in the calm mode was not as unrealistic as it might sound on paper. We arrived back at the station only five minutes later than had we driven aggressively, five minutes that soon evaporated into our memories as they blended with the rest of the day.

It was somewhat of a relief to cruise at 77 mph in the final test of this section, since this was closer to the speed of traffic flow, and we didn't have the stress of faster cars constantly on our bumpers.

Aerodynamics and Fuel Economy

These tests were designed to show the impact of aerodynamics on fuel economy. We looked at the effects of adding a roof rack and luggage, dropping the tailgate on a truck and drafting behind an 18-wheeler.

Buick Enclave: During the summer, when families are on vacation, it's a common sight to see SUVs with roof racks loaded with luggage. What does this do to fuel economy? We broke this test into two parts: with empty roof rails (many of which are left up all year long) and then loaded with a cooler and suitcase.

  • Aerodynamic effects of roof rails: When compared with cruise control set at 65 mph, adding the cross rails of a roof rack (and driving with cruise control set at 65 mph) caused a 1 percent loss in fuel economy from 27.2 mpg (without cross rails) to 27 mpg.
  • Aerodynamic effects of carrying luggage on the roof: When compared with cruise control set at 65 mph, adding a suitcase and cooler (and driving with cruise control set at 65 mph) caused a 21 percent loss in fuel economy from 27.2 mpg (without the luggage) to 21.6 mpg.

Lexus ES 350: We wanted to see if "drafting" in the slipstream of a truck would improve mileage but we also didn't want to tailgate or follow too closely. The Lexus is one of several new cars that offer adaptive cruise control, a radar-enhanced feature that allows the driver to follow the vehicle ahead at a variety of distances. In our test, however, we found our fuel economy unchanged while drafting behind a truck. Our average speed on this run was lower than 65 mph to match the speed of trucks. This lower speed should have given us better mileage on that fact alone. That it didn't is an indication that following the truck actually made the mileage worse. We weren't close enough to enter the slipstream, so we must have been riding in the turbulence.

Toyota Tundra: We performed two aerodynamic tests for this pickup truck: windows open and tailgate down.

Windows open instead of using air-conditioner: Often people will cite the increased efficiency of running the air-conditioner rather than opening the vehicle's windows. We were surprised to find that, for this truck, anyway, it wasn't true. We recorded a fuel economy increase of 9.5 percent when we rolled the windows down (at 65 mph) rather than turning on the air-conditioner under the same conditions. With the windows down, driving with cruise control set at 65 mph we got 20.9 mpg and with air-conditioner on and windows up we got 19.1 mpg.

Since our results run counter to popular wisdom, we looked for a possible explanation. First of all, the aerodynamics of a big truck are going to be highly individualized. Secondly, we theorized that the large side mirrors might have blocked aerodynamic drag that usually results from opening the windows. Still, this test showed that the air-conditioner produced a measurable drain on the engine and a resulting drop in fuel economy.

Tailgate down with cruise control set for 65 mph brought us a very slight improvement to 21 mpg. Compared to the base run that gave us 20.8 mpg, this was a 1 percent improvement. This test would have been interesting to do at different speeds to see if the effect was more significant. Also, different trucks have different bed lengths, which would change the outcome. Anyone with a pickup truck could try this test for himself to see what works best for his truck.

Additional Notes on Aerodynamics Luggage: Putting luggage on the roof of a vehicle hurts economy significantly because it represents an aerodynamic double-whammy. Objects on the roof increase the "frontal area" presented to the wind. The presence of such objects also spoils the aerodynamic shape of the vehicle, resulting in an increase in the coefficient of drag. Often we hear this in the car as wind noise. These two factors are compounded because the drag force acting on an object is equal to the frontal area times the drag coefficient. A rounded aerodynamic roof pod is better than exposed luggage.

Speed: The speed you choose to drive will directly determine how much gas you burn, especially at highway speeds. Aerodynamic drag increases in proportion to the square of speed, so doubling speed from 40 to 80 mph results in a quadrupling — four times more — of drag. Using more typical highway cruise figures, the drag force at 85 mph is twice as high as it would be at 60 mph. Drag is not the only force acting against a vehicle, but as the vehicle gets larger and boxier (like the Tundra), the drag component becomes a more significant fuel economy factor.

Tire Pressure and Fuel Economy

Most people pay little attention to the tire pressure in their cars. How far below specification are the typical car tires? And how much gas does this waste? We decided to see what driving on tires underinflated by 25 percent below spec would do to fuel economy. We chose 25 percent because this is when the tire pressure warning light illuminates.

We recorded fuel economy while driving with tires 25 percent underinflated and the cruise control set at 65 mph. We then compared the data to driving at the same speed with tires inflated to spec, and found these results.

Lexus ES 350: 4.6 percent loss in fuel economy if tires were 25 percent underinflated, from 32.5 mpg with tires inflated to spec to 31.1 mpg with underinflated tires.

Toyota Tundra: 2.9 percent loss in fuel economy if tires were 25 percent underinflated, from 20.8 mpg with tires inflated to spec to 20.3 mpg with underinflated tires.

Overall Fuel Economy Results

If you are currently an aggressive driver (cruising speeds from 75-85 mph, constantly accelerating and changing lanes and braking sharply) and you decided to calm down (driving with the cruise control set to 65 mph), your fuel economy would improve an average of 35 percent.

If you want to drive at higher speeds (77 mph with cruise control on) but you eliminate midrange acceleration, lane changes and harsh braking, your fuel economy will improve from 12.5 percent.

Adding a roof rack (cross rails) and driving with cruise control at 65 mph caused a 1 percent loss in fuel economy than driving at the same speed without cross rails.

Adding luggage to the roof rack and driving with cruise control set to 65 mph caused a 21 percent loss in fuel economy.

Driving with the windows open instead of using the air-conditioner gave us a fuel economy increase of 9.5 percent.

Driving a pickup truck with the tailgate down (with cruise control set for 65 mph) brought us a 1 percent improvement in fuel economy.

Driving with tires underinflated by 25 percent caused a loss of fuel economy on an average of 3.75 percent.

These results apply to the steady-state cruise portion of a highway trip on flat and level ground in calm conditions. We didn't test these effects in a city driving situation this time out, as we were not able to create a repeatable city driving pattern. We'll leave that for another day.

Editors' Final Thoughts

Dan Edmunds, Director of Vehicle Testing: My family and I recently took the Buick Enclave (the same one we used in this test) on a long trip to Oregon. On the way up, I was anxious to get there. I wasn't the most aggressive of my fellow motorists, but I wasn't timid either, constantly looking for a way to advance my position in traffic. I thought the Enclave's fuel economy was respectable when I calculated it at 21 mpg.

On the way home, I wasn't in nearly as much of a rush, so I set the cruise control at 65 mph and watched the "crazies" sail past. This probably added 30 or 40 minutes to our trip, but I was astounded to see my mileage jump to nearly 26 mpg. Was this a fluke? I wanted to put driving style, and other factors, to the test under more tightly controlled conditions.

Our aggressive driving technique in these tests not only included factors and bad habits that are common to those in a hurry, but more than anything it reflected the "I've gotta get there" attitude that is common in our busy society. As you drive toward any large city on a freeway, you'll often notice how speed and impatience seem to steadily increase.

Speed is a big part of the impatient-driver mindset, but following too closely to prevent the other guy from cutting in front results in a constant on and off the gas pedal. The need to get around the slowpokes and delivery trucks also creates deeper throttle applications and extra transmission kick-downs.

City freeway commuters find it hard to avoid falling into these traps, as everyone wants to sleep as late as possible and still get to work on time.

Brent Romans, Senior Automotive Editor: So it seems that the biggest effect on fuel economy is — surprise! — you. Well, it was a surprise for me, anyway. Dink around all you want with tire pressure and driving with the windows down on a 100-degree day. If you want to maximize fuel economy, the best way to do it is simply to drive in a relaxed manner.

Unfortunately, this aspect of improving fuel economy is also the hardest for some people to pull off. Today's lifestyle of the typical American is such that there isn't time — or we don't allow ourselves the time — to drive in a relaxed fashion. Even if you know that you can save money and fuel by slowing down, will you?

It's easier to do if you think long term. If you drive relaxed all year, you could improve your fuel economy by 30 or 40 percent as compared to driving aggressively. And that means saving the money spent on gas, too. If you drive 20,000 miles a year in a typical car, you'd be saving about $650 a year in fuel costs.

Philip Reed, Senior Consumer Advice Editor: My father is a chemist, and he became interested in fuel economy during the oil embargo of the early 1970s when he was asked to study the use of methanol as a gasoline supplement. As a teenager, I watched him testing the effects of different conditions on fuel economy. Since then, I've realized that there is much we can do as individuals to take control of our future. It starts with better understanding of the world around us.

This list of tips we have printed here will give you simple ways to boost fuel economy without buying new automotive equipment or popping for a new car. I also hope that this will encourage you to run tests with your own car. Understand that you have a choice. Make an informed choice based on testing, rather than taking as gospel what the media serves up. You will be amazed not only at the amount of money this saves you and your family, but at the feeling of power it will give you as your understanding of fuel efficiency grows.

We Test the Tips

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