fuel economy

New study details the mileage you lose by driving too fast

Although consumers are increasingly calling for more fuel efficient vehicles, they are refusing to drive slower on highways. A new study by the Oak Ridge National Laboratory quantifies the consequences of these actions and draws a surprising conclusion.

Motorists want to have their cake and eat it too, says a new study conducted at the Oak Ridge National Laboratory, in Knoxville, Tenn. Not only do drivers want more fuel-efficient vehicles, they also want higher speed limits. Not surprisingly, fuel efficiency drops exponentially by increasing your speeds as little as 10 mph. What’s interesting is that decreased fuel economy may not vary as much amongst different vehicles, according to the new study.

In fact, the research points out that there are relatively few differences between a massive SUV and a sleek aerodynamic sports car.

“People really like rules of thumb, and if you’re increasing your speed from 50 to 60 miles an hour, we find for the largest number of vehicles fuel economy will go down about 12 percent,” said Brian H. West, a researcher at the Oak Ridge National Laboratory.

Although 12 percent is a significant drop, that’s just the beginning, says West. Simply driving 10 mph faster (60 to 70 mph) will cause fuel economy to decrease 14 percent, while driving 80 mph can cause a 16 percent dip. Simply put, a vehicle that gets roughly 40 mpg at 50 mph would only get about 25 mpg at 80 mph.

The correlation between excessive speeds and decreased fuel economy is not anything new, but researchers did stumble upon a surprising discovery. Researchers had assumed that different vehicles would fare better than others, but ultimately found that there is little variation. In fact, fuel economy is consistent across all vehicle segments. Interestingly, the study found just a one to two percent deviation between the heaviest vehicle and the most fuel-efficient model.

The study also concluded that even hybrid vehicles fell subject to declining fuel economy. Although hybrids are engineered for stop-and-go city driving, highway driving proves detrimental when it comes to fuel efficiency. Specifically, hybrids cannot regenerate enough energy to counteract the gasoline they use when traveling at highway speeds. This is primarily the reason why hybrids have lower EPA estimated highway mileage ratings compared to city.

Only time will tell if studies like the one conducted by West deter calls for increased highway speeds. Highway speed limits have substantially increased since Congress lifted federal restriction in the 1970s. Most recently, Texas opened the nation’s fastest highway, which boasts a top speed of 85 mph.

In the meantime, automakers will be forced to meet increasing fuel economy requirements, while also battling the lead foot of the consumer. In August, the Obama Administration finalized a groundbreaking new standard that increase fuel economy to 54.5 mpg for cars and light-duty trucks by model-year 2025.

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Comments

The old ... '55 MPH' economy limit, wasn't an arbitrary figure pulled out of a hat, it was the result of physical reality. A vehicles drag _'squares'_ at just over 55 MPH. There is a lot of potential MPG by driving at 50-55MPH and accelerating slowly up to speed. Most people never hit the EPA figures because of the way they drive. They waste fuel and brakes and increase pollution by accelerating from one stop light to the next. I drive like I have no brakes... saves a lot of money. And I never drive my big pick-up over 55MPH. If you drive a manual, shift at around 1,800 to a max of 2,200 RPM, for best economy.
@Three Duece said "I drive like I have no brakes." This is how I drive as well. I like 57 mph when traffic allows. I just get in the second lane from the right, the "truker's lane" and cruise. I drive a Nissan LEAF, a fully electric car, and get the equivalent of 180 mpg. I average about 5.5 miles/kWh. I drive about 12 miles on the 10 freeway from Santa Monica to downtown Los Angeles. The whole way, I never touch my brakes, heavy traffic or light. It's a game I play and I've gotten very good at it. With an EV, you can hypermile like crazy!
Kudos for your efforts and the reduction of pollution due to your considered actions. It all pays back. And, yes, mileage has always been a top priority for me. I have fast sporty cars which I enjoy, but for general transport and long trips, considerate discipline and the challenge of scoring a high MPG, is a game that played well, has positive payback. Regards
All I can say to Three Deuce is he must not live out west. When I go back home for vacation it's a 10hr. drive. It would take Three Deuce a extra 3hrs. and for the little gas he might save, I would be drinking a cold one sitting next to the pool while he or her is trying to keep there eyes open driving 13hrs.
Well, I principally live in the NW, and spend considerable time in NE Texas. I also drive many trips a year of varying lengths from 250 miles to 1,500 miles, and some all the way across the country from the NW to Ft Lauderdale/Tampa. I just finished a 28 hour round trip of 500 miles today...At 45-55 MPH. The 45 MPH was over the passes in the truck lanes. Going up a steep hill in a heavy vehicle, at the speed limit, consumes one hell of a lot of fuel in a short distance, for little gain in time. My old work trip, in my pick-up, from Portland, Or. to LA was about 950 miles. At a 55 MPH that calculates out to around 17 hours. At 65 MPH, it would be nearly 15 hours. At 55 MPH, the truck would get 15-16 MPG. At 65 MPH, it dropped to 10-11 MPG. 63 gallons at 55 MPH, versus 90 gallons at 65 MPH. I will trade a nearly 50% increase in fuel consumption for a 13% increase in travel time, every time. And at $4.00 a gallon gas, the increased cost is $108.00. And I have found, that higher trip speeds are often negated by actual conditions... Traffic, stops for food, and at least one more stop for fuel,and whatever. Now in a high MPG car, I could of course drive faster, depending on the effect of increased speed on fuel consumption. But I generally don't. Even in Texas on 70-85 limit roads, I generally keep it somewhere between 55 and 60 MPH and enjoy the conversation, music and scenery. Besides I like too look for old cars and trucks, might miss one at the go nowhere fast speed for more money. And fuel mileage isn't the only cost factor with a vehicle, higher speeds consume tires faster, wear out engines and transmissions, bearings, faster, and a lot of other consumables. Not to mention that column of pollution created by every burnt gallon of fuel. One gallon of fuel burned creates approximately 19 pound of pollution. I care about my impact on the planet and the condition of the environment I leave my kids, and yours. Too many, are in a big hurry, to go nowhere fast... and we all pay for their willful lack of consideration.
All I can say there is no truck lanes here in the west and you would be a traffic hazzard. I think going faster to save lives is better than saving a few gallons of gas. There's plenty of oil for gas for 100 of years. I least my kids will see me alive than trying to save the planet.
No Truck lanes in the West. Apparently, you never leave town, and your certainly not a truck driver or drive an RV, or pull a race trailer. Almost all mountain passes have truck lanes, and I-5 has them even on low rises where a truck would slow. The far right lane is the slow/truck lane anywhere, pass on the left. Going faster to save lives, is a specious argument, especially in a truck lane. Pay attention when your driving, that is your responsibility. Trucks in Oregon are required to drive at 55MPH, and it is enforced. I live in Grants Pass, Oregon with a lot of mountain passes, North and South, all have truck lanes. Many trucking companies that own and fuel their own trucks, are now requiring drivers to drive at 55 MPH, even when the legal truck speed, is higher. Saves fuel and equipment..and is safer, which the insurance companies recognize with lower rates If I'm on a two lane road and traffic is backing up, I pull over at the nearest opportunity, it's courteous, and the law. Apparently you don't have much consideration for your 'Kids' or grand kids with your self serving polluting attitude.
I guess you dont live in the real west where you have to drive 25 to 30 miles just to get to another town. We have no mountains here the trucks go 70 to 75 MPH. Like I said before you would be a hazzard on the road.
While I will say this is true for most cars, especially cars with 3 and 4 speed autos (God help you if you're still driving a 2 speed Powerglide around) and even my '04 Cobra that in 6th gear I'm @ (I think) 2200 rpm's @ 65 mph. The 2013 GT500 has such a high rear, 3.31 that you cant even put it in 6th gear until over 70 mph...so keeping it in 5th around 2k rpms @55 mph uses more gas than putting it in 6th gear @ 80mph while "just" seeing 1500 rpm's. Can't argue with me, I already tested it, and the difference is +4 mpg in 6th gear @80 mph @1500 rpms.
The range of RPM is a general comment, but widely applicable. There are variations to the norm, but not much, this is due to a number of factors and would have to be considered individually. Higher compression ratios raise the volumetric efficiency at higher rpms. Higher altitudes lower volumetric efficiency. For example, my BMW 328is(10.2 CR) gets as good as, or slightly better mileage at 65, then 55 @ near sea level. It also exceeds the EPA Hwy figure by 5-6 MPG. This because it's volumetric sweet spot(efficiency @ near sea level), is such that it over comes the negatives of increased coefficients of drag, mechanical frictions, External frictions, internal frictions, and rolling frictions, But a 65MPH it is only turning over 2,300. Beyond 65 the combined cumulative forces severely reduce efficiency. Overdrive sixth gear(s) are used by MFG's to get good EPA figures and avoid the Gas Guzzler tax. They are generally useless in normal use, unless your going down a long steep hill at speed. What the MFG's don't tell you, is, the sixth gear(and most 5th gears) are generally very weak, comparatively, and misuse or abuse will cause premature failure. In sixth gear, the GT 500 won't pull max RPM or top speed, that is attained in 5th gear with the stock final drive ratio. The 2000 Cobra 'R' would not pull max RPM or top speed in 6th gear with the 'as delivered' 3.55 final drive ratio, while my SVO( same 'OD' ratio 'aftermarket' 6-speed T-56/3.45 FDR) would, due to its higher rpm torque, due to turbo charging, and its final, overall, drive ratio. I have yet to see few cars that would, even given enough room on a calm day at reasonable temperatures. Land speed cars, have to be geared for their target speed plus a little more, based on their RPM @ max HP, factored barometrically for air pressure/density, and temperature, and length of speed course. Same for drag cars. RPM at a given speed, can vary widely in an automatic without a locking converter. The final high gear, 'low' ratio, whether a 2-speed or 3-speed auto, was always 1-1, and is not factor as to engine efficiency at a given RPM. My last 2-speed glide was in my Altered roadster. Gearheads commonly confuse the gear/ratio issue. Low gear is a high ratio, High gear is a low ratio
While I will say this is true for most cars, especially cars with 3 and 4 speed autos (God help you if you're still driving a 2 speed Powerglide around) and even my '04 Cobra that in 6th gear I'm @ (I think) 2200 rpm's @ 65 mph. The 2013 GT500 has such a high rear, 3.31 that you cant even put it in 6th gear until over 70 mph...so keeping it in 5th around 2k rpms @55 mph uses more gas than putting it in 6th gear @ 80mph while "just" seeing 1500 rpm's. Can't argue with me, I already tested it, and the difference is +4 mpg in 6th gear @80 mph @1500 rpms.
Hmm! How do I put this without offending. Thanks for supporting my contention about RPM efficiency brackets. But then you go on and use that to contend that because of the much reduced RPM in 6th gear of a GT-500 at 80 MPH, that it is getting better MPG. Apparently using low RPM as the only factor for good mileage. The forces at play at 80 MPH would severely negate the efficiency of the engine at those low RPM's. The power it takes to maintain 55 MPH in a vehicle like a Mustang is around 55-60 HP. To maintain 80 MPH you need about 120+ HP. The caloric intake to produce 120 HP is significantly higher then 55-60 HP... It just doesn't pencil out in the real world. In all practical terms/regimes, the 6th gear in the GT-500 is useless, unless your driving in Utah or Texas with their 80 to 85 MPH limits, and then...barely. And, you can't even reach top speed with it. But it helps to avoid the tax.