At Highway Speeds, Mileage Drops More for Fuel-Efficient Models

The EPA fuel ratings are a useful tool for generally judging what kind of mileage one can get out of a car, but real-world driving often changes the formula due to a number of variables. One such variable is the actual speed at which you drive on the highway.

Consumer Reports wanted to find out how quickly fuel efficiency drops for certain models as highway speed increases. Obviously the faster you drive, the lower your car’s mileage, but when CR tested several different models it found that hybrids often experienced the steepest drops in fuel economy.

The Honda Insight was the biggest loser in the testing, losing almost 15 mpg after going from 55 to 75 mph. Hybrids were not alone in losing mpg. The Toyota Camry’s efficiency also turned sluggish once the needle crept up, going from 40.3 mpg at 55 mph, to 35 mpg at 65 mph and finally falling to 30 mpg at 75 mph.

The smallest differential between 55 and 75 mph turned out to be the least fuel-efficient model tested. The Mercury Mountaineer returned 23.8 mpg at 55 mph and fell only to 17.8 mpg at 75 mph.

Check out the results from all seven vehicles after the jump.

Tested: Speed vs. Fuel Economy (Consumer Reports)

Comments

the percentage of milage drops seems to be consistant with all th models. all the cars have 25 - 30 percentage dromin milage. the report seems to skew the preception of people

I'm never going over 55 again. If I have to drive on the shoulder to do so, so be it.

I knew after ~60, MPG drops off steadily but I never knew it was so dramatic!

Billy4202-

Yep. Above a certain speed (usually around 45mph), air resistance is the largest contributor to your fuel consumption. Theoretically, air resistance increases as the square of your speed (so if you double speed, resistance quadrulples), but experiments have shown it to be closer to the cube (so if you double speed, resistance octuples) So increasing speed from 55 mph to 65 should increase your air resistance 40-90%.
This is the original reason why a 55mph speed limit was established in the 1970's to help reduce the impact of the oil embargo.
Still, please don't drive 55mph if everyone else is going 70-80mph, because then you are a danger on the road. Safety is probably better than frugality.

And the force from deceleration in a crash is proportional to speed.

Higher speed = more force exerted on your body in a crash = higher probability of serious injury.

Going with the flow is not safer. And yes speed doesn't kill but stopping does.

I've found just the opposite on my 95 Neon. During my latest trip down to North Carolina (500 miles one way) I went between 75-80 the entire way and got on average 42mpg. Whereas going slower I generally only get between 35-37mpg. Thats all due to where the engine is running though. At higher speeds I have more wind to contend with but the engine is at more of its sweet spot for power thus making it work less.

Peppy-
Yep, there is more to fuel efficiency than just air resistance. I'd say your Neon was probably the exception rather than the rule. Most cars are designed to have the engine's sweet spot somewhere around 55mph, especially cars built before 1996 when the speed limit was still 55mph. That's pretty neat that your car does better at higher speeds!

H-
Stopping doesn't actually kill you, it's the deformation of the metal of your car disemboweling or otherwise injuring you that does. (since we're going down that line)
Interestingly, NASA has done tests on the blackout condition for acceleration versus deceleration. While it doesn't really make sense from a physics standpoint, for some reason the human body does much better during deceleration. I haven't been able to determine if they ever tried reproducing the results with the seat turned around backward. Would be interesting.
However, going with the flow of traffic is absolutely safer. Any highway safety expert will attest to that. The main goal is to avoid an accident all together. If your speed is vastly different from those around you, you disrupt what drivers expect to see, causing a panic reaction which can cause accidents even if they don't hit specifically you. Not to mention how much the chance of accident increases as drivers attempt to change lanes to get around you with different speeds in the lanes.
To assume that going slower is safer is a good rule of thumb, but too simplistic to be universal.

Try drafting behind a truck-- you'll get stellar mileage and cool rock chips on your windshield.

Dan,

We got so much traffic here it'll be a cold day in hell that I actually reach 55 on my commute. I was kinda kidding about riding the shoulder though. But on those rare days that everyone else is going 80-100...nah. Go around me.

This is all dependent on make/model/engine. My car (Mercedes E550) gets 25.5mpg whether I'm going 60mph or 80mph. Articles like this can be misleading and should have a "your mileage may vary, literally" disclaimer since different vehicles do have vastly different sweet spots.

Highway driving is also a place where Hybrids fail miserably...typically getting no or very little improvement from their standard gas-powered counterparts. Very few people (except urbanites) do city driving as defined by the EPA, and a similar sized efficient standard car will do as well as the hybrid in many mixed driving scenarios.

DJ,

The claim that hybrids have minimal advantage over their gas-powered counterparts isn't quite true though. Granted, they do have a smaller advantage on the highway, but put a non-Atkinson-cycle 4-cylinder and the same Atkinson-cycle 4 up against each other and the Atkinson's gonna be more frugal (of course at the expense of power output), since it allows for fuller gas burning.

I also get better mpg the faster I drive. I used to have a 91 Civic 5-spd and I always got the best mileage out of it if I drove above 80 mph. I'm beginning to notice the same with my 04 Mazda3.

The Mazda is also a manual. I wonder if this has anything to do with it. When I drive rental cars with automatics, I notice major drops in the mileage the faster I go.

How many of us drive more highway than rural or city roads? I think less people drive all highway on a daily basis than the later. Yes, there are plenty of exceptions to this. I'm talking about all Americans as a whole. Hybrids excel in non-highway driving and I think that's there main purpose.

Let's see the fuel economy results for all in-town/rural driving and the Hybrids will shine a little brighter I think.

Just a l'il bit of logical/intuitive supposition on my part but wouldn't it make sense that a peppy little 4cyl would be working its "ace" off to achieve that 55mph - 75mph differential vs a larger engine that's almost begging to go that fast...ok not begging but wouldn't have to expend percentage-wise as much effort as the itty-bitty 4-banger?

DJ,
Stop deluding yourself, you really did pay too much for your gas guzzler. My Prius hits 55 mpg on the highway while running the air conditioning and with three people on board...and it will do it all day long. City mpg varies by season but hovers around 50. Cold weather and heavy AC use knock it down three to four mpg. Get off you high horse and drop the uninformed opinions about hybrids.

I just saw the Motor Week program with the mileage comparison, that included some of the editors from this site - over the 200 mile route, the test Prius like mine averaged 49.2 mpg. So DJ, would you say the Prius "failed miserably" in that controlled, real world test?

So the Mercury Mountaineer drops from the worst mileage tested by far to the worst mileage by far?

Sounds like the mileage drops at roughly the same percentage in all those cars tested. The lesson of generally better mileage as wind resistance decreases isn't rocket science.

I don't think you can make 40mpg with a Camry, just put a spouse, a couple of kids and 2 suitcases and you will see the fuel economy drop.

I'll bet if the high-horse urbanites actually got a driver license and took a road trip across boonieland, maybe then they'd realize the foolishness of dictating speed and traffic laws from anything other than an informed perspective on local road conditions.

I drove I-95 from Miami to NY. Around the time I left rich NY liberal "civilization" of south and central FL, the speed limit was 70, with acres of tobacco farms, truck stops, and no such thing as "rush hour" until I temporarily emerged from the wilderness near the DC suburbs.

I drove I-80 across country. Same thing. The speed limit was 75 in Nebraska, yet I was practically the only car on the road. There were no tall buildings or other liberal constructs of "density" until I got near the Chicago suburbs, where I could see the Sears tower and planes flying into O'Hare. Then it was into Indiana and back to boonieland until after the Delaware Water Gap in NJ.

Thinkerdude - Cars.com achieved 40.3mpg in their test with the Camry so no thinking on your part is required. And by the way putting a spouse, a couple of kids and 2 suitcases in any vehicle will make the fuel economy drop. It's not rocket science.

Odd, my efficiency goes UP with higher speed until about 80 mph. I get about 18 mpg doing 30 mph thru neighborhoods, 23 doing 45 mph thru the city, hit 28 doing around 65 on the highway, and got a stellar 34 mpg doing about 75 on the interstate. I drive '99 Mits Eclipse RS Auto, btw. Just another perspective.