Tested: Speed vs fuel economy

We all seem to be in a rush these days. Life’s time demands seem to only increase, with the delicate balance of work, family, and recreation pressuring us to maximize every minute. As a result, there is real temptation to edge beyond the speed limit to shave seconds off each mile. But while driving fast may get you there sooner, there are some real downsides, such as diminished safety for yourself and fellow motorists; risk of a speeding ticket; and decreased fuel economy.

To quantify the impact a heavy right foot can have on your wallet, Consumer Reports recently conducted a seven-vehicle test comparing the fuel economy of each at speeds of 55, 65, and 75 mph. The results show the faster you drive, the more fuel you burn—no surprise there. But the most significant change in fuel economy comes from the most fuel-efficient vehicles we tested.

The Honda Insight hybrid showed the largest drop in fuel economy—over 15 mpg going from 55 to 75 mph. The Toyota Camry returned 40.3 mpg at 55 mph, but that reduces to almost 35 mpg when the speed moves up to 65 mph and drops to almost 30 mpg when speeds reach 75 mph. That’s a drop of about 5 mpg for every 10 miles over 55. Vehicles with lower fuel efficiency had the smallest drop. The V8-powered Mercury Mountaineer has a fuel economy of 23.8 mpg at 55 mph and that drops to 21.2 mpg at 65 and 17.8 at 75 mph. See the chart below for more details on all the vehicles tested.

Make & model	55 mph	65 mph	75 mph
Acura TSX 2.4-liter 4-cyl.	39.9 mpg	35.5 mpg	30.7 mpg
Honda Insight 1.3-liter 4-cyl.	51.9	44.8	36.5
Lexus RX350 3.5-liter V6	30.9	27.4	23.0
Mercury Mountaineer 4.6-liter V8	23.8	21.2	17.8
Toyota Camry 2.5-liter 4-cyl.	40.3	34.9	29.8
Toyota RAV4 2.5 liter 4-cyl.	34.6	29.3	25.9
Toyota Yaris 1.5-liter 4-cyl.	42.5	37.9	34.0

In this economy, saving money is important. But it’s not that simple. As the adage goes, time is money and it can sometimes be worth spending more to get somewhere faster. Another consideration is traffic. There is debate as to whether driving slightly over the speed limit is more dangerous than going slower. Some say it is the difference in speed of vehicles that contributes to accidents—not speeding—and that it’s best to move with the flow traffic.

What do you think? Are speeders or slow drivers the cause of accidents? Would you be willing to drive slower to save a few bucks and reduce national fuel consumption? Let us know your thoughts in the comments below.

See our guide to fuel economy for more on saving gas and alternative fuels.

—Liza Barth  

Comments

[Cale]

I can tell you this much, I wouldn't want to collide with a deer at 75 mph in a Honda Insight, aside from that setting artificially low speed limits just seems to turn more motorists into speeders.

[Jack]

"The Honda Insight hybrid showed the largest drop in fuel economy—over 15 mpg going from 55 to 75 mph."

This is one place where the difference of the real cost is much better understood in gallons per mile, rather than miles per gallon. The insight's cost for speeding is far less significant than the Mountaineer's loss of "only 6 MPG." The difference between 23.8 MPG and 17.8 MPG is a horrible waste of gas, while the Insight's drop from 51.9 to 36.5 is relatively not as bad.

Here is a demonstrative spreadsheet, using your table as a starting point.
http://spreadsheets.google.com/pub?key=tjEQaa2gtBEbRiWqqZkVgAA&output=html

[Dan]

Have CU testers ever found the ultimate "sweet spot" for speed as it relates to fuel economy? City driving with its frequent stops obviously lowers fuel economy, but driving too fast also lowers it. I was just wondering if there is a particular speed range that maximizes fuel economy without being too slow or fast.

[Keith]

I would like to have seen the test results at 40, 45 and 50 to see what the optimum speed for the best fuel mileage would be per vehicle. It would be interesting to see how all the vehicles best mpg speed would vary.

[bwilson4web]

Thank you! The Insight mileage at 55, 65 and 75 mph was exactly the missing piece of information needed to understand some of the highway mileage reports. Combined with the Prius data for the NHW11, NHW20 and ZVW30 we can finally map what happens when high mileage vehicles meet the 70-75 mph test zones that automotive writers often use.

Please share this with your future vehicle reports for all vehicles including the diesels.

[Eric]

Are speeders or slow drivers the cause of accidents? Yes. In the right situation either one is dangerous. If I'm driving 65 in a 35 mph zone then I'm dangerous. If I'm driving 35 on the freeway, I'm just as dangerous.

Am I willing to drive slower to save money? Sometimes. I find when making the argument for lower speed limits there is an incorrectly assumes that the driver's time has no value. Placing a national speed limit at 55 back in the 70s was fine for people living on the east coast or big cities where things are close together, then that works. But the same 55 in western states where distances are measured in hours on the Interstate and not in minutes is a completely different situation. It also wasted productivity of people (and generated more pollution and greenhouse gasses) by having them in cars longer. While it may only be a few minutes, if you are going to make the argument about saving national consumption of fuel by aggregating savings, you also have to add in the loss in prodcutivity in aggregate by doing the same.

[John G.]

Jack makes a good point.

Driving faster gets you off the road faster too. An engine that is off doesn't burn much. I realize 'miles per gallon' is based on distance, but missing lights and idling at lights wastes gas too. Has anyone done a study on this?

Driving a light car with a higher risk of accidents wastes lives.

How much carbon debt is there by buying a new fuel efficient car? That may offset the milage differences too.

The big thing is to get off of fossil fuels altogether. Arguing over milage differences is like trying to bail water out of a raft as it goes over a cliff. No matter what you do, you're going over.

== John ==

[Burpnrun]

This may be a stupid question, but:

If one "saves" gas by going 55 vs 65, doesn't one go less distance in an hour at 55, and therefore have to drive an extra 10 miles (assuming trip was to be 65 miles) after an hour is up, and therefore has to use gas to do those 10 remaining miles? Where is that calculated in?

[Dave L.]

Jack: Thanks so much for taking the time to put that spreadsheet up!

[Dave L.]

... although, gallons per 1,000 miles would be easier to read.

[Neil]

Where is the prius for comparison? Using only one hybrid is a poor judge of the breed and does not compare objectively Hybrids in general.

[Joseph]

Cale is right that looking at the delta in MPG is misleading and the insightful (no pun intended) way of looking at the data is to convert it to gallons per 100 miles. I did that and the delta from 55 mph to 75 mph should corrolate to the aerodynamics of the vehicle. To my surprise the Toyota Yaris and the Acura TSX had smaller penalties for the faster speed (0.59 and 0.75 gallons respectively). The Insight was third at 0.81 gallons. The Mountaineer was predictably last with an extra 1.42 gallons of fuel consumed to travel 100 miles at the higher speed.

[Mike]

My experience is to go with the flow. It's the difference in speeds that creates dangerous situations, especially when distracted drivers are thrown in the mix. Consider a situation where traffic is stopped in the right lane due to an off ramp being overcapacity and continues in the remaining lanes, we see those darting out or stopping to get in creating especially dangerous conditions. A similar situation applies here in CA where trucks are limited to 55, but remaining traffic can move at 65-70 (although trucks are limited to the right two lanes thankfully). The variation in speed between lanes creates havoc.

[Adam Bernard]

Well, thanks for confirming your pro-Japanese bias. Two Hondas, one Toyota, and an out-of-date American V8 SUV. Yes, I know that the Camry was supposed to be a 'typical' midsize sedan--but couldn't you have switched things out by including (for example) a rwd car (Mercedes E, Cadillac CTS)? Or a minivan (Chrysler Town & Country)? I've not even sure what the point of including the Acura TSX--another midsize, four-cylinder sedan which, predictably, scored within 1mpg of the Camry--except to show that you love the Japanese.

And, yes, I work for an American automaker. Which means I'm extra-annoyed.

[DanO]

I would like to see some results for a diesel vehicle alongside these, perhaps a 2010 VW Jetta TDI or 2010 Audi A3 TDI. It would also be beneficial to include the percentage change in MPG (or GPM as stated by Jack) from a baseline speed.

[Erik]

I don't see how this test can be fair when you are comparing Apples to Oranges and the Testers themselves see a larger drop for the most efficient cars and the smallest drop for the least efficient. At Least Compare similar cars so we can make sense of the readings.
If I were reading about all Hybrids ie. Prius, Insight, Malibu Hybrid, Civic Hybrid. THEN we could actuallt Compare Cars......

[Matt]

Why was this only presented as raw mpg and not % reduction? Of course the Insight lost the most mpg, it has the most to lose. That 15 mpg number looks big but as a % loss it's not much different than the others.

[Jim]

This is to you guys at Consumer Reports.

The Mercury Mountaineer is a design dating back how many years? Five? It's also the least aerodynamic vehicle in your test, the biggest and heaviest, the only one with a V8 engine and, significantly, the only American vehicle in the test. The Ford Escape, the Saturn Vue, the Ford Fusion Hybrid (especially) or the Mercury Milan Hybrid, if you insisted on a Mercury, all would have been much more representative. It's even more deceitful that the only fact your report displayed was the raw mileage of each vehicle, and not the percentage of mileage lost, because even the Mountaineer V8 would have done well, there, and a Ford Fusion Hybrid would have shined.

Your continued efforts to trash American vehicles by, in this case, testing one of the two worst American hybrids against six of the latest Japanese vehicles only serves to further discredit you as a source of information for auto buyers.

A Ford Fusion Hybrid might very well have won this test. It is a fresh new design with excellent aerodynamics and comes with a modern, powerful drivetrain. The fact that you chose a V8 powered two ton SUV as your only American entry speaks clearly what your point was, in publishing this article.

You people are one of the reasons American automobile manufacturers are in trouble. Your misleading publication has been driving people to buy imported cars for years, with very little justification. I hope your parent company's advertising profits are worth it.

I suppose we should be grateful you didn't use a Cadillac Escalade Hybrid.

[Ken]

This is interesting information, but it does not give us a total picture. First, it would help a lot to test various cars with both high and low coefficients of drag. (I would expect a Corvette to have a smaller drop in efficiency at high speeds than a Chevy pickup, even though both may have the same engine.) Second, expressing the results as a percentage efficiency change or as costs per mile might help.

Several studies have disproven the safety-hype myths about speeding. I don't drive faster than I feel is safe, regardless of what a local municipality might deem lawful in a specific location. I have a vehicle with extremely high braking and handling capabilities, and I don't drive distracted or otherwise impaired. Thus I don't agree with standard speed rules which apply equally to semi trucks and 20-year-old SUV's.

I avoid traveling at excessive speeds because I am aware of the increased gasoline consumption, although I'm sure our definitions of excessive might differ. And no, I am not remotely convinced that driving slower would have safety benefits anywhere close to eliminating impaired driving and mandating stability control.

[Dan]

It would have been interesting to know at what speed these cars returned the best mpg. Could you do such a test?

[CCWeems]

How fast we drive is a political not technical issue. Run the tests again but at 45 and 35mph (aerodynamic drag is generally not a factor below 40mph). In all likelihood you would see a continued increase in mileage by most if not all vehicles, the hybrids will really go up. Then which do you choose? What if driving at 35mph saved the most fuel and was the safest? Were the government to set a 35mph speed limit there would surely be loud protests by nearly everyone. The fact is that the faster you go fuel economy goes down and injuries go up. There is not however anything magic about 55mph. It is but one point on a continuum. Those whose proffer that the 55mph limit is too slow have as much reason to support their arguments as those who complain about a 35mph limit.

To restate: there is no technical support that a 55mph speed limit is optimal. A decision driven strictly on technical issues could roll us back to 35mph. The original 55mph was originally chosen on purely political reasons. Were it to be reinstated it would be by politics again.

This is just about highway transportation. When are they going to focus on all other forms of transportation and the effect of increasing speed has on them? Trains at 200mph use far more electricity than they do at 70mph – this is for the trip not just for a moment in time. Should we slow them down? Is the loss of time by highway users less valuable than rail users? There is not enough time to look at planes in detail but it is undeniable that a turboprop at 275mph is much more efficient than a turbojet at 550mph. Should we turn our backs on progress and make trains and planes go slow? If you answer no why should we do the same on cars and trucks? They get much better mileage, they have reduced their pollution by 90% and are far safer than cars only 10 years ago. How can we justify making them go at a slower speed than has been the accepted norm?

Politics.

[Ander]

It is slow drivers who sit in the left lane that are the cause of accidents. As for dropping fuel economy at higher speeds it is unsurprising that a hybrid does not do well: they have all the extra weight of the hybrid system and none of its benefit when you are on the highway. They become heavy cars with small engines.

[Steve W]

Hitting things is what causes accidents. America has some of the lowest speed limits in the western world, yet has higher auto accident rates, and more fatalities than other countries with higher speed limits.

[IvanG]

The reduction in the number of miles per gallon is not a proper measuring tool for the waste of fuel. Both increase in total fuel consumption and percentage decline in miles per gallon are more meaningful. As to the cause of accidents, speeding can lead to accidents, but driving faster than the flow of traffic and maneuvers around slow-moving vehicles are more likely to cause accidents. Obstructing multiple lanes by driving roughly in parallel at about the same speed creates a moving hazard.

[Walter]

Interesting debate. Interesting comparison. But where are the diesels? Lets see a Jetta TDI or a MB Bluetec ML on here...