Updated on 11.07.11

# Saving Pennies or Dollars? Going Below Speed Limit

Saving Pennies or Dollars is a new semi-regular series on The Simple Dollar, inspired by a great discussion on The Simple Dollar’s Facebook page concerning frugal tactics that might not really save that much money. I’m going to take some of the scenarios described by the readers there and try to break down the numbers to see if the savings is really worth the time invested.

Gayathri writes in: Driving 1mph slower than posted speed limit. Yeah, that’s a myth.

Actually, it’s not a myth. Most cars made in the United States maximize their fuel efficiency at about 55 miles per hour and drop off rapidly above that limit (this is actually from a study – West, B.H., R.N. McGill, J.W. Hodgson, S.S. Sluder, and D.E. Smith, Development and Verification of Light-Duty Modal Emissions and Fuel Consumption Values for Traffic Models, Oak Ridge National Laboratory, Oak Ridge, Tennessee, March 1999).

This means that if you’re tooling along on the interstate at the speed limit of 65 miles per hour and drop that back to 64 miles per hour, you’re actually improving your gas mileage by about 1.5%, according to fueleconomy.gov.

So, let’s work out what that’s really worth.

Let’s say you have a typical car that gets 25 miles per gallon at 55 miles per hour. At 65 miles per hour, it’s going to get roughly 15% worse gas mileage, or 21.25 miles per gallon. If you trim that back to 64 miles per hour, your gas mileage is a bit better – you’ll be getting 21.625 miles per gallon, more or less.

Now, let’s say you’re going on a 400 mile trip on the interstate and that gas is available for \$3.25 a gallon.

If you go 65 miles per hour, it will take you 6 hours and 9 minutes to make the trip. You’ll burn through 18.82 gallons of gas, which will cost you \$61.17.

If you go 64 miles per hour, it will take you 6 hours and 15 minutes to make the trip, six minutes longer. You’ll burn through 18.5 gallons of gas, which will cost you \$60.13.

In short, driving one mile per hour slower will add six minutes to the trip and save you \$1.04 in gas. Your savings simply by driving one mile per hour slower is \$10.40 per hour. That, of course, is after-tax money.

That figure, as mentioned above, assumes a 25 mile per gallon car, but other mileages have similar savings. It also assumes that you’re slowing down a bit from a speed above 55 miles per hour.

So, should you just go 55 on any road you’re on? I wouldn’t do that. Instead, I’d stick to the posted speed limit and maybe go a mile an hour or two below that in the slower lane on an interstate.

Doing this serves three purposes. One, you’ll put cash in your pocket for the extra time you spent driving. Two, you’ll never get a speeding ticket. Three, you’re sticking more or less with the flow of traffic (going much slower would disrupt that), so you’re not disrupting traffic flow and endangering yourself that way.

The next time I’m rolling along some flat four lane road in southern Iowa, I’ll just set the cruise to a couple of miles per hour below the speed limit and roll along. Sure, I might get there five minutes later, but I know I won’t get pulled over for speeding, I’ve got something entertaining on the radio, and that bit of extra time will put a bit of money straight into my pocket.

1. lurker carl says:

My calculation reveals the realized savings is only 18 cents per hour. Your mileage may vary.

2. Baley says:

Actually, if you drive 64 mph in a 65 speed limit on the freeway, you will be driving approximately 11 mph slower than the flow of traffic. You may not be “endangering” yourself, but you sure will be ticking off other drivers. :)

3. Josh says:

@Baley, actually driving slower than the flow of traffic is indeed endangering yourself, as it has been proven time and time again that slow drivers are a hazard on the road. You can argue if it is moral/ethical to go over the speed limit or not, but either way driving slower than the flow of traffic is a hazard.

4. Courtney20 says:

I wouldn’t drive slower on the interstate, because as Josh points out it’s a hazard. But I did notice when I moved and commuted to work a different way that didn’t involve as much highway, my gas milage went up about 2mpg. YMMV, literally.

5. Julia says:

If you’re driving less than 5 mph over the posted limit, stay to the right. For that matter, if you’re just keeping up with traffic, stay to the right. You’re always safer if faster drivers are in front of you than you are if they’re behind you, so let them pass.

6. AnnJo says:

I would question the current validity of a study published 12 years ago, probably using data even older than that. Between the change in maximum MPH from 55 to 65 on most interstates and the changes in car engine technology during that time, today’s cars may well be more efficient at higher speeds than cars back then.

If I were aiming to improve my highway MPG, I would concentrate on avoiding the use of the brake pedal as much as possible in heavier traffic, i.e., leaving more space between me and the car in front so that I could reduce my speed when needed by taking my foot off of the gas pedal without having to brake. No study to back it up, but I’d think that would save considerably more gas, and in congested areas is a more likely scenario, than driving a mile or two under the posted speed limit.

7. Julia says:

Courtney20, that’s a really good point.
In more populated areas, there’s always more than one way to get there. Evaluating your regular routes may reveal another money leak that can be plugged without any pain.

8. Matt says:

Speed limits on most interstates are 65+… and most people on them go 70+. Going 54 in a 65mph zone, even in the right lane, means you’ll be going quite a bit slower than those around you. It’s certainly an option, but I think I’d rather be going closer to the speed of traffic and take a small hit to the wallet than go slower and potentially get a big hit to the car from someone who’s not paying attention!

9. jackie says:

\$1.04 divided by 6 hours and 15 minutes is 17 cents/hour. Which is I think a more realistic what of thinking about it.

10. chuck says:

the “cost” of driving slower is arriving 6min later, not the entire trip time. those 6min saved you \$1.04 in gas thus it is \$10.40 an hour. trent has it right.

11. Steve says:

I have a 12-mile commute each way. I haven’t done the math but I have found that driving 65 (in the far right lane of the freeway) vs 70+ in the other two lanes gets me at least one more round trip per tank.

12. kristine says:

I find that driving 55 instead of 65 (posted is 55) slower does indeed save me gas. Half a tank on a 4 hour trip. But I do have to right lane it.

AnnJo- yes, using the break is literally throwing away gas. The gas creates momentum, and using the break throws away that momentum, or a significant portion of it. When I am driving around town, if no one else is behind me, I will let my car naturally decelerate to stop signs and red lights. I also coast down hills. Boosted my MPG from 28 to 33.

Though some of the practices are unsafe, you can google hypermiling.

13. Steven says:

It’s people like you…

Word of advice. If you’re going to drive on the interstate, stay in the right lane, and stay out of my way! I have places to be, and after a couple thousand miles of interstate, I’m sick of looking at it.

14. NMPatricia says:

Who would have thought of such responses. I had to laugh at some of the responses. On one bypass on which the speed limit is 55 mph, I set my cruise control and chuckle as nearly everyone passes me. Though I did have some thoughts as semi trucks and school buses pass me. The freeways have 70 mph speed limits. We tend to go 65 mph in order to not mess up too badly the flow of traffic but have some appreciation for our gas mileage.

15. krantcents says:

I do not drive on freeways to work anymore! I now drive on surface streets and try to drive 1-3 MPH because I brake less. It helps with my gas mileage.

16. Courtney20 says:

If you do drive on highways, you’ll get pretty much an equal boost from setting the cruise control as you will from driving under the speed limit. This also gives you the ‘savings’ (both money and time) because you’re not going to inadvertently find yourself going a little too fast on a hill and getting pulled over.

17. Jules says:

In Colorado, they’ve started pacing traffic–cop cars going at 50 mph or thereabouts–along one of the highways. No more traffic jams, far fewer accidents.

In terms of safety, it’s the difference between the speeds of the individual cars that makes the biggest difference. One car going 64 and while the rest average 65 isn’t going to matter than greatly, but one car going 55 while the rest average 65 is taking a risk.

18. Fr33d0m says:

“the “cost” of driving slower is arriving 6min later, not the entire trip time. those 6min saved you \$1.04 in gas thus it is \$10.40 an hour. trent has it right.”

If you drove 60 hours you would have saved \$10.40.

Or perhaps for every 60 hours of similar driving….

19. littlepitcher says:

Savings includes: 1-cost of tickets, court costs, and time lost from work, if you are on hourly wage; 2-cost of insurance premium increases; 3-wear and tear on tires and wheel bearings is greater at higher speeds. Now figure in loss of prestige when perceived social responsibility is reduced from court visit for a bad habit shared by testosterone-overdosed 16-year-olds.
And, sugar, our Southern cops really are that bad.

20. Jonathan says:

“I would question the current validity of a study published 12 years ago, probably using data even older than that. Between the change in maximum MPH from 55 to 65 on most interstates and the changes in car engine technology during that time, today’s cars may well be more efficient at higher speeds than cars back then.”

The benefit doesn’t come just from where the car’s engines runs most efficiently. It may be that newer cars run more efficiently at a slightly higher RPM, or that the gearing has changed to allow them to go faster at the optimum RPM. The differences at high speeds is related to drag. As speeds increase it requires more energy to overcome the effects of drag. In other words, it takes more gas to drive 65 mph instead of 64 mph than the difference between driving 35 mph and 34 mph. 55 mph seems to be the sweet spot for most cars, but there are exceptions. I have seen anecdotal evidence that a Corvette gets almost the same fuel economy at 55 mph vs 75 mph. This is, of course, a car that is built for speed and is more aerodynamic than most passenger cars.

21. Paul says:

Is there ever a speed decrease that works against fuel efficiency? That is to say, if, let’s say, I have an hour to do a 20 minute drive (and arriving early means sitting and twiddling my thumbs) and other cars aren’t a consideration, do I actually lose mileage by driving 40 MPH instead of 55?

22. jim says:

AnnJo, You’d think new cars would allow us to drive faster like that but no. New cars are also less efficient at higher speeds. Several people have tested modern cars and gotten the same results. You are right that driving sensibly (less braking, less aggressive acceleration) can have as much impact on MPG as driving reasonable speeds.

Jonathan, I found someones test results for a 2008 Corvette and the max MPG was around 35-40MPH. Course thats just one data point.

23. Kerry D. says:

In the SF South Bay Area: The last couple years have seen a huge increase in ticketing and monitoring for speeding tickets. (I think maybe it’s the police department’s fundraiser.) So… driving at or under the speed limit is a HUGE money saver. While moving with the flow of traffic on a four lane road with a 35 mph speed limit, I and most of my fellow drivers were given a \$380 speeding ticket–turns out that when the HIGH school gets out, it’s a “school zone” and we were exceeding 25 mph! They’re out ticketing like that all over here, anywhere traffic is flowing faster than the speed limit.

24. slccom says:

I routinely go slower than the speed limit on the Interstates, often 55 MPH. Stephen, YOU are the problem! It is the speeders who rationalize their illegal behavior and blame those of us who choose to drive according to the laws and common sense.

What I do when I go 55 is use my hazard flashers generously. When I see someone even far behind me in the (right lane, of course), I put them on. This alerts the jackasses who are on the phone or listening intently to interesting entertainment rather than paying attention to the fact that they are behind the wheel of a moving vehicle that I am going slower than they are.

No crashes, no near-misses, and I don’t cause crashes. But then, when I drive, I DRIVE.

25. Geoff Hart says:

I can confirm what Trent said: a few years ago, I worked on developing the SmartDriver training program for truckers while I was employed by a Canadian NGO, and the savings were as good or better than the amounts Trent reports.

The problem isn’t with vehicle technology; it’s that wind resistance increases with increasing speed, and the increase isn’t linear: the faster you go, the more the resistance increases. The savings are particularly significant if you’re up here in Canada, where fuel prices are up to twice the U.S. cost (depending on the exchange rate at any given time).

It’s also worth noting that you’re also cutting greenhouse gas emissions significantly. That’s also compelling. Ditto for the fact that the slower you’re driving, the more likely you are to survive an accident, avoid driving off the road if you hit a patch of ice (a concern at this time of year), or avoid an animal that runs out in front of you. Also not a negligible benefit.

It’s true you shouldn’t drive excessively slower than the rest of traffic, but if you’re driving slow in the slow lane, people coming up fast from behind can simply go around you. If they can’t figure out how to do that, they shouldn’t be driving.

On another note, maintaining a constant speed is another huge fuel saver. The more often you have to accelerate back up to speed, and the harder you accelerate, the more fuel you waste. Keeping a safe following distance lets you reduce the frequency of braking hard and having to accelerate back to speed.

26. lurker carl says:

The minimum speed when the transmission shifts into the highest gear and the torque converter locks is when a vehicle typically obtains it’s best fuel mileage. That speed is as low as 30 mph for some vehicles and as high as 60 mph for others, the speed for that final shift point depends upon the engine, transmission, final drive ratio and tire size.

27. AnnJo says:

Thanks, Jonathan and Jim, for the reminder about drag. Reading a little more about that, I see that really is a big factor on MPG. So what’s with all the boxy new vehicle styles? Those can’t be aerodynamically very efficient.

28. Rebecca says:

All I can add is the folks in my neighborhood must get fantastic gas mileage. To get to my carpool point, I have to drive on two lane back roads. Invariably, when I’m trying to get to work in the AM, I get behind someone going 20 in a 40 MPH zone with no way to get around them. **sighs**

29. Jess says:

I hope I’m not the first person to mention this, but do be careful of going below the speed limit – you CAN get pulled over for not going fast enough (here in MA, anyway). Also, please make sure to stay in the right hand lane if you have the choice so the rest of us can pass you!

30. Allison says:

I think the greatest benefit of going 5 mph or so below the speed limit on the interstate is that you don’t have to constantly speed up/slow down. Maybe it’s just where I live, but here people have no idea how to use an acceleration lane and merge when entering the interstate. You always end up having to adjust your speed to accomodate them. If I stay a bit below the speed limit, they seem less confused :) then I can usually leave my cruise control on and avoid all that braking/re-acceleration.