To entertain myself on the long drive from my Mom’s house in the Bay Area to my home in LA, I conducted a science experiment. How much more efficient was my 2003 MINI Cooper at various speeds and how much money did that translate into?
Step 1. Measure the average miles per gallon (a statistic my car tracks) at six different speeds for a distance of two miles. Repeat twice so there are three sets of data for each speed (see chart below).
Step 2. Average the miles per gallon at each speed.
Step 3. Extrapolate the cost of the trip per mile at each of the six speeds based on the average miles per gallon for that speed. I paid $2.63 a gallon when I filled up before driving home so this is the price I used. The formula looks like this:
1 mile x 1 gallon/average miles at a particular speed x $2.63/1 gallon of gas =
Driving one mile at 85 mph costs $.095.
Driving one mile at 80 mph costs $.085.
Driving one mile at 75 mph costs $.086.
Driving one mile at 70 mph costs $.075.
Driving one mile at 65 mph costs $.068.
Driving one mile at 60 mph costs $.068.
Step 3 raised a few questions, namely why the cost of driving at 75 mph was slightly higher than driving at 80. And is my car really as efficient when I drive at 65 mph as when I drive at 60 mph? More testing is needed to answer these questions.
Step 4. Determine the time it takes to drive one mile at each speed. The formula looks like this:
1 mile x 1 /miles per hour x 60 minutes / 1 hour x 60 seconds / 1 minute =
At 85 mph one miles takes 42 seconds.
At 80 mph one miles takes 45 seconds.
At 75 mph one miles takes 48 seconds.
At 70 mph one miles takes 51 seconds.
At 65 mph one miles takes 55 seconds.
At 60 mph one miles takes 60 seconds. (This is also the speed at which trucks will pass you en masse.)
Step 5. Compare gas costs with time savings.
Driving at 85 mph costs $.027 more per mile than driving at 65 mph but saves you 13 seconds.
For a 30 mile commute, this adds up to $.81 more at a savings of 6 minutes and 30 seconds.
For a 400 mile road trip, this adds up to $10.80 more in gas but cuts one hour and 27 minutes off the trip.
For a driver who puts 10,000 miles on their car each year, driving at 85 mph costs an extra $270 but saves 36 hours.
Conclusion: The cost of driving fast – at 85 mph instead of 65 mph – is roughly $.03 more per mile, or about 40 percent more. The actual difference, $.027, looks small but adds up quickly. Slow down and drive at 65 mph, which is the speed limit on California state highways (with a few exceptions), and you will save roughly 30 percent.
During the tests the terrain was mostly flat and wind was minimal. I collected data while driving but did the calculations in the safety of my non-motorized home.
Questions. What are the indirect costs of driving fast or of driving slow? Did you cause an accident or get a ticket for driving at 85? If you slowed down to 60, did other drivers honk at you? I noticed my car was so much quieter at 60!
Welcome to Bargain Babe! I’m Mara, and I’m so glad you’re here. I am an obsessive bargain hunter and penny pincher, and I am determined to live a full life without EVER paying full price. 
Thanks for the experiment and your time. No wonder why most of the road has a speed limit at 65.
Very interesting, I like the way you approach the problem. Now I will drive at 65!
Driving the speed limit on freeways, usually 65 or 70, seems the most prudent. The higher the speed, the higher the risk of an accident. While YOUR driving skills may be up to the task of driving at high speeds, the drivers who share the road with you may not be. So.. drive sanely and save some money, too.
With such a short test distance (2 miles) it doesn’t seem to me that this is representative of the mileage you’d get in highway driving; you’ve spent a disproportionate amount of fuel accellerating. I suspect if you tried testing at longer distances you’d find the magnitude of the savings would decrease (though certainly not to zero).
Perhaps 2 miles is too short of a distance, but I did not spend any of that time accelerating. I waited until I was at speed to start.
We did a somewhat less scientific experiment with our 11,000lb RV (our home at the time) and found savings of over 35%. We were doing a lot of driving at the time so we DEFINITELY drove slower…in the right lane. I suspect make/model/weight plays a significant role. Thanks for the validation!
Yes, you bring up a good point. Each vehicle is different and so the numbers I came up with are really a guideline for other vehicles.
I slowed to 65 mph when prices were so high and I did find that I felt safer, and I love to go fast. I do go slower than 80 now, but your experiment is a good reminder for me to keep it under 70. I think you should repeat this article every few months as a reminder. Save money. And maybe save a life.
Drove cross-country New Hampshire to Austin in a Penske 16 foot truck (booked online with AAA discount and saved 22%). It was brand new and said it would do 15 miles per gallon in the owner’s manual. We (him) hyper-miled the entire way and got 25 miles to the gallon.
I don’t care if people honk at me now. Driving like an old person saves money, and is (in most ways) safer. I’m not quite a hyper-miler, but I don’t drive fast, I try and plan ahead, and I drive defensively…..
I can think of 3 things for now, and I list them in order of certainty. Of course, this assumes you used cruise control.
1) You weren’t driving over the same stretch of road, giving the experiment a variable that is not accounted for. Hills really make a difference.
2) Your tracker is not calibrated properly, and in my own experience, usually incorrect. It’s my own experience and in no way speak for anyone else, yours can be correct.
3) The gears on your car are set up in a way where you’re at a lower RPM at the higher speed, as opposed to higher RPM on the lower speed. From the energy the engine generates, more goes towards speed in the former, and more towards torque in the latter.
I applaud your use of an experiment to show that yes (as we were all told) we should slow down!
I’ll agree with cmadler that perhaps this experiment isn’t rock solid, mainly based upon how driving at 80 mph was “cheaper” than 75 mph… however I think that’s probably just an anomaly and if you took more measurements that the cost would have followed our intuition.
On a related note: When you’re driving on the freeway, you’re mostly just trying to fight against aerodynamic drag (this is why they tell you to roll UP your windows and use AC on the freeway)… and so this with in mind, using just standard aerodynamic drag equations, driving at 85 mph creates TWICE the drag than at 60 mph…your calculations show only about 1.4 times, so obviously there are other things going on, but still! Slow down == money in da’ bank!
I’d be interested in seeing what other people get…ya know, for science’s sake — not condoning speeding 😉
Good point, Robert. The wind could have shifted during the test and affected the results. More testing is needed! If anybody else conducts this kind of test in the future or has in the past, what did you find?
Have found that driving in city streets usually the lights are timed according to the posted speed limit.
Have experimented with this many times and
it is true almost all of the time. So if I go the legal limit I can make almost all the green lights for long stretches.
I like doing things legal now since tickets are incredibly
expensive and the likelyhood of accident goes up
when driving overlimit.
All of that to say I now try to go legal limit to avoid
any contact with the Law.
I feel safer as some of the comments have concurred.
So I translate that to freeway driving as well.
It is good to know that going the legal limit also
saves gas and money (and no expensive tickets!)
Saving time by going over speed limit may
cost more in the end when tickets can be
several hundred dollars.
You calculate the time savings, but then ignore them in your conclusion. A $10.80 savings in gas equates to a time savings of one hour and 20 minutes – in effect you are paying yourself $7.44 an hour to drive slower. That is less than the minimum wage in California! I value my time more than that so I would take the time saving over the gas saving any day of the week.
My time is worth considerably more to me than $11/90minutes ($7.33/hr). I’ll cut off that time every chance I get.
-d
“For a driver who puts 10,000 miles on their car each year, driving at 85 mph costs an extra $270 but saves 36 hours.”
So…36 hours of your life is worth only $270? If you want to make arguments about safety, that’s another story. Based just on the two factors you’ve presented though, time and money, I can’t say I’m convinced even a little! My time is much more precious to me than the $10/hour I’d save.
Our recent drive from Missoula, MT, to San Francisco corroborates these numbers. The gas mileage in our 2007 Subaru Forester consistently exceeded the manufacturer’s estimate of 29 MPG. Since we were mostly on two-lane highways instead of freeways, going around 60 made safety sense as well.
But there was also a another benefit. Driving was a lot less tiring! We both found that slowing down made it more pleasant and relaxing and that we felt less compelled to grip the wheel as the days rolled by.
And when you’re driving over 2100 miles, that makes a big difference.
Based on your experimental data, the average mpg numbers are accurate to within plus or minus 1 mpg.
A little fact that most people overlook is that SPEEDING not only costs more but it is the REASON for most of the traffic jams out there. Driver usually speeds until the car in front of him. Hits the brake causing the car behind him to hit the brake and causing a ripple effect. Hence, sometimes you get to the point the traffic speeds up to 65 mph and nobody has any idea why there was a traffic jam in the first place.
That’s not a cost of speeding, that’s a cost of not paying attention or not adjusting your speed to prevailing conditions.
-d
My immediate thought was right along with Cameron. In most cases I would much rather have 36 hours of my life at the expense of $270. Unless I deem the time traveling worthwhile, ie, with my bride. Other than that I can’t think of a time/reason I would rather have that $270 over 36 hours of my life.
On second thought I do listen to a lot of audio – books/music/conferences and that’s a pretty good way to ‘get paid’ to do something you enjoy (not at the expense of anything else though – you’re driving, you’ve got nothing else to do).
There is an underlying fallacy in sticking the speed as the best measure for economical driving. Long distance truckers will tell you that driving at contant optimal rpm is what provides the best mileage. These professionals should know as it is their daily bread and butter. An internal combustion engine, whether diesel or gasoline, gives optimal performance at a constant rpm, usually around 2000 rpm for petrol engines.
I love how this “save the environment” fad has turned people into “safe, considerate drivers.” Why did it take $4/gallon gas prices for all of you to finally learn that driving like an @sshole isn’t necessary. I still see way too many people risking their lives and others going 80+mph because they’re late for work. Every day. Leave a few minutes early, have a peaceful drive to work, and stop being an idiot, I say.
That being said, I don’t believe for a second that Penske trucks get 25mpg, you’re only fooling yourself, it’s physically impossible, you must have had very poor data. And hypermiling isn’t safe. Your idea of saving a few bucks by risking other people’s lives is selfish and is a great example of why brakelights and traffic even exists on the highway in the first place.
Madhu Murthy has an excellent point. I know that I will sometimes go a bit faster so that I can shift to a higher gear rather than run a lower gear at higher rpms. It’s way more efficient. My understanding is also that higher gears are generally better for gas mileage.
another way to look at it:
according to these numbers, you spend 28% more in fuel to save 23% in time (going 85 vs 65 – I didn’t do 60mph as highways are mostly 65 mph now).
Looking at it that way, I think I would rather spend 23% more time to save 28% in cost. Not only is it safer to drive slower, but you get a ‘free’ 5% savings thrown in there.
Of course, every car is different and some might be closer to equal savings (ie, 10% time savings = 10% cost increase) in which case, I would naturally drive faster….
Andrew,
That’s a really stupid way to look at it. What if gas were $0.01/gallon? Is that 28% still worth more than the 23% decrease in time?
-d
Dan,
It would be stupid if gas were indeed $0.01/gal. When it gets there we can revisit the topic, where I will then admit to my ‘stupidity’. Until then, lets keep things out of the hypothetical realm.
I just want to thank the writer she approached the topic wisely …