I experimented with my car (1989 Honda Accord SE-i) and when I spent a week driving 75 on the interstate I got an average of 24.9 mpg, when I moved down to 55 for the next week I got an average of 25.5 mpg. So yeah, for me there was a difference but I consider it to be small enough that I now just drive 75. Especially because driving 55 when everyone else was doing 80 was just plain scary.
Wow, great discussion here. Now if only they would teach this in drivers ed to the rest of the millions of drivers who will never see this web site. The great advantage of really knowing your personal vehicle. What it can and cannot do, it’s own perameters and how to get the most out of it. most drivers don’t know the machine they just turn the key and go. If they would teach car drivers like the teach us airplane pilots there would automatically be less accidents, less waste, less road rage, less need for the baby-sitting safety equipment overkill. It’s also nice to see here the acceptance of this open discussion among strangers.
Understood. But the OP is only asking about fuel economy.
You are correct, driving slower would certainly reduce the average fuel consumption.
You’d probably see some amount of improvement in gas mileage, but while a beautiful idea in concept, the execution would prove impossible.
I’ve said before that the one and only way to get everyone to drive no faster than a certain speed is to include speed limiters on every single vehicle on the road. As long as the capability exists to exceed the speed limit, it will always happen. It’s human nature to want to go faster than the guy next to you.
I should also add that, if you insist on driving 55 while most everyone else is doing 80, you are creating a dangerous situation. It could be argued that it’s all the other folks flying along that are creating the dangerous situation, but there’s a whole lot more of them than you.
Some data points:
Consumer reports:
4cyl 2005 Camry:
55mph: 40mpg
65mph: 35mpg
75mph: 30mpg
Mercury Mountaineer:
55mph: 24mpg
65mph: 21mpg
75mph: 18mpg
Wayne Gerdes (yes, the (in)famous hypermiler)(See www.cleanmpg.com)
Toyota Prius
50mph: 67.6mpg
60mph: 60.7 mpg
Ford Escape Hybrid
30mph: 59 mpg
40 mph: 56 mpg
50 mph: 51.4 mpg
60mph: 42.3 mpg
Scrabbler
I have no idea how much better mileage you will get, all I know is I get 20% more mileage by locking cruise on 55 and driving the 21 miles to work 5 nights a week. I work graveyard and traffic is less than when I leave work for home every morning. I’m just trying to get my monies worth for my fuel buck these days. I will say, in Colorado, my route, nobody, and I mean nobody stays behind me. JMHO
The difference is significant.
The problem is there is a significant safety issue with cars driving 55 and others going 80 on the same road.
I like to drive 55, and on long trips I try to chose routes that will allow me to do it without aggravating those drivers who want to drive faster.
Personally I would like to see the max limit reduced to 60 and see that it is really enforced. However I know that is not likely to happen any time soon. Maybe there should be a type A personality test as part of getting a drivers license so we can weed them out. 
Hee, hee, Ken, you are funny. That is, assuming you are joking. Cars have complicated profiles,and efficiency does vary dramatically over the rpm range v. speed.
My Sienna gets 24 mpg at 70 mph. At 65, it goes up to 26 mgp. At 50 mph in a place in Mexico with slow speed limit, I cannot see the gauge moving at all, so I am assuming it must approach 30 mpg. I believe the enclosed tables totally.
By the way, some trucking companies are requiring their drivers to slow down to save gas. It was on web news not long ago.
Think about how much you will save by driving 55 mph instead of 75 and how important it is to you. Plan a fairly long trip to visit a relative or take a vacation. Calculate how much you will spend for gas if you drive 55 mph and how much more it will cost to drive 75. You will see two things:
(1) The extra cost of driving fast is much smaller for an economical car than it is for a gas hog. The extra cost is much smaller for a Honda Civic than for a big van, SUV or pickup.
(2) The extra cost of driving 75 instead of 55 is substantially less than the cost of the trip at 55. If the cost difference between 75 and 55 is a burden, how can you afford to go at all?
My wife and I just returned from a vacation at a place several hundred miles from home. We drove most of the distance at or above the 75 mph speed limit. Even at today’s prices, gas amounted to only 6% of the cost of our vacation. It wasn’t the cost of gas that determined whether or not we could afford to go.
If you really want to make time on a long trip, never stopping will do a lot more to increase your average speed than going 75 or 80 where ever you can get away with it. Don’t drink coffee or anything else that makes you have to stop to pee before you need to refill your tank. Pay for your gas at the pump and have the kids do their business in the restrooms while you are pumping.
Pick routes that don’t go through large towns with extended low speed zones and red lights.
If you go 10 mph for one mile and then go 30 mph for the second mile, your average speed for that two mile trip is not 20 mph, but only 15 mph. If you want the average speed of that two mile trip to be 20 mph, you could go 10 mph for the first mile, and then go the speed of light for the second mile, or, you could do the first mile at 15 mph and then go 30 mph for the second mile.
Agreed. It’s not how fast you go, it’s how slowly you don’t go. My typical procedure is
(1) drive 3-4 mph above the speed limit, which is as fast as I think I can go without risk of speeding tickets, and
(2) combine food and toilet stops with refueling at intervals of 300 to 350 miles.
On interstates with a 75 mph speed limit, this lets me average at least 70 mph.
That isn’t the point I wanted to make. Suppose a trip costs $100 at 75 mph. Using Consumer Reports’ 33% lower consumption at 55 mph, you can divide the $100 into $67 for driving at 55 and $33 for going faster. If the $33 is important to you, the $67 is twice as important. If driving 75 instead of 55 forces you into poverty, driving 55 instead of staying home will force you into bankruptcy. When you include all costs associated with your trip (lodging, meals, activities, etc.), the difference between 55 and 75 becomes minimal.
As you can see, the savings depends on the car. I did a few quick calculations recently. One was that driving my Toyota Avalon at 60 rather than 75 would earn me $6 and hour for my extra driving time. The other was that if the national speed limit were lowered to 55, the lost time would be equivalent to killing about 3000 people per year.
I?ve started driving frugally and now go 25% further on a tank of gas than my husband in a nearly identical car. Here’s how I do it: http://www.ehow.com/how_2309496_save-gas-realistically.html.
I really don’t know where you people live at. When I am on the highway and speed limit is 70mph. I see cars driving crazy, 85mph is crazy. Cars travel here at between 55 and 75mph. People drving faster, and I have no idea why they do, cause more problems then slow drivers. Is this a control issue to rule the highways, the other thing is not wearing seat belts and getting a $65.00 ticket for that. We drive around 60 mph and in the slow lane. I watch all the faster driver cutting in and out of traffic and down the road usally are stopped by the police for driving crazy. If you feel you need to drive fast and be a control freak, just keep your check book out because I do call the police and report you. If you can’t figure out you get better gas mileage driving slower, you really need to give up your drivers license and get a bike. I read these blogs and think there are just to many stupid people driving in this world.
On a 100 mile trip, if you drive 75mph, it will take you one hour and twenty minutes to travel the distance. Slow down to 60mph and the same trip will take one hour and thirty minutes, 10 minutes more…With my car, I would burn about 1.2 gallons less going 60 mph over 100 miles. If EVERYONE slows down to 60, that’s lots and lots of gasoline we don’t have to import…
An interesting sidebar. If you drive in the left lane at ANY speed, SOMEONE will try to pass you no matter how fast you are going…
That’s why you should always drive in the travel lane (center) or right-hand lane. Left is for passing and crazy drivers only:). I stay out of that lane as much as possible and try to maintain 60mph where fuel economy is maximized. I think the States overall have done a good job of posting appropriate speed limits where needed. The problem is enforcement of the limits…perhaps a spy satellite from space that photographs your license plate and sends you a nice fat speeding ticket for every violation…people would soon learn (like Pavlov’s dogs) to reduce their speed accordingly.
I really don’t know where you people live at. When I am on the highway and speed limit is 70mph.
I just drove across west TX on I-10/I-20 where the posted “limit” is 80 mph and most people seemed to be driving within the limit. The point is that if the speed limits were increased to reasonable values they would be ignored less. Posting a 60 mph limit on an interstate will be about as effective as posting a 25 mph limit, it will be completely ignored.
New math?
You’re correct, his example shows that driving 60 mph instead of 75 mph for 100 miles will cost and extra 20 minutes of your life. If the fuel savings is 1.2 gallons (assume $5.00), I can pay myself $15/hour to sit in a car by diving more slowly. I don’t think that’s going to happen unless the cost of fuel triples, there simply is not enough incentive at these prices.
At $5/gal, 1.2g and 20 minutes works out closer to $18/hr by my calculations. I do not believe that any of my regular, daily driver type vehicles have that high of a difference in level of consumption between those two travel speeds. Regardless, adding 20 minutes to my commute time isn’t worth it to me either even if it were to cost 2x that amount.