I remember reading some time back that researchers had determined that 55 m.p.h is the most economical speed to drive one’s car, coincidentaly the legal speed limit on most roads. I remember thinking at the time that they would hardly tell us if the most economical speed were 83 m.p.h, and that fuel economy must differ depending on the type/weight of the car, and even enviornmental conditions (such as if you’re driving into a hard wind.)
Is there a clear standard to this? What actually is the most economic speed to drive, if such a thing can be determined?
My mother claims that you save gas if you drive at a steady speed, such as on cruise-control without a lot of stops and starts, but as far as I can tell, there’s little difference. She also claims that running the air conditioner makes you use up fuel faster, and you should roll down your windows instead. I replied that the drag created by having the windows down seemed even more inefficient. Any truth to either of these claims?
Here’s Cecil’s column on your second question (window open vs. AC on).
As for the first question, it would have to depend a lot on the car. The aerodynamics of a car plays a major part, as well as the engine size and the gear ratio for the top gear. One rule of thumb I heard is that the best fuel economy is at the low end of the top gear, but I have no idea if that’s accurate at all.
and numerous other things. So without going deep into the details, the short answer is no, no reputable researchers determined that there is one ideal speed for all, or even most cars.
That having been said - in general, the most economic speed one can drive at is as scr4 guesses - the lowest speed one can go without lugging the engine in the highest gear is typically the one that will give you the best fuel economy at (mostly) decent speeds.
isn’t this why they have “city/hwy” mpg ratings? driving cross-country i always get a higher mpg than driving around the city. and i’m pretty sure (wouldn’t bet the house on it or anything) that my best mpg is around 70 mph, with my tachometer hovering around 3000rpm (don’t know where that lies in the “low end of the top gear” spectrum). besides, i thought that dumb 55 thing was invented for the gas shortage way back in the 70’s. i would think it’s totally obsolete now.
Well, I would hazard that you would likely get better mileage at a lower speed than 70 mph at 3000 rpm, knowing in general what I know about cars. But like I said - it all depends on many things. The main difference between 70 mph and 55 mph is that the wind resistance of the car increases roughly as a square of the velocity increase. To simplify - if it takes 8 hp for a car to overcome the wind resistance at 30 mph, it takes about 32 hp to overcome the wind resistance at 60 mph ((60/30)[sup]2[/sup] = 4, times 8 hp = 32 hp). And about 128 hp to overcome the wind resistance at 120 mph. And all that extra power needs extra gas to generate it. But this is a rough rule of thumb, not to be taken exactly. For example -
My model of FIAT X1/9 had a detailed road test done in England, where they did long-range steady-state driving at constant speeds in 5th gear to see the difference in MPG. The absolute best values were realized at 40 mph, and decreased steadily up to 100 mph (the highest they tested). In fact, at 40 mph IIRC they were getting 62 MPG, whereas at 55 mph they got 41 MPG, and at 70 mph they got 35 mpg. And an amazing 21 mpg at 100!
Why are so many people worrying about fuel economy? Let’s assume you do the majority of your driving to commute for work. If you drive in the city only (i.e., where there are “city roads”), there’s not a whole lot you can do to get better economy, and you have to live with it. Or buy a smaller car. Of course, if you already have a paid-off-car, there’s no sense in dropping $13,000 to save a couple of nickels on gasoline.
If you have to commute more than 15 miles or so to work (still on surface roads), then obviously the hassle of that commute makes the job well worth it. In that case, you make more than enough to fill your tank without having to worry about a couple of bucks. If this isn’t the case, work at the McDonald’s next store instead of the McDonald’s in the next city.
If you have to travel extensively on the freeway to commute, then you have it good - you get worse rolling mileage, but no wasted fuel stopping and going (that acceleration really hurts you). Plus, as above, the drive must be worth it and you can afford the gas anyway. Otherwise, ibid the McD’s thing.
Don’t get me wrong, if I could get a significant improvement of my mileage, I would worry just a little bit more. But it’s silly to change preferred driving habits for the sake of a few nickels. So, if you have a Ford Excursion, go ahead and trade it in for a subcompact Ford Fiesta.
Also, consider the value of your time versus the cost of fuel. You may save a few cents driving at 55 instead of 75, but a one-hour commute at 55 saves you 40 minutes per day at 75.
Because I’m a bit of an environmentalist, and I’d like to conserve fuel wherever possible.
I have a small car . . . a Ford Focus, to be exact (which is paid for.) I’m not trying to save a few nickles, I just want to use as little gas as possible, and try to cut down on my contributions to the hole in the ozone layer.
My next car is going to be one of those gas-electric hybrids. I’m planning to buy that in the next few years.
My work is about five miles from my home. Most of it is highway driving.
As I said, it’s not the money . . . jut trying to conserve wherever possible.
Balthizar, Lissa, you both make some good points. While it is true that on a local, single-case scale the difference in fuel savings is very small, when one multiplies that savings by hundreds of millions of miles driven by the entire population, the savings will be quite significant.
I do agree that sometimes people stress too much over it tho. I am always floored when each year I hear the News Droids doing their same old schtick about “gas prices went up by 1 cent a gallon this Memorial Day, and this could put a damper on holiday travel…” Give me a break. Anyone who cancels the trip to see grandma before she passes on because gas is even 20 cents more a gallon has much more serious problems then worrying about a vacation. You could drive 2000 miles in a 20 mpg car, and the differential gasoline cost would only be (2000/20) * 0.2 = $20.00 I can see it now - “Well kids, DisneyWorld is cancelled - your father can’t come up with the extra $20.00…” :rolleyes:
One nit-picky note - auto exhaust is not deemed to be a primary culprit of ozone depletion, or in increasing the size of any holes in the ozone layer.