What kind of driving offers the best fuel economy per time?

Suppose you go for a drive but your goal isn’t to make it to a specific destination but rather to kill a certain amount of time. What kind of roads should you take to use the least amount of fuel in that time?

I’m thinking urban driving is clearly better than highway driving. Just using estimates of my car’s typical fuel economy, it gets around 25 mpg on the highway going around 70 MPH which comes out to 2.8 gallons per hour. With urban driving, it’s closer to 20 mpg which at an average of 30 MPH comes out to 1.5 gallons per hour, almost twice as economic. However, these are just estimates and don’t go into much detail.

Has anyone done some more serious calculations on this? I’m curious about not just highway versus urban, but highway with sparse traffic versus highway with a lot of speed changes versus major city streets with occasional red lights versus small streets with frequent stop signs. Has anyone come up with tips for increasing fuel economy with respect to time? I’m sure a lot of it is the same as with respect to distance, like don’t punch the throttle from a stop, but some is bound to be different, like maybe stop and go is better since you’re burning time but not gas while stopped.

Think of it in terms of the various factors that expend energy:

  • You want to drive relatively slow as aerodynamic drag increases with speed
  • You want to drive at consistent speeds as braking will expend your kinetic energy and require additional power for acceleration
  • You want to minimize idle time, so avoid stop lights and stop signs (which also increase braking losses)

So you basically want roads that provide very consistent but relatively low speeds.

Today’s NASCAR winner was almost out of fuel. He conserved fuel by depressing the clutch and giving no gas going into the turns. Interestingly, he was not losing much track position by using this technique, although the followers were saving fuel too.

If you don’t care about ending up at a specific destination, I’d say just sit in your car at home with the engine off.

Next best would be to roll downhill somewhere. Even with the engine on, you’ll get better mileage than idling.

Next best is to get stuck in traffic or some such, idling.

Finally, if you actually want to maintain some minimum average speed, and end up in the same place you started, I would say go as slow as is reasonable, and maintain that constant speed. Avoid hills if possible. I found that my car, which normally gets about 32 mpg hwy, can go up to about 45 mpg at 35 mph (in 6th gear) if I don’t have to stop. Per-hour, this is nearly triple what I get on the highway.

My car, a Subaru Outback, gets its best gas mileage at about 50 mph. I seem to recall the Car Talk guys saying that this was typical.

As Dr. Strangelove suggests, a parking lot is better than any road - the car whose engine is off will win this competition.

If that’s not allowed, choose a deserted road where you can chug along in low gear at idle rpm, keeping your foot off the gas pedal.

Like others said, you want to maximise idle time.

I suggest the nearest major freeway at rush hour.

This sounds just odd enough for Mythbusters to take a crack at.

If I’m understanding correctly, the idea is to have the engine running and the car moving, while using the least possible amount of fuel in an hour. My suspicion would be to have the car running at idle (eg: foot off the gas) and in drive. Almost any car with an automatic transmission will move forward - find an empty parking lot and have fun.

If you want to actually get somewhere and be efficient at it - find a road with a 45 or 50 MPH limit and set the cruise control. As mentioned above, somewhere around that speed is the “sweet spot” for most cars.

This is going off in a whole other direction than I intended. No, sitting in your car with the engine off isn’t an option. If you’re going to do that, why even leave the house?

Let me set up a scenario. Say I’m geocaching to kill time. I have a certain amount of time before I want to be back home. I’d like to get several of them, but I don’t care to burn extra gas racing to get as many as possible. At the same time, I’m not going to creep at 2 MPH either. I’m going to do roughly the speed limit on whatever road I’m on. And be realistic, there aren’t roads with 10 MPH speed limits going all over the place.

So around here, I can see four common scenarios. One is travelling on a freeway with sparse traffic doing about 60 to 65 MPH. I’ve already explained why I don’t think that’s ideal. Two is travelling on a freeway with dense traffic going slowly. I’m not going to include traffic that’s completely stopped as I might as well stay home if that’s the case. It is frequently speeding up and slowing down though so a lot of time is spent accelerating. Three is travelling on a major street where the speed limit is 35 or 40 with occasional red lights. Four is travelling on minor streets doing 20 to 30 with frequent stop signs. If you can think of another common scenario, throw that in there too.

And remember, we’re talking about fuel consumed per hour, not per mile.


Well, the thing is that contrary to the usual forum post, you have a clearly stated question with a straightforward answer… you just don’t like it :). Not trying to be insulting here–but that’s the reality. But let’s go through your scenarios one by one.

This is just about the worst you could do. Even though cars have a peak mpg at some non-zero value (probably in the 30-50 range), the peak hpg value is at 0 mph, and goes down from there as speeds increase. Almost universally, cars consume fuel at a higher rate the faster they go.

Possibly the best of your proposals. The slower the better, and the more constant the speed the better. Might not be that great if it’s really stop-and-go, but you can often improve this by leaving a large gap in front of you.

This might be ok if the lights are long enough. You burn fuel accelerating again, but if the lights are long (like over a minute), you’ll probably make up the difference.

Not so good. You burn fuel accelerating, but you probably only pause for a couple of seconds at the stop signs, so you don’t get a chance to burn time at idle. Probably the second worst.

I’ll propose two other scenarios with better results:

  • A backcountry road with a low speed limit, like 35. Might be hard to find, especially since you want an area that’s flat.
  • A suburban development with those nasty, twisty, cul-de-sac dominated layouts, but no stop signs. These seem harder to find these days (people seem enamored with installing 4-way-stops everywhere), but they do still exist. Putter around at 25 mph.

If the goal is lowest fuel-per-unit time, then you just want the lowest engine power setting, since this consumes the least amount of fuel per unit time. That would be idle; it’s not possible to run the engine with any less fuel moving through it. Put the car in drive, and leave your foot off of the accelerator pedal. Choose the roads with the lowest speed limits so that you’re less likely to get rear-ended.

Going back to your OP question:

You should choose the slowest roads/speeds possible that will allow you to reach your destination in that “certain amount of time.” If you’re trying to kill 24 minutes, and you have a choice between covering:

A) 20 miles at 50 MPH or
B) 10 miles at 25 MPH,

you should choose B. Same amount of time, lower engine power requirement, therefore less total fuel burned over the course of the 24-minute drive.

Well my question included the statement that you’re “going for a drive”. It’s hard to count sitting in a parked car and staring at the driveway as that.

Yeah, that’s what I was thinking.

Thing is I’ve never been in slow traffic here and it not be really stop-and-go (or slow-down-and-speed-up at least). And if you allow a gap, someone’s going to fill that gap guaranteed. So with all the acceleration and deceleration, I was thinking this alternative wouldn’t be so great.

Most of the lights will probably be green. You’d probably only have to stop every mile or so.

But on the other hand, you could accelerate more slowly than the other scenarios since there aren’t likely cars behind you and wouldn’t have to accelerate to as high of a speed. Surely that would make it at least comparable to some of the others.

That sounds like it may be the best. Unfortunately those are rare in this area, but back home they’re abundant. Most of them are dead ends though.

I could certainly see those being ideal, but like you said, they’re hard to find. I do know of some that use roundabouts so they don’t require a complete stop except in the rare case that there’s another car.

It has to be reasonable. I mean if you’re trying to get away from home and go for a scenic drive, are you going to just put it in drive and idle everywhere at 5 MPH?

And obviously slower is better, but constant speeds are too and slower roads usually have more stops on them.

You’ll want to avoid slowing down and stopping, because you lose momentum, and having to get back up to speed uses more fuel.

Also, you’ll want to cruise at the lowest possible speed that allows you to stay in the highest possible gear, which keeps engine revolutions at a minimum.

Ordinarily, that would call for a nice, straight, limited-access highway, but you could probably keep going even slower than the 40 mph minimum typical of an Interstate. In that case I’d recommend something nearly deserted like U.S. 41 through the Florida Everglades or maybe Cal. S.R. 190 through Death Valley.

Actually, a fuel-efficient way to cover distance is to gently accelerate to a decent speed (say, 50mph) then shut off the engine and coast to a stop.

It is certainly true that braking to a stop and strong acceleration are inefficient.

Actually engines (particularly spark-ignited gasoline engines) tend to be more efficient when operating at a large percentage of their full load, rather than at a small percentage. Example, the Nissan Altima, available with a 2.5-liter 4-cyl engine or a 3.5-liter 6-cyl engine. With the four-banger, MPG is 23/32; with the V6, MPG is 20/27. Why? Cuz the V6 engine is underloaded.

Upshot of this is (in terms of engine efficiency) you’re better off accelerating briskly, putting a substantial load on the engine to achieve higher efficiency.

Note that

The optimum is sitting still. If that doesn’t count as “driving”, then the optimum is whatever gets closest to sitting still while still meeting whatever your minimum standard of driving is. If you want to say that the slowest speed that “really counts” is 15 MPH, then the optimum is 15 MPH. If you want to say that it has to be at least 25, then that’s your answer. It’s that simple.

No, it’s not that simple because in the real world, there aren’t roads where you can go 15 MPH nonstop. In fact, the slower the road, the more stops it tends to have. I think you would agree, acceleration uses a lot of fuel. So with regards to common real-world roads, it’s a matter of whether you consume the most fuel on low-speed roads with stop signs every block or two, medium-speed roads with occasional stops, or high-speed roads with constant speeds. This isn’t a theoretical scenario where you can say that there are 5 MPH roads going all over the place with no stops.

Very possible. But there are clearly other possible factors (e.g. the V6 engine weighs more).

There will indeed be some load at which the engine is most efficient; beyond that, efficiency drops. You’re saying “brisk” acceleration is good, I’m saying “strong” is a bit too much. We probably need actual data from a real car to decide what’s optimal.