What's the most efficient speed to drive at?

This question (whether it’s more efficient to drive with the windows up and A/C on, or windows down and A/C off) http://www.straightdope.com/columns/read/1010/what-gets-the-better-gas-mileage

reminded me of a similar thing I’ve been wondering and can’t find an answer to:

What speed should you drive at to get the absolute maximum MPG? Too fast, and air resistance will drag your mileage down. Too slow, and parasitic losses in the engine, transmission, etc. will also reduce it. Where’s the sweet spot?

Of course it will vary from car to car, but I’d guess it’s somewhere around maybe 30 mph.

(Interestingly, at slow speeds, air drag is much lower, so you should probably opt for “windows down, no A/C”.)

In my car (2011 VW CC with turbocharger and DSG transmission) it seems to be in the neighborhood of 65 mph.

That’s surprising, given that air resistance increases with the square of speed.

That said, the highest actual mileage I’ve ever measured with my 2000 Saturn LW1 wagon (2.2l 4cyl) was at 65–70 mph (~34 mpg). Which doesn’t prove anything, because I’ve never measured the mileage at a sustained low speed. I’d no doubt get pulled over (or rammed) if I tried to go a steady 35 mph on the freeway.

What’s a DSG transmission?

Direct Shift Gearbox

It’s going to vary car to car, but it will generally be at typical highway speeds, somewhere around 60MPH because car engines are optimized for performance at those speeds.

I should have said ‘drive train’ there, not just ‘engine’. Transmissions are a factor there. With a standard transmission the lower speeds could be more efficient if you ever maintain a steady speed over distance in the sweet spot for a particular gear the way you would with highway driving.

According to the Mythbusters it’s around 45 MPH.

I did a google search for speed mpg graph. It looks like for most cars, the sweet spot is around 40 - 45 mph:


One of the reasons in mpg ratings that slower city driving is lower in most cars than highway isn’t the speed. It’s stop and go driving.

I remember back in the 1960s and 1970s that there was a lot of interest in hypermilage.

These people would take stock cars and then modify them (such as removing the suspension) to get the best mileage they could. If I remember, they usually drove somewhere between 15 to 20 mph on a closed course.

Wait: Wikipedia to the rescue: According to this article, a 2004 Chevy Impala’s most efficient speed is 42mph.

And, of course, there’s a webpage devoted to Hypermiling. So tell me, what’s the secret?

  1. Running the car without air conditioning or any of the electrical equipment in the car such as the radio.

Okay, Count me out.

I’d say the highest gear at the lowest rpm before the engine starts lugging. Well, maybe a bit more rpm but not much more.

AC/open windows? Tome and Ray discussed this on Car Talk and for once agreed the cutoff is 45 mph. Slower; windows down and AC off. Faster; roll the windows up and AC on. I guess the drag isn’t much of an issue below 45 but at higher speeds, the drag makes a difference.

Put me in the 40-45 MPH group.
Below that you can’t run in the highest gear.
Much above that aerodynamics becomes a factor.

(bolding mine)

Count me out, also!
I live in Houston, Texas and for a large part of the year the weather here isn’t very conducive to “Running the car without air conditioning…”, IMHO.
For yours truly, it’s ‘windows up and AC on’, gas mileage be damned! :wink:

Interesting chart: According to it, only one appears to get slightly better gas mileage at 45 compared to 40 which is the Mazda 3. Another appears to get the same gas mileage at 40 and 45 which is the Elantra.

Two cars got better mileage at 35 than 40, while another two got better mileage at 40 than 35. One showed no difference, while for some reason the Focus didn’t have any data below 45.

I suspect some of this varying on cars sweet spot is due to computerized fuel leaning built it to be optimized at certain speeds for the various car manufacturers.

Most I ever got out of a car was 69.4 mpg highway (driving about 52 mph), and 63 mpg city. It was NOT a hybrid. It was a 1984 Honda CRX. EPA sticker showed 67 highway and 51 city. It had the 5-speed manual, and only weighed 1,713 lbs. Sure miss that car. Many hypermiled these cars close to 100 mpg.

That would be my guess as well.

If the dash on my Prius (which has a CVT) can be trusted, I get near 100 mpg around 35-40 mph which appears to be the peak.

I figure that if I drive at the maximum legal speed, I will use less fuel. It stands to reason - less time driving = less fuel used.

Wouldn’t that be nice. Too bad it’s bunk.
Powers &8^]

It depends on a lot of factors, but in general the sweet spot tends to be between 40 - 45 mph.

I seem to remember back in the 1970’s there were government studies that showed that cta.ornl.gov/data/tedb32/Spreadsheets/Table4_28.xls‎ (a newer study that included some older data). It was during the 1970’s “fuel crisis” and cars were much bigger and about as aerodynamic as a brick and the government was trying to force people to save fuel, but even the government felt the public wouldn’t drive that slow on the highways so they went with the 55 mph speed limit.

Not just “one of”, but THE reason. Acceleration (if more than gentle) is a bit less efficient than maintaining speed, but braking is throwing energy away: turning kinetic energy into heat, which is dissipated. (Exception: regen braking like a Prius.)

If you want to get great mileage, avoiding using those nasty pedals!

Bingo. There’s a bit more to it than just driving in the lowest gear without lugging, but that’s a big factor.

I’m confident you’re right. I worked in engine controls for Ford, at their SciLab in Dearborn, back in late 70’s early 80’s. The algorithms for calculating things like fuel and spark advance were called “strategies”, and even way back then (even on 8-bit processors) involved a lot of factors, and a lot of compromises between performance, emissions, and economy. (One of the trickiest parts back then was modeling the fuel layer thickness in the intake manifold.) Once the strategy was established, there were still lots of tables of “calibration” data that were massaged by technicians to suit a given car/engine combination.

I wouldn’t be surprised to find that the ideal gas mileage speed might depend on which technician did the cal for your particular vehicle. (Before cal, cars were barely driveable, despite all the science and math that goes into calculating the tables in the first place.)

No doubt things have come a long way since then, and I wonder if there are many central fuel injection models (which would still have an intake manifold). But I bet they still have technicians who calibrate the tables, and even if not, the optimum speed would depend rather strongly on the control strategy. Of course, it would also depend on transmission shift points and gear ratios, and aerodynamics.

BTW, even back then, the research showed that at highway speeds (55), most cars were more efficient with AC on and windows up. That was before they even got rid of the drip rails, but after they’d curved them inwards to reduce drag, IIRC. I’d expect the benefit to be bigger today, with sleeker cars.

The most efficient speed to drive at is 40 km/hr if you are driving you’re a car then the maximum speed should be 50-60km/hr

The most efficient speed to drive at is 40 km/hr if you are driving you’re a car then the maximum speed should be 50-60km/hr.