Highway speed to optimize gas mileage


#1

If the biggest waster of gas is going 80, than what speed should I go if the speed limit is 80?


#2

And just how many places do you drive that 80 MPH speed limits ? Most limited access roads ( Turnpikes ) have a posted minimum speed also.
The simple answer is drive at what you feel comfortable with while getting decent fuel usage.


#3

South Dakota and other interstates are now 80 mph limit. But drive at the speed you want. I have noticed little difference in mpg based on speed alone, with today’s high gear ratios and 8 speed transmissions, you can cruise along at a little over 2000 rpm compared to 2500 to 3000 some years ago. I never put much faith in Nixon’s 55 mph thing back then but seems to have taken hold in people’s minds.


#4

I get 40mpg driving at 60mph on the highway. 99 Toyota Corolla automatic


#5

My opinion? 85 mph if you are in the middle or left lanes. Maybe 60 mph in the right lane only, ever.

If you aren’t scared to death doing 60 on an 80 mph highway, get OFF the highway or speed UP.

Lower speed will give better mileage IF you think its worth your time to take longer to get where you are going.


#6

The biggest hazard on superhighways is related to the disparity in speeds between the fastest and the slowest drivers. While you could undoubtedly get better gas mileage by driving 20 mph slower than that 80 mph speed limit, you would be subjecting yourself and your passengers (as well as drivers of other vehicles) to a GREATLY increased risk of injury and death by driving that slowly.

My advice is…
You can’t afford to save money by driving drastically slower than the prevailing speed of traffic on any road.
:thinking:


#7

As long as there is no minimum speed limit posted, I’d set the cruise control on 60 or 65 if fuel economy was my main goal. However, I think 70 mph would be safer as there would be less of a difference in speed between you and the other cars.


#8

Just as a warning, when South Dakota went to 80, they warned that that was an absolute and don’t go beyond that. They said the same thing on the two lane roads when the limit went to 65. You can drive at the limit but don’t expect the normal 5-7 mph leeway. 80 is plenty fast though and you have to pay good attention to the road at that speed. But it does make a difference cross country across South Dakota.


#9

Not going to SD anytime soon but the info is appreciated!

Florida is 70 mph and some actually go 60 but most hammer 79 in the left lane. Troopers don’t even blink if you pass by at 78 mpg.


#10

Eighty. Or with the flow of traffic. Don’t hold up traffic. It can be dangerous.


#11

I have not noticed any significant mileage drop going 78 vs 70.


#12

There is no question that lower speeds increase MPG. The best speed is the lowest you can go in your top gear without lugging your engine. Which is usually about 30 MPH.

I know many will disagree with me, but what measurements I have made bear this out. I normally get 28-30 MPG on the highway, but on occasion I have driven that same route at 40 MPH in heavy rain/snow, and MPG went well over 40 MPG. However, I do not advocate driving at that speed on a 65 MPH highway, it’s unsafe.

Wind resistance goes up with the square of speed, so that is a major factor at higher speeds.


#13

Slightly less than whatever the prevailing speed of traffic is, keeping in the right lane of course. If you’re by yourself with no other visible traffic around, 65 MPH-70 MPH. Fast enough so that a person traveling at 80-85 MPH won’t run up on you suddenly. Basically fast enough so that the closure-rate isn’t going to be unsafe.


#14

Limited access highway - maximum speed 80 MPH - many over the road carrier semi’s governed at 62 mph - therefore a person in a regular vehicle should be able to drive at 65 or 70 without catastrophic event.


#15

The power needed to overcome wind resistance goes as velocity to the third power. So you’d be using about 3 times as much engine power to overcome wind resistance at 80 mph compared to 55 mph.


#16

Who’s to argue with math an engineers but suppose it depends on the wind direction, shape of the car, hilly or level terrain and so on but next time I’m on the highway I’ll see what the computer says at 70 and 80. Seriously though, even in the VW I was never too concerned with saving a few pennies and just drove normally.


#17

I find that driving at 65 I get better gas mileage than at 75.


#18

The power used to overcome wind resistance goes as velocity to the third power, but the energy needed to cover a mile goes as velocity to the second power.

Generally, the slower you cruise, the better your gas mileage will be, but there is a speed where you realize diminishing returns and if you go slow enough, negative returns. This diminishing returns speed is determined by how large your engine is and the reduced thermodynamic efficiency of your engine as it’s throttled down to a low power output partially offsets the savings realized because of the lower energy needed.

My Kawasaki Ninja 300 motorcycle will get around 70 mpg at suburban and country road speeds, 45-55 mph, but if I get on the highway and open it up to 70 mph, I pay dearly getting only mid 50’s mpg.
I came back from Wichita Falls TX to Austin TX with a Toyota Yaris and those 75 mph speed limit highways caused my gas mileage to plummet to around 35 mpg. Normally I get 42-44 mpg on my 60-65 mph commute to work.

Because electric motors have a nearly flat efficiency curve, they can put out 5 horsepower nearly as efficiently as they can put out 100 horsepower, going fast with an electric car dramatically reduces range. Or maybe it’s better to say that going slow dramatically increases range. I can’t go slow enough to make my Yaris get 80 mpg.


#19

I’ll second this. It wouldn’t be fair for you to be a hazard just to save a little money.


#20

Well, in an era when cars had the general aerodynamic properties of a brick, slowing them down would be a pretty good boost to gas mileage. Combine those aerodynamics with a 2 or 3 speed transmission that would be screaming along at 4,000 rpm at 80 and… Yeah, slowing down was a good idea.

It’s not nearly as necessary anymore because cars are more aerodynamic and we’ve discovered how neat this thing called overdrive is. :wink: