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To Get Better Gas Mileage, How Much Should You Cut Your Speed?

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2008 Toyota Prius

2008 Toyota Prius

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It’s fair bet that by now, we all know gas mileage depends on many factors, including how well-maintained a car is, the pressure of the tires, how full the car is, and the road and weather conditions at the time. 

How fast you drive also affects your gas mileage, with the energy required to push the car along growing exponentially the faster you go. 

On paper, that particular rule represents itself as a neat equation, but what does it mean in real life?  How much can you improve gas mileage by slowing down a little, and how fast should you drive?

To find out, we made a 500-mile round trip in a 2008 Toyota Prius, driving the outward leg at 70 mph, and the return leg at 60 mph to see just how different our gas mileage would be.

70 mph = 45 mpg.

With a full tank of gas, we headed out on the outward run, driving where possible with cruise control engaged at 70 mph. 

Although we stopped twice en-route, the car was not left long enough for the engine to cool down, while our start and stop locations were within 50 feet of each other in terms of elevation.

60 mph = 51 mpg

On the return trip, with similar traffic patterns, we kept our cruise control set to 60 mph. 

The drop in speed may have resulted in the return trip taking some 30 minutes more, but it resulted in a gas mileage of 51 mpg, far higher than our outward journey. 

Big savings

2008 Toyota Prius

2008 Toyota Prius

Enlarge Photo

Calculate the gas mileage increase over our Prius’ entire 11.9 gallon tank, and you’d be looking at a total range of 607 miles per tank.

That’s nearly 114 miles more range per tank than the official EPA estimate for that particular car. 

Put it another way, that’s the equivalent of getting $9 of extra fuel per fill.

The perfect speed?

On the roads we travelled on, 60 mph allowed us to keep up with the flow of traffic in the slow lane, ensuring we didn’t hinder the progress of 18-wheelers and other slower-moving traffic. 

But it also meant that we were traveling at a speed slow enough to maximize the gas mileage of the car we were driving. 

For most cars, that’s somewhere between 45 mph and 65 mph. 

What speed to you drive at on long-distance trips, and why? 

And have you saved gas by doing so? 

Let us know your experiences in the Comments below.

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Comments (11)
  1. Probably some reasonable data there.

    However, I am not sure any cars gets maximum MPG at 65 MPH. The data I have seen suggests it may be more common at 35 MPH with more aerodynamic hybrids at 45 MPH.

    Of course, there is probably no practical way to drive long distances at 35 MPH.

    Also have to be careful about elevation changes in these types of testing. I am been amazed at the MPG difference between my commute to-work, and my commute home.
     
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  2. I tried 50 mph with my 2010 Prius. The best result is 70.5 mpg after 46 miles, including 9 miles local. The temperature was 66F. About the same altitude starting and ending points. No wind.
     
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  3. Ecomodder.com has a decent collection of speed vs mpg charts from users who have actually gone out and tested their vehicles. Johm is right that the optimal speed is usually around 35-45 mph. In general, it seems to be the lowest rpm you can drive in the top gear you can drive in.

    http://ecomodder.com/forum/showthread.php/speed-vs-mpg-charts-post-em-if-you-15182.html
     
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  4. Tim,
    Thanks for the data. That is awesome.

    Some of the data only goes down to 45 mph which is the maximum MPG for the vehicles. That suggests that there was no practical way to test (or drive) long distances at slower speed.

    John C. Briggs
     
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  5. ABSOUTELY NO.

    I will NEVER lower speed for MPG. Time is just as valuable if NOT more valuable.

    Slowing down for MPG is silly. Why NOT just get a more efficient car?

    For example, at 70mph, Prius get 45mpg. Well, at that speed, Volt would easily do 3.5 miles per KWh. That is about 119 MPGe (34KWh per gallon). More efficient!.

    For cost, 3.5mi/kwh is about $0.051 per miles @ $0.18/KWh (most electricity rate are lower than that). 45mpg @ $3.50 per gallon is $0.077 per mile.
     
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  6. I have a 1994 Hyundai Excel 5 speed that gets 45MPG highway by driving with a light foot, and staying at 60MPH or less, when I use it! Most of my trips are with my electric recumbent bike - 6,000 miles/year, car - ~3,500 miles/year.
     
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  7. I forgot to mention, WHAT EVER HAPPENED TO THE 55MPH NATIONAL SPEED LIMIT??? We use 19,000,000 BPD of oil. Slowing to 55mph will cut that down significantly!
     
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  8. Seven or 8 years ago a friend and I went skiing in Ruidoso, NM from Austin, TX and got about 45-46mpg at 75mph average speeds on the way there. There was a 6600 foot elevation gain on the trip. On the way back we set the cruise at 70 for the whole trip and got 54 mpg. It wasn't easy going that slow for 630 miles. There is so much desolation that you want to go faster just to get through it. The speed limits have been raised since then to 80 mph on I-10 and 90 mph is allowed by the DPS state troopers. In recent trip in a gas turbo 4cyl car I travelled 90 and got 30 mpg so this is a rare instance where I went fast just to be done with the trip. The roads are good and fast out there. Nothing to do but cook with gas. Not too green.
     
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  9. The original trip at 45-46 mpg was ina 99 Jetta TDI. The 30 mpg trip was ina 1.8T powered Focus.
     
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  10. I know it is not comparable to ICE and hybrids, but here is some efficiency data for Tesla Roadsters and Model S.

    Best efficiency is at 17-22 MPH. Ouch. Let's just say, slower is more efficient.
    http://www.teslamotors.com/blog/model-s-efficiency-and-range
     
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  11. Great article on how to save by reducing your speed. I spend a lot of time wondering how to get fleets more engaged with "driving green and saving green". This article was a great testimony to the benefits
     
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