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Speed vs Fuel Efficiency
Old 10-02-2014, 09:13 AM   #1
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Speed vs Fuel Efficiency

Anybody here trying to optimize fuel efficiency on long distance driving trips? I've recently done a few trips with cruise control and got the following approximate MPG on my honda element:

25mpg @70-75mph
30mpg @60-65mph

Although I knew driving faster reduced efficiency, I was surprised at how much a difference it made (~20%). This works out to about $25 per 1000 miles. However to get the better efficiency I have to put up with my wife saying I drive like an old man.

Interestingly the following site suggests that the decrease in fuel efficiency is not monotonic at highway speeds for all cars. E.g. the civic has lower efficiency at 60mph than at 70mph.

Fuel Efficiency & Speeding: The True Cost | Automatic Blog
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Old 10-02-2014, 10:17 AM   #2
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While I do understand the science behind it, I just can't stand to drive slow. I will take the slight hit in cost for the time and aggravation savings.

Most of my vehicles are lower mpg to start with, so my effect is less than 20%. My big V8's are still operating at fairly low throttle to go fast vs go slow.

On the small engines, it is a more significant effect since you are using a lot more of the total power available.

Also, peak engine efficiency (at full throttle) is at the rpm where peak torque occurs, however this rpm and power is a lot higher than required to maintain highway speed. So lower rpm with high OD gearing and less throttle provides better mpg.
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Old 10-02-2014, 10:24 AM   #3
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DOn't do a lot of highway driving. What is known is that past about 55 to 57 mph air resistance becomes the major factor in fuel economy.

For my car, 95 Jag XJ6 mostly around town I get 23.1 mpg. I had left the counter averaging over the whole summer. In winter it drops to 22.3. Colder air, higher resistance. Modern fuel injected engines maintain 14.7 AFR within about 2 minutes of starting.

Tire pressure is another factor, I run 45 PSI on the car.

Next up, with auto trans whenever possible get to the speed where the torque converter clutch is locked.

Of course driving style is also factor, leadfoot etc.. Though engine is most efficient at high throttle opening, do tradeoff in speed vs lugging the engine.

My suburban, 7000+ lb, 7.4 litre engine gets around 13 MPG around town.
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Old 10-02-2014, 10:28 AM   #4
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I'd like to see this chart for my vehicles (Honda CR-V and Volvo S40). Skimming the article and comments though, his numbers seem highly suspect. There was something about aggregating numbers from different drivers and making some assumptions about driving habits? I just want a basic chart - mpg versus speed.


The effect of air drag at speed can be calculated from vehicle specs I think, but the effect of gear ratios gets more complicated.

And if you are going into a 15 mph headwind, 65 is like 80.

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Old 10-02-2014, 10:30 AM   #5
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My 2010 Camry SE @ 70-75 mph =30 mpg. I'm happy with that, and like 38Chevy454, hate going slow!
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Old 10-02-2014, 10:33 AM   #6
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With MPG as one priority when choosing what cars to purchase for over 30 years, we're well aware of speed vs MPG for any given car. The chart in the OP's link is conceptually true for production cars at least, of course the inflection points will vary though. Unless we are traveling a great distance, we tend to drive posted speed limits or slightly under and use cruise control as often as possible. The difference between 55 mph and 70 is huge for every car we've ever owned, and I suspect most production cars.

In addition, once we started driving hybrids, we also found that for hybrids ambient air temps make a huge difference, much more than conventional ICE cars. Nothing we can do about it, but our cars get 40-55 mpg in warm months and 35-45 mpg in dead of winter.
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Old 10-02-2014, 10:33 AM   #7
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Quote:
Originally Posted by ls99 View Post
... Though engine is most efficient at high throttle opening, do tradeoff in speed vs lugging the engine. ...
Yes, and this is something some people don't consider when they say a hybrid shouldn't make a difference at highway speeds, since you aren't braking/accelerating.

But all else equal, a hybrid will have a smaller engine (the motor/battery make up for that with their boost for acceleration). And a smaller engine will run at a more open throttle at a given highway speed, and therefore will be more efficient at that speed. Another minor advantage for a small engine is lower friction for smaller parts. This could be offset some if the total battery/motor/engine weight is greater than the weight of the larger engine.

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Old 10-02-2014, 10:37 AM   #8
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Either measure it on your own or do some research on your car. As you noted, the tipping point is different for different cars. That Civic curve is odd.

I used to have a Honda Pilot, and on an owners forum for that people noted that over 70mph gas mileage dropped off quite a bit. I started paying attention on long trips and wouldn't notice much change from 65-70, but at 75 or even 72-73 it got very noticeably worse.

With a lot of new cars there is a screen option to show current mileage. I haven't quantified anything on my new Forrester but when I'm cruising on a flat road I get great mileage at about any speed. But with even a bit of an incline, it's a lot better at lower speeds (50s) than high (70s).

I'm not going to drive 55 on a 70 mph interstate though. It doesn't seem safe to drive at too different of a speed from everyone else, plus it would drive me nuts.
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Old 10-02-2014, 10:39 AM   #9
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I think 2 hours time saved is worth $25


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Old 10-02-2014, 10:43 AM   #10
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.....
I'm not going to drive 55 on a 70 mph interstate though. It doesn't seem safe to drive at too different of a speed from everyone else, plus it would drive me nuts.
Yes, I alway pay to play, hate 55.
Besides the torque converter clutch on my car does not lock up until 57 MPH. The suburban's 4l80E monster tranny's TCC locks up at 63 MPH. I am mindful of those little features.
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Old 10-02-2014, 10:49 AM   #11
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Either measure it on your own or do some research on your car. ....
Without a known dead-level stretch of road and zero wind, it's pretty tough to measure. As far as research, are actual curves posted anywhere? I bet the mfg has this data, and maybe some car site? A dynometer loaded by an air drag calculation could probably get you very close w/o needing a track. The dyno would account for gear ratio and clutch locks.

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Old 10-02-2014, 10:59 AM   #12
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I drive pretty slow compared to most. In a 55 i'll go 55-60. In 65 i'll go 60-65. I rarely drive in areas with limits of 70+ but if I do I usually drive 5 mph under the limit if busy and slower(60-65) if few cars around. In some areas driving 65 in a 70 or 75 will piss off a lot of people but those are the maximum speeds not the minimum speeds so I let them be pissed. I leave plenty early for wherever i'm going so there is no reason to be in a hurry.
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Old 10-02-2014, 11:04 AM   #13
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I seldom go on long trips any more (except for hurricane evacuations), and when I do my top priority is not MPG, or speed, but not being stopped for speeding. The last speeding ticket I got was back in 1999 on I-10 in St. James Parish, and even though I was only 6 mph over the speed limit the ticket was outrageously expensive. I mean completely ridiculous!!! Yes it was a speed trap but I was also the only car on the road at 7 AM on a Saturday morning, and simply wasn't watching my speed that closely. However I don't have any intention of ever, ever surrendering that much money to government entities just for speeding, ever again. NOT gonna happen.

So, I assume that every mile of every interstate is a speed trap, and I drive at exactly the speed limit.

To save on gas, I try keep my speed smooth and steady as much as possible.
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Old 10-02-2014, 11:07 AM   #14
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Quote:
Originally Posted by ERD50 View Post
I'd like to see this chart for my vehicles (Honda CR-V and Volvo S40). Skimming the article and comments though, his numbers seem highly suspect. There was something about aggregating numbers from different drivers and making some assumptions about driving habits? I just want a basic chart - mpg versus speed.
I agree there's not much detail on the chart and I don't know anything about the source so it's difficult to assess the reliability. A pet peeve of mine is that the chart does nothing to show the variability in results.

Quote:
Originally Posted by Midpack View Post
With MPG as one priority when choosing what cars to purchase for over 30 years, we're well aware of speed vs MPG for any given car.
Highway MPG is going to be one of the biggest factors when we get a new car. This might be a while though as we kept our other car for 17 years.

Quote:
Unless we are traveling a great distance, we tend to drive posted speed limits or slightly under. The difference between 55 mph and 70 is huge for every car we've ever owned, and I suspect most production cars.
I haven't done on tests at 55 but will try to do so if it's safe. My car has a huge front surface area (honda element) and should benefit more than other cars.


Quote:
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Either measure it on your own or do some research on your car
As ERD50 mentioned, I'm not really sure how to do this accurately. So far I use a trip odometer (reset when gas tank is refilled) and compare to the amount of gas at the next refilling. This results in a lot of slop from driving conditions and possible differences in the level the gas tank is refilled. I'll try to exercise my google-fu and see if I can find existing results on my car.

Quote:
I'm not going to drive 55 on a 70 mph interstate though. It doesn't seem safe to drive at too different of a speed from everyone else, plus it would drive me nuts.
Usually I try to set the cruise control so to maintain position in the slow lane of the highway (so I do not need to pass or be passed). On a 70mpg interstate I think this usually results in somewhere between 60-65.
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Old 10-02-2014, 11:36 AM   #15
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It helps if you tuck in behind a big rig. up to 40%
Mythbusters: drafting 10 feet behind a big rig will improve mileage 39 percent

So, going with the flow of traffic is probably best.
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Old 10-02-2014, 11:40 AM   #16
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It helps if you tuck in behind a big rig. up to 40%
Mythbusters: drafting 10 feet behind a big rig will improve mileage 39 percent

So, going with the flow of traffic is probably best.
I do a lot of things to save small amounts of money but this isn't one if them. I avoid semis on the highway as much as possible. I don't get close behind them and I never am along side of one for more than a couple seconds. Drafting 10 feet behind a semi may improve mileage by 39% but it also reduces life expectancy(I assume).
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Old 10-02-2014, 11:50 AM   #17
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My Prius gets around 50 both in town and at freeway speeds (70-75).
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Old 10-02-2014, 11:55 AM   #18
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I don't track it that closely, but according to the car's computer, I've never gotten below 50.
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Old 10-02-2014, 11:57 AM   #19
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Wind resistance is about nil until about 45 mph, then it's a steep curve on a "dirty" vehicle ( bad aerodynamics ) and less so on a slippery shape.

Most vehicles get best mileage at about 45-50 mph.

I wasn't around, but someone who was told me during WWII ,the national speed limit was 35mph.
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Old 10-02-2014, 12:04 PM   #20
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A sister site to Early-Retirement.org is Fuelly - Track and Compare your MPG. Fuelly is a crowdsource project that gathers information on hundreds of thousands of fuel-ups each month so people can see the real world gas mileage that people are getting from their vehicles (many folks complain they do not get the EPA estimates).

Our plan is to evolve the site to include more Total Cost of Ownership data. For example, I am a low mileage driver and own a Toyota Prius & Honda Odyssey. Although the Prius gets better fuel economy, the insurance is more expensive even though it's overall value is lower than the Honda. The reason insurance is more is because a small fender bender is often more expensive to fix then a regular (non-hybrid) car. The Prius is also depreciating faster than our Odyssey because the minivan holds is value quite well because less used units end up on the market (family mobile where owners tend to keep them much longer). So the fuel economy savings are driving slowly are quickly eaten up by other ownership costs.

It's going to be a few years until the total cost of ownership data is significant on Fuelly but I suspect it will support what many of us have known for a long time. Buy a lightly used vehicle that is highly reliable and it will be the cheapest to own in the long run. The diesels are looking pretty attractive with better than average MPG longevity and lower depreciation because there is such limited supply.

When you look at a pie chart of the expenses related to owning a vehicle, fuel is just of the slices. If you take a vehicle which is dropping in value at $225/month and the owner has finance the car for 60 months and paying $225/month plus their insurance payment (lets say $80/month) and fuel might cost them $150/month. All to often people focus on how to save a few dollars on the fuel costs but to save the most green ($) you have to choose the right car to buy in advance (used and reliable).
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