snopes.com  


Go Back   snopes.com > Urban Legends > Automobiles

Reply
 
Thread Tools Display Modes
  #1  
Old 13 May 2011, 06:02 PM
snopes's Avatar
snopes snopes is offline
 
Join Date: 18 February 2000
Location: California
Posts: 109,640
Driver Driving faster decreases mileage?

Comment: A common factoid that gets quoted as gas prices rise or as states
consider raising speed limits is the following:

For every 5mph you drive over the 55mph speed limit, you lose 5mpg of
mileage.

Sometimes, this is quoted as 1mpg or some other number. I have yet to
experience this myself and have seen no significant difference when
driving 55 vs. driving 70. What do you think?
Reply With Quote
  #2  
Old 13 May 2011, 06:10 PM
Troodon Troodon is offline
 
Join Date: 06 January 2004
Location: Waltham, MA
Posts: 8,077
Default

Driving more quickly does lower MPG, because air resistance increases non-linearly with speed.
Reply With Quote
  #3  
Old 13 May 2011, 06:23 PM
overyonder overyonder is offline
 
Join Date: 03 March 2010
Location: Charlotte, NC
Posts: 2,056
Default

There's a little bit more to it than the plain "every 5 mph over 55mpg" story.

Some cars are more fuel efficient at 55, while others are more efficient at 65 or at 45.

OY
Reply With Quote
  #4  
Old 13 May 2011, 06:38 PM
Der Induktionator's Avatar
Der Induktionator Der Induktionator is offline
 
Join Date: 18 April 2005
Location: Luzern, Switzerland
Posts: 1,296
Default

there's certain fixed consumption (electrical accessories, A/C, etc.), so at very low speed the efficiency is poor, but once you get past that, driving faster increases fuel consumption. It's something like air resistance and correspondingly power needed increases with the square of the speed.

The way the post is written is wrong, though. It might hold for a certain car for a certain narrow range of speed above a certain speed, but fails in the general case.

Consider an SUV that gets 5MPG at 55MPH. Driving 60MPH you loose 5MPG: that would give you 0MPG - the engine would consume all fuel you gave it as fast as you could pour it in, and not go anywhere. Go another 5MPH faster and you loose another 5MPG, and the situation reverses. If you could go 65, you now get negative 5 MPG. For every 5 miles you drive, a gallon of gas magically appears in your tank. 70MPH gives you 2 gallons of gas for every 5 miles you drive.

We should all drive massive gas unguzzling SUVs at crazy speeds far above the legal speed limit and solve the world's energy crisis.
Reply With Quote
  #5  
Old 13 May 2011, 07:06 PM
jimmy101_again jimmy101_again is offline
 
Join Date: 29 December 2005
Location: Greenwood, IN
Posts: 6,825
Default

Quote:
Originally Posted by snopes View Post
For every 5mph you drive over the 55mph speed limit, you lose 5mpg of
mileage.
I believe this is generally stated as "for every 5 MPH over 55 MPH you decrease the fuel efficiency by 5%. Since most people have trouble understanding percents they tend to remember the quote as a change in MPG. Different vehicles have different percentages and most vehicles, like overyonder said, have an optimal speed, in MPG terms, near 60 MPH.

Given the on board electronics and nature of modern fuel injection systems it should be fairly trivial to get accurate measures of fuel consumption under various conditions for any modern car. Indeed, many cars come with fuel MPG displays which, if done properly, should be fairly accurate (probably extremely precise but somewhat less then exactly accurate). Or, with a bit'o wire and a laptop and a few other bits you can do it yourself, as was done here.
  #6  
Old 13 May 2011, 07:43 PM
overyonder overyonder is offline
 
Join Date: 03 March 2010
Location: Charlotte, NC
Posts: 2,056
Default

Actually, cars with MPG displays are notoriously inaccurate. The problem lies in the fact that the gasoline used (in some cars) is measured at near the fuel-pump, but does not take into consideration the fuel return lines to the gas tank, which is a fair amount actually.

Additionally, some cars measure MPG by using the mass airflow sensor readings, which measures how much air h`as been used. It's not a very precise measurement unfortunately, so the MPG display is not all that accurate.

OY
Quote:
Originally Posted by jimmy101_again View Post
Indeed, many cars come with fuel MPG displays which, if done properly, should be fairly accurate (probably extremely precise but somewhat less then exactly accurate). Or, with a bit'o wire and a laptop and a few other bits you can do it yourself, as was done here.
Reply With Quote
  #7  
Old 13 May 2011, 09:06 PM
UrbanLegends101 UrbanLegends101 is offline
 
Join Date: 24 November 2010
Location: Florida
Posts: 1,314
Default

Too bad the various car performance magazines don't graph fuel consumption vs speed, as an additional performance category.
Reply With Quote
  #8  
Old 14 May 2011, 12:17 AM
jimmy101_again jimmy101_again is offline
 
Join Date: 29 December 2005
Location: Greenwood, IN
Posts: 6,825
Default

Quote:
Originally Posted by UrbanLegends101 View Post
Too bad the various car performance magazines don't graph fuel consumption vs speed, as an additional performance category.
That would be interesting, even for regular cars. It would be nice to know what a particular car's optimum speed is. And, how that speed changes with windows up/down, the AC on/off, ... a couch sticking out'a the trunk.
Reply With Quote
  #9  
Old 14 May 2011, 12:56 PM
UrbanLegends101 UrbanLegends101 is offline
 
Join Date: 24 November 2010
Location: Florida
Posts: 1,314
Default

The reason I bring the graphs up is that I have seen anecdotal accounts claiming that some of the high performance vehicles do get better fuel consumption numbers in the 70-80 MPH range rather than the more typical 40-50 MPH block. The reasoning is based on better air flow at more open throttle positions, less air resistance, thus more power per pound of fuel.

There are some interesting studies in the general aviation realm, weighing the maximum endurance vs maximum range performance of the aircraft. Maximum endurance meaning greatest number of minutes of engine operation per fuel load compared to maximum range for the same fuel load, which isn't the same number of ground miles, in no wind conditions.

one typical discussion:

http://www.eaa1000.av.org/technicl/p...s/perfspds.htm

I am not sure we need to get to such detail for an automobile, but it would certainly be nice to see a graph plotting steady state speed vs fuel consumption, perhaps more correctly, miles per gallon at that speed.
Reply With Quote
  #10  
Old 14 May 2011, 04:42 PM
jimmy101_again jimmy101_again is offline
 
Join Date: 29 December 2005
Location: Greenwood, IN
Posts: 6,825
Default

Quote:
Originally Posted by overyonder View Post
Actually, cars with MPG displays are notoriously inaccurate. The problem lies in the fact that the gasoline used (in some cars) is measured at near the fuel-pump, but does not take into consideration the fuel return lines to the gas tank, which is a fair amount actually.
I believe the general approach is to count the number of times, and the dwell, that the fuel injector(s) fire. They don't measure fuel flow from the tank or use the fuel gauge as part of the calculation, no need to since the much more precise (and relevant) fuel injector data is available. The only uncertainty is the exact volume of fuel that a single pulse of an/the injector injects. That conversion factor is probably fairly accurately known, and most of the inaccuracy is probably a constant factor. That would mean that MPG calc's for a given vehicle might be off a bit but relative measurements (e.g., MPG as a function of speed) for a particular vehicle would be more reliable. In the link I posted the MPG is calculated using wheel speed (probably form the ABS sensors) and the injector data.
Reply With Quote
  #11  
Old 14 May 2011, 04:45 PM
snopes's Avatar
snopes snopes is offline
 
Join Date: 18 February 2000
Location: California
Posts: 109,640
Ponder

Quote:
For every 5mph you drive over the 55mph speed limit, you lose 5mpg of
mileage.
Then if you drive a 20 MPG vehicle at 75 MPH, your fuel tank will instantly empty.
Reply With Quote
  #12  
Old 14 May 2011, 04:47 PM
Der Induktionator's Avatar
Der Induktionator Der Induktionator is offline
 
Join Date: 18 April 2005
Location: Luzern, Switzerland
Posts: 1,296
Default

Quote:
Originally Posted by UrbanLegends101 View Post
There are some interesting studies in the general aviation realm, weighing the maximum endurance vs maximum range performance of the aircraft. Maximum endurance meaning greatest number of minutes of engine operation per fuel load compared to maximum range for the same fuel load, which isn't the same number of ground miles, in no wind conditions.
This reminds me of the Lockheed SR-71 Blackbird that used less fuel per mile the faster (and higher) it flew. Wikipedia: "One unique characteristic of the SR-71 is that the faster it went, the more fuel-efficient it was in terms of pounds burned per nautical mile traveled..."

Most cars don't turn into a ramjet the faster they go, though.
Reply With Quote
  #13  
Old 14 May 2011, 05:12 PM
Der Induktionator's Avatar
Der Induktionator Der Induktionator is offline
 
Join Date: 18 April 2005
Location: Luzern, Switzerland
Posts: 1,296
Default

Quote:
Originally Posted by snopes View Post
Then if you drive a 20 MPG vehicle at 75 MPH, your fuel tank will instantly empty.
But at 80, your tank will start to fill again by one gallon every 5 miles.
Reply With Quote
  #14  
Old 14 May 2011, 05:54 PM
Jahungo's Avatar
Jahungo Jahungo is offline
 
Join Date: 23 May 2007
Location: New York, NY
Posts: 5,351
Default

Or you'll only be able to drive backwards.
Reply With Quote
  #15  
Old 16 May 2011, 12:00 AM
Alchemy Alchemy is offline
 
Join Date: 05 March 2001
Location: Plymouth, MI
Posts: 4,407
Default

Quote:
Originally Posted by jimmy101_again View Post
I believe the general approach is to count the number of times, and the dwell, that the fuel injector(s) fire.
Gasoline engines normally operate on a lean-rich frequency around 2-10 Hz, which is substantially controlled by injector dwell time. Sampling and integrating all these dwell time values represents a pretty high computational overhead, assuming these values are even accessible to an external recording device. A moving average or historical dwell time would be useful to calculate mileage but I'm not sure the PCM calculates or provides this value.

In the link you posted, the person used the OBDII-reported MAP (and manifold air temperature?) and RPM to estimate mass airflow (although I can't see how, as the equation he lists is incomplete) and used that along with an air/fuel ratio estimate to determine mass fuel flow. This should be accurate to two or possibly three digits, perhaps repeatable to three or four digits, but certainly not the ten (!) digits he reports. (Is he seriously reporting mpg to the nearest micrometer per gallon? For those of us who need to know how much gas we need to drive from one side of a red blood cell to the other? ) But in any case, mass airflow and AFR allows a (closed-loop) estimation of fuel consumption about as accurate as anything else. Most importantly, it's based on values that will remain fairly consistent on a straight, flat, constant-speed stretch, so sampling does not need to be done on the order of milliseconds to assure statistically random sampling.

I also suspect a lot of the inaccuracy of dash-integrated MPG indicators is not an issue with the PCM data itself, but with how it is sampled and integrated by the MPG calculator, but I'm not familiar with the devices so I can only guess.
Reply With Quote
  #16  
Old 16 May 2011, 05:31 PM
Debunker's Avatar
Debunker Debunker is offline
 
Join Date: 29 September 2003
Location: Yorba Linda, CA
Posts: 420
Default

The MPG readout on our Suburban is reasonably accurate. When I fill the tank, it's about 5% optimistic in amount of fuel consumed. The real-time MPG readout shows a definite drop in MPG as speed increases. It's not 5 mpg per 5 mph, but it's there.
Reply With Quote
  #17  
Old 17 May 2011, 09:44 PM
Diabolus Ex Deus's Avatar
Diabolus Ex Deus Diabolus Ex Deus is offline
 
Join Date: 19 August 2006
Location: Los Angeles, CA
Posts: 110
Default

I think it's not really a good idea to cite actual numbers if you're trying to speak broadly about fuel efficiency. There's plenty of truth in the claim that faster driving can decrease fuel efficiency in the sense of simple mechanics, though. The more you have the gas pedal depressed, the more gas is being fed into the engine. Pretty simple.
The problem with quoting actual numbers is that there are a vast number of variables to take into account. Aerodynamics, gear ratio, number of gears, and valve timing are just a handful I can think of right off the top of my head. For instance, I drive cars with manual transmissions, and because of this there are more gears than the automatic counterparts (typically speaking, though there are exceptions) - With a 5th gear, my cruising RPMs are lower than the 4-speed counterpart, therefore requiring marginally less fuel to maintain speed.

This is just a simple observation from personal experience, but like I mentioned in this post, I don't see there being a very clear way to quote an actual percentage on the increase since it depends on so many things. That number, at best, would be an average, and at worst, a guess.
Reply With Quote
  #18  
Old 17 May 2011, 09:52 PM
GenYus234's Avatar
GenYus234 GenYus234 is offline
 
Join Date: 02 August 2005
Location: Mesa, AZ
Posts: 25,424
Default

Quote:
Originally Posted by Diabolus Ex Deus View Post
With a 5th gear, my cruising RPMs are lower than the 4-speed counterpart
Not necessarily. The 4th gear (overdrive) on the AT in my wife's car is a higher gear ratio than 5th gear on the MT in my truck.
Reply With Quote
  #19  
Old 17 May 2011, 10:51 PM
Diabolus Ex Deus's Avatar
Diabolus Ex Deus Diabolus Ex Deus is offline
 
Join Date: 19 August 2006
Location: Los Angeles, CA
Posts: 110
Default

Quote:
Originally Posted by GenYus234 View Post
Not necessarily. The 4th gear (overdrive) on the AT in my wife's car is a higher gear ratio than 5th gear on the MT in my truck.
Indeed, but that's why I was referring to "automatic counterpart" - If the exact same model of my car is given a side-by-side comparison of automatic and manual, you'd see a difference in RPMs at speed. But, there are exceptions to this. I'm just making the statement to illustrate that there are too many variables to cite an actual number across the board.
Reply With Quote
  #20  
Old 17 May 2011, 11:03 PM
jimmy101_again jimmy101_again is offline
 
Join Date: 29 December 2005
Location: Greenwood, IN
Posts: 6,825
Default

Quote:
Originally Posted by Diabolus Ex Deus View Post
I think it's not really a good idea to cite actual numbers if you're trying to speak broadly about fuel efficiency. There's plenty of truth in the claim that faster driving can decrease fuel efficiency in the sense of simple mechanics, though. The more you have the gas pedal depressed, the more gas is being fed into the engine. Pretty simple.
Simple perhaps, but wrong. The more you have the gas pedal depressed the faster the car is moving (at least while cruising). Since MPG is miles/gallon and not minutes/gallon ("more gas is being fed into the engine") your comment is irrelevant to the MPG question.

For most modern passenger cars the MPG versus speed gives a fairly broad hump shaped peak with the maximum MPG occurring near 60 MPH or so. Lower speeds, down to say 30 MPH, give slightly worse gas mileage. Above the optimal speed the mileage drops off much quicker than it does at speeds below the optimum.

For example;

from http://www.fueleconomy.gov/feg/drivehabits.shtml
Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Last 1/4 tank gets worse mileage than first 1/4? purpleiguana Automobiles 24 04 January 2010 03:21 AM
Gas mileage - asphalt vs. concrete Mr. Furious Automobiles 14 15 October 2007 11:32 PM
Better Gas mileage with Acetone? Dondi Automobiles 2 16 April 2007 07:26 PM
How to dry clothes faster snopes Old Wives' Tales 27 11 April 2007 12:44 AM


All times are GMT. The time now is 03:58 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2018, vBulletin Solutions, Inc.