AutoExpress looks at MPG myths, little impact from tire pressure
Shortly after the crew at Britain's AutoExpress magazine squeezed 50 mpg out a MINI John Cooper Works special, they decided to evaluate the real impact of various factors on fuel consumption. In order to make the test results as accurate as possible, the AutoExpress team headed to the Millbrook Proving Ground, an independent test facility originally built by General Motors. The engineers ran a series of tests over a 7.75 mile loop with varied driving conditions in a Vauxhall Astra. The engineers monitored engine parameters to calculate fuel consumption. For a baseline they ran the loop at an average speed of 29 mph with everything turned off and the windows up. In this condition they recorded an average of 25.4 mpg. They then proceeded to try different combinations such as turning on the air conditioning, lowering the tire pressure to only 20 psi, rolling down the windows, carrying three adult passengers in the back seat and putting on a roof box and bike rack.
All the variables increased fuel consumption to some degree but there were some surprises in the magnitude. With the tire pressures reduced to 20 psi, the mileage dropped just over 0.6 mpg to 24.73 mpg. This may be due to the same factors that Michelin's Bob Massa talked about during the Audi Mileage Marathon. Modern steel belted radial tire construction actually results in very little deformation of the contact patch with tire pressure. Just as pumping up the tires above recommended values would yield little improvement in mileage, so too it seems at least at this pressure it has little impact. Perhaps if the pressure dropped down to 10 psi more difference would be apparent. Check out the rest of the results at AutoExpress.
[Source: AutoExpress]
Reader Comments (Page 1 of 1)
Barry 9:44AM (10/27/2008)
The real test will be to determine if this applies to all 57 states given differing environments.
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George 10:16AM (10/27/2008)
Yes, more tests should be done. Whatever helps increase gas millage is welcomed by many. It may also help in other areas like electric cars where you need all the extra help you can get to increase the milage of a charge.
http://gas2electric.net
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Phil L. 10:13AM (10/27/2008)
Note that these tests were run using a car powered by a small diesel engine and a manual transmission, a rare combination in the US. I'd be curious to see if the relative percentages would work out the same were they to test a gasoline/automatic car.
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Jo Mama 10:21AM (10/27/2008)
57 states? What country do you live in?
Who tests fuel mileage at 29mph? That's the dumbest thing I've ever heard of. There's no aerodynamic resistance that would significantly effect any car (windows up or down) at 29 mph.
This test should be considered null and void.
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Sam Abuelsamid 10:33AM (10/27/2008)
29 mph was the average. The test loop included simulated urban driving with stops and starts and acceleration and braking. The maximum speed was higher than 29.
Brent 12:23PM (10/27/2008)
Our next president said there are 57 states. I think that is what he is talking about.
Bolen 10:52AM (10/29/2008)
"57 States" is not as dumb as you might think. The US Postal Service lists 57 2-character abbreviations for US States and Possessions. The 7 "extra" codes are:
AMERICAN SAMOA -- AS
DISTRICT OF COLUMBIA -- DC
FEDERATED STATES OF MICRONESIA -- FM
MARSHALL ISLANDS -- MH
NORTHERN MARIANA ISLANDS -- MP
PUERTO RICO -- PR
VIRGIN ISLANDS -- VI
Herm 4:22PM (10/27/2008)
Excellent test, now it should be repeated with a run-of-the-mill Ford F150 pickup, gas V8 with auto transmission.. why? there are a lot of those in the world, and a 10% improvement in their gas mileage would save many many gallons of gas worldwide. Try a rigid cover for the bed, try different tires and speeds..
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DasBoese 6:57AM (10/28/2008)
(Fullsize) pickups are a predominantly American thing. There are hardly any in the rest of the world, so the impact wouldn't be that big.
stevejust 9:05PM (10/27/2008)
I can say that I drive a honda civic hybrid hybrid that displays my real time mileage, and tire pressure matter far more than .06 mpg. The truth is that when I start to see my mileage averaging fall, the first thing I do is check my tire pressure. Aside from weather changes (cold weather not so good for mileage) the most noticable thing I ever see impacting my mileage is my tire pressure.
So my real world driving experience tells me they're just wrong.
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Chris M 10:22PM (10/28/2008)
I've noted the same thing on my Prius, I could tell when the tire pressure was down by the drop in gas mileage.
Of course, the weight of the vehicle might make a difference, as a really light vehicle wouldn't deform the tire as much as a heavier vehicle when the pressure was low, thus less effect on fuel mileage.
Will 9:42AM (10/28/2008)
Aerodynamic mods such a roof racks and roof boxes have been shown in test and in actual driving conditions to significantly affect fuel economy, to the tune of 10 to 30%, depending on how many bikes you've got strapped to it and the speed you travel.
Roof racks don't affect fuel economy very much, if at all, in city driving. At highway speeds, like traveling to a race out of town, roof racks be really drag down fuel economy. A car we used for the team (2001 VW Passat Wagon, manual) would get 30mpg stripped down, but with 4 bikes on the roof would only get 21 mpg. This is from pump to pump, not measured from the trip computer.
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BoneHeadOtto 12:34PM (10/28/2008)
My physics and real word testing tells me they are wrong. The whole point of the added tire pressure is to decrease rolling resistance. Therefore it makes no sense to test this in a stop and go situation. This needs to be tested in a highway situation. Their 29mph average just means they tested too much stop and go. Plus stop and go is nearly impossible to test accurately. But cruising at 60mph is easier to test within reasonable errors. Plus 0.6mpg difference is probably within error tollerances due to the shoddy testing.
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Herm 1:04PM (10/28/2008)
The point of this test is that it is computerized, thus repeatable but not nessesarily accurate.
The Ford F150 has been the BEST selling vehicle in the US for 23 years, and that means a lot of trucks that usually get 15mpg.. oil that gets used in the US is not available in Europe, thus everyone should conserve.
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