[TheInterloafer]

The New York Times reported the other day that the EPA fuel efficiency ratings are usually too optimistic:

"The E.P.A. test was developed in the 1970's and is not reflective of the way cars are driven today," said David Champion, director of vehicle testing at Consumer Reports in East Haddon, Conn. For the highway portion of the E.P.A. test, for example, the vehicle's average speed is 48 miles an hour, with the air-conditioner off and no braking for traffic.

This raises the question: Why does one's gas mileage go down if they run their car's air-conditioner?

[pilferage]

An air conditioner is connected to the car's engine (Either by pulley, or via the electrical system.), and when turned on, uses some percentage of the engine's output. So, to go the same distance the engine must use even more energy... And gets slightly worse mileage. The same thing applies to anything else drawing power.

Think of an air conditioner as a backwards engine, it uses energy to change the temperature of air, as opposed to changing the temperature of air to make energy.

I believe the EPA uses atypical driving circumstances because the car's engine hasn't been broken in thoroughly. If they were to test it a few thousand miles later they'd get much better results. For instance, my car supposedly gets 35/45mpg, but I get 45/55mpg. They're pushing the scale in one direction to compensate for the increase in efficiency later in engine life.

[Aedo]

The New York Times reported the other day that the EPA fuel efficiency ratings are usually too optimistic:

No tests really reflect daily vehicle use and as such they cannot be used to predict actual (and individual) usage. The tests are useful, however, in that they are all done to a standard and as such can be used to for direct comparisons between different vehicles.

[rockdoc123]

Do the tests take into account that when the air conditioner is turned off generally windows are opened which should increase drag on the vehicle and I suspect would hinder performance?

[TheInterloafer]

Pilferage, thanks. I knew that AC energy had to come from somewhere. RockDoc, yes, I've heard that having open windows decreases fuel efficiency.

[PhilBiker]

Do the tests take into account that when the air conditioner is turned off generally windows are opened which should increase drag on the vehicle and I suspect would hinder performance?

No. They don't bring wind into account at all whatsoever, they are done in a closed room on a treadmill.

[Caoimhan]

Which is also very unrealistic, given that the drag coefficient of a particular chassis is a major factor in real-world gas mileage. High profile vehicles, such as SUVs will have a higher drag coefficient than sub-compacts, simply because they are presenting a larger cross section to the wind. If you were to put the exact same engine into a Ford Focus and an Expedition, the Focus would likely get 25% better gas mileage. The Expedition chassis also weighs more, and would therefor also suffer due to acceleration of that extra mass.

[strider3700]

Do the tests take into account that when the air conditioner is turned off generally windows are opened which should increase drag on the vehicle and I suspect would hinder performance?

No. They don't bring wind into account at all whatsoever, they are done in a closed room on a treadmill.

That would mean that all that is being measured is the efficiency of the engine and the drivetrain. Wouldn't pretty much every vehicle out there running the same engein get exactly the same mileage.

Ford offered the 5.0 L in both the mustang and the explorer one year. Obviously the explorer should get worse mileage. If I go back and find those numbers will they be the same?

[strider3700]

damn it looks like in 95 the 5.0 (4.9) was not offered in the explorer but was offered in the mustang. There it got 17 city 24 highway. in 96 it was in the explorer and it got 14 city 18 highway. Even with minor tweaks to the engine I can't see it making that big of a difference to the mileage

[Aedo]

Do the tests take into account that when the air conditioner is turned off generally windows are opened which should increase drag on the vehicle and I suspect would hinder performance?

No. They don't bring wind into account at all whatsoever, they are done in a closed room on a treadmill.

That would mean that all that is being measured is the efficiency of the engine and the drivetrain. Wouldn't pretty much every vehicle out there running the same engein get exactly the same mileage.

Ford offered the 5.0 L in both the mustang and the explorer one year. Obviously the explorer should get worse mileage. If I go back and find those numbers will they be the same?

As far as I know rolling road tests (the 'treadmill' PhilBiker mentioned) are set to take into account the weight of the vehicle, hence the difference in the fuel consumption of the Mustang vs Explorer. Drag doesn't actually have that much effect below 40-50mph so doesn't really matter in tests for operation at legal speeds .

[rockdoc123]

Drag doesn't actually have that much effect below 40-50mph so doesn't really matter in tests for operation at legal speeds

that's interesting...do you have the calculation? The only reason I ask is that when I look at flow through pipelines as an example turbulence and shear are major factors, and oil flows at speeds similar to what we are talking about.

[Aedo]

Drag doesn't actually have that much effect below 40-50mph so doesn't really matter in tests for operation at legal speeds

that's interesting...do you have the calculation? The only reason I ask is that when I look at flow through pipelines as an example turbulence and shear are major factors, and oil flows at speeds similar to what we are talking about.

Sorry - didn't have the figures to hand (ie: should have found proof before I posted!) but things like rolling resistance, resistance in wheel bearings and brake drag have a bigger impact at low speed. I've round a reference for aerodynamic drag related to the design of solar cars (where rolling resistance is very low and aerodynamic drag has more impact) and the short quote from them is:

Aerodynamic Drag is very non-linear with speed. At very slow speeds, below 10mph, it doesn't have too much effect, but as the speed increases to more than 30mph, aero drag gets important.