Forums

Does a car's drag coefficient impact its EPA range or mileage estimate?

Does a car's drag coefficient impact its EPA range or mileage estimate?

Many of you have noted that the M3 should have better range than other EVs (such as the Bolt) because it has a lower drag coefficient. Will that actually be realized in the EPA rating? It seems like the EPA rating is based on putting a car on a dynamometer and driving it through EPA's protocols. There is no mention of a wind tunnel to simulate the drag effect.

Thanks,

PhillyGal | July 28, 2017

Very very very much so!

carlk | July 28, 2017

Yes. You will be getting the EPA number probably by driving no faster than 60 mph or so.

Frank99 | July 28, 2017

They adjust the load on the dyno based on rolling resistance and aerodynamic drag; for example, this post:
http://ecomodder com/forum/showthread.php/aero-drag-epa-mpg-23769-2.html#post335960
discusses it.

El Mirio | July 28, 2017

https://www.fueleconomy.gov/feg/how_tested.shtml

they state that they adjust the dynamometer to account for drag coefficient.

Frank99 | July 28, 2017

Another good link:
http://jalopnik com/how-fuel-economy-is-measured-and-why-you-get-different-1716232721

Robocheme | July 28, 2017

I guess that I didn't word my question correctly.

I'll put it another way. If there are two cars that are identical (weight, motor or engine, etc.) except that one is built like a box while the other is very streamline, won't their EPA mileage or range ratings be identical?

I ask because others have said that they expect the M3 to have a better EPA rating than the Bolt primarily because it is much more aerodynamic. I would think that it won't because the EPA tests don't seem to take drag into account, Fortunately, in real life the M3 should be better.

Frank99 | July 28, 2017

No, because the manufacturer runs a "coastdown" test - in still air, they accelerate the car to 85 or so and then kick it into neutral, and then measure the decelerate rates as the car coasts to a stop. They calculate the aero and rolling resistances at various speeds, and program those into the dynamometer before the test.

Robocheme | July 28, 2017

Thanks, Frank, El Mirio,

Frank99 | July 28, 2017

No problem, it was an interesting question that didn't have an obvious answer. I learned something looking it up.

Robocheme | July 28, 2017

Yeah, the website that I looked up on the EPA test didn't mention the aero correction, only that the car was on a dyno.