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Old 11-20-2010, 12:20 PM   #16 (permalink)
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Reviving an old thread, hoping for some clarification...

Originally Posted by CapriRacer View Post
According to US Department of Energy, the effect aero has on the EPA fuel economy test is 3% for the Urban cycle and 11% for the highway cycle. (For rolling resistance of tires it's 4% / 7%)
I had no luck searching for info about the aero drag relationship to the EPA tests. Don't suppose you recall where you got it?

That means the effect on fuel economy of a 10mm change is 0.03% to 0.11% - wider being worse.
Don't forget to account for changes in CdA, not just Cd. It's a small additional difference, but going wider by 1 tire size (10 mm) adds approximately 25 cm^2 (3.9 in.^2) to frontal area, assuming a ride height of 12.5 cm / 4.9 in. (my assumption).

In addition, a potential problem with using the EPA highway test cycle to estimate the impact of fuel savings from aero changes can be seen in its format:

- The test's max speed of 60 mph is only briefly touched
- The average test speed is only 48 mph
- The test duration is 765 sec. / 12.8 minutes

source: Detailed Test Information

I don't think it's a stretch to say estimates based on those figures would underestimate actual real world results of aero changes on highway fuel consumption. (The EPA itself has effectively conceded that point by adding an additional fudge factor to its 2008+ MPG ratings.)

So if instead we plug the delta RR and delta CdA numbers into the rolling + aero drag formula, we can calculate the impact of the change over a range of speeds.

Using this tool (default values are for a 1996-1998 Geo Metro hatchback): rolling + aero drag calculator

To go 60 mph, the vehicle requires 9,183.7 W (~45.82 MPG US)

Increase tire width by one size:

- Cd increases 0.003 (from 0.34 to 0.343)
- A increases 25 cm^2 (from 1.8581 m^2 to 1.8606 m^2)
- Crr decreases by 3% (say from 0.008 to 0.00776)
- (link to calculator with these values changed)

And now, to go 60 mph, the vehicle requires 9,206.8 Watts (~45.71 MPG US)

So at this speed, fuel economy is fractionally worse with the wider, lower Crr tire.

Looks like the "tipping point" is ~45 mph, where below that, the wider, lower Crr tire offers a slight efficiency benefit, and above that speed, the narrower, higher Crr tire offers a slight efficiency benefit.


Also: did you mean to say "INCREASES" in this section?

So if you combine that with the effect RR has on the EPA test, then a 10mm increase in tire width DECREASES the fuel economy by 0.12% to 0.21%, which is significantly larger than the effect caused by aero (0.03% to 0.11%)
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