tire width vs. drag (Cd)

[sbdeadelf]

Does anyone have actual A-B-A test results on this? When I went from 155s to 185s on my Fiesta my mileage seemed to drop a lot, but I never did any reliable testing. A 155/60/15 is the biggest tire that will fit on an early Sprint and I LOVE them, but if somebody with a test-able Metro (96 or newer I guess) lives near enough to me in SoCal and has some wide tires and wants to swap and test, I'm game!

[aerohead]

Originally Posted by sbdeadelf

Does anyone have actual A-B-A test results on this? When I went from 155s to 185s on my Fiesta my mileage seemed to drop a lot, but I never did any reliable testing. A 155/60/15 is the biggest tire that will fit on an early Sprint and I LOVE them, but if somebody with a test-able Metro (96 or newer I guess) lives near enough to me in SoCal and has some wide tires and wants to swap and test, I'm game!

A comparison of your present tank mileage with your baseline mpg with old tires will show any change if present.

[winkosmosis]

Originally Posted by sbdeadelf

Does anyone have actual A-B-A test results on this? When I went from 155s to 185s on my Fiesta my mileage seemed to drop a lot, but I never did any reliable testing. A 155/60/15 is the biggest tire that will fit on an early Sprint and I LOVE them, but if somebody with a test-able Metro (96 or newer I guess) lives near enough to me in SoCal and has some wide tires and wants to swap and test, I'm game!

Any change in diameter?

[sbdeadelf]

Not a lot of change in diameter:
For the 155R12 the diameter was SUPPOSED to be 21.8, but actually measured 21.3 if memory serves.
For the 155/60/15 the diameter was around 22.3 inches. From actual driving, the speedometer which USED to be absolutely dead-on, is now about 6% under the actual mileage. With the 12's, max pressure was around 35psi and with the 15's max psi is 44. Also, I trust Continental more than most of the 155R12 makers, but they do cost more.
Handling is exeptional, especially compared to the 12s. Another nice thing about the skinnies, is that even at 1650 lbs, you just do NOT hydroplane.
As for testing, there are just too many variables to get good results in a tank-fill test. I just can't use up fuel fast enough to get a result that's meaningful. By the time I'm ready to fill up more than a gallon, the temp has changed, the winds have changed, etc.
I keep drooling over the 135/70/15's they have access to in EU!..... unless someone proves the 'wider is better' thing correct. Then I'd have to re-think a lot of things! Oh NO!

[CapriRacer]

Originally Posted by sbdeadelf

Does anyone have actual A-B-A test results on this? When I went from 155s to 185s on my Fiesta my mileage seemed to drop a lot, but I never did any reliable testing. A 155/60/15 is the biggest tire that will fit on an early Sprint and I LOVE them, but if somebody with a test-able Metro (96 or newer I guess) lives near enough to me in SoCal and has some wide tires and wants to swap and test, I'm game!

CAUTION:

If you are going to do A-B-A testing, EVERYTHING has to be the same, except for the item under test.

There are differences in RR between new and worn tires, tires of different speed ratings, tires with different load carrying capacities, not to mention tires of different makes and models.

Just swapping out 155's with 185's isn't going to tell you anything of value unless you control a whole lot of other things.

[MetroMPG]

Reviving an old thread, hoping for some clarification...

Originally Posted by CapriRacer

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%)

[MetroMPG]

Edit: and of course in my theoretical example above, it doesn't account for a very slight increase in tire diameter (likely positively affecting gearing) & ride height (likely negatively affecting Cd).

[aerohead]

Darin,I believe that Hucho's book has CdA vs MPG tables/curves for both N.America and European test cycles.Sorry,don't have my book with me.
If one presumed that the Cd was un-changed and only the frontal area went up with wider tires,then the drag relationship would be arithmaic.Say,a 2% increase in frontal area would mean a 2% drag increase.
At 55 mph ( 88 km/h ) mpg would suffer by 1%.
At 70 mph ( 112 km/h ) mpg would suffer by 1.2%.

[mcrews]

I have 255/45/18 instead of 245/45/18. on my 02Q45
THe diamiter is 3.4% more than the 245. my mileage did infact increase with the increase in diameter. the 255 is also about 3/4 inch wider than the 245.
the hieght increase was just under 1/2 inch.
THe issue is complex.
If you do alot of short distance driving then the increased diameter can hurt mileage (ignoring the cd issue) because of the extra effort to get the car moving each time you stop
If you do alot of freway, then you'll see results because of the lower rpm at cruise.

[CapriRacer]

Originally Posted by MetroMPG

........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?.......

I got it from the "Tires and Passenger Vehicle Fuel Economy" Report put out by the Transportation Research Board. I used it on my web site. It's the first image on the page:


Barry's Tire Tech

Originally Posted by MetroMPG

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

Yes. That is not the only mistake, but the others are small and don't change the conclusion.