tire width vs. drag (Cd)
 

The question of the effect of tire width on drag seems to come up once in a while. So here's more data to add to the pile, from Hucho, 1998.

---

Influence of the tire width on drag, lift, and yawing moment, after H Kerschbaum Fig 5.72

http://www.goauto.com.au/mellor/mell...if?OpenElement

CD --- tire & rim size (for a 1991 BMW 318i)

0.293 --- 155 R 15; 5 1/2 Jx15 St. with wheel covers
0.294 --- 165 R 15; 61 2 Jx15 St. with wheel covers
0.297 --- 175/70 R 15; 6 Jx15 St. with wheel covers
0.305 --- 185/65 R 15; 61 Jx15 St. with wheel covers
0.311 --- 205/60 R 15; 61 2 Jx15 St. with wheel covers
0.314 --- 205/60 R 15; 7 Jx15 LM
0.319 --- 225/55 R 15; 7 Jx15 LM

Keep in mind the increase in Cd comes with an increase in A (frontal/projected area) too, so you're getting a double whammy.

Add to this from Phil's notes:

• 1986, wind tunnel development work for Subaru XT show a drag increase of 5.1 % when tire size is increased from 155 to 185 series radials.

• 1984-1987 HONDA CRX shows jump from 165 to 185 series radials increase drag 9.3 %.

Phil's notes also point out that the increased drag from wider tires can be partially offset by managing the airflow ahead of them (dams/spats).

Wow, I wouldn't have guessed the CRX' drag to go up that much. :eek:

Couldn't a low rolling resistance tire that takes much higher air pressure cancel out the CD loss of a larger foot print? I am throwing that out there because the 13's on my 96 Geo 1.0 call for a max of 35 psi (I put in 39) while the 195/50/15 size that I want to put on in the distant future has a max reading of high 40s.

Couldn't
 

When I went from a 35 psi 165 70R-13 all-season steel radial,to a wider 175 60R-14 Michelin MXV4 Green-X 44-psi all-season radial,There was no loss in mpg,so I know,at least for the CRX,that the relationship worked.The ride improved,wet and dry traction improved remarkably,and tread is supposed to go 83,000 miles.From a safety aspect,I think it's a good way to go.And with fairings and strakes you can still probably compensate for the increase in frontal area.Net gain.

I'm skeptical about the impact of wider tires on fuel use. A wider tire with the same contact patch as a narrower one is also deforming less, which has to count for something.

Me, too! But rather than just wonder, I pulled out my Excel spreadsheet and did some calculations.

According to the first post (not sure where the data came from!) The average for a 10mm increase in tire width was 0.003 or about a 10% change in Cd.

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%)

That means the effect on fuel economy of a 10mm change is 0.03% to 0.11% - wider being worse.

Smithers reported to the California Energy Commission on a study of tire sizing and its effect on RR. From that data, the effect a 10mm increase in width has on RRC is about 3% (if you assume there are no other changes)

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%)

This means the aero effect of the width of tires is more than offset by the improvements in RR.

This may seem counter intuitive as wider tires ought to have more RR, but most of the effect is coming from the less deformation as winkosmosis suggested.

I always wondered how Fred Flintstone was able to propel his car with just his feet. I guess those ultra-wide tires on his car produced some really low RR values.

I'm sure the width vs. RR curve has a point of diminishing returns.

I think I saw something about extremely low RR for stone tires (not Firestone), and a significant flywheel/gyroscopic effect once in motion.;)

I think wind tunnel test are done with the wheels stationary. A spinning wheel wouldn't show a change in frontal cA, but there must be a dynamic change in cD for an eggbeater style of wheel. Any quantitative studies on this?

So that's a typo? Wider tires really do decrease rolling resistance? I knew it...

Got a link to an article about the width/efficiency?

Edit: Is this it? Tire Fuel Efficiency Consumer Information Program | Federal Register Environmental Documents | USEPA

case-specific basis
 

I don't know if sweeping generalities serve the greater good.Certainly exceptions will abound as soon as any claim is made about any topic.
With respect to my experience,Michelin's claims about fuel-savings were so clever,I went for the bait.
And after spending $1,000 (US) for their tires and 14-inch wheels to put them on,I was rewarded with Zero-mpg gain!
The only reason I didn't immediately blow my brains out was that I rationalized that I had gained excellent dry and wet traction,a quiet smooth ride,and long tread life AT NO EXPENSE to mpg,in spite of the aggravated frontal area.
It could have been different for a different vehicle,different tires.With a 345,000 mile data base on the CRX I'm pretty confident with my numbers.