tire width vs. drag (Cd)

[MetroMPG]

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



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/projecte 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).

[Frank Lee]

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

[ATaylorRacing]

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.

[aerohead]

Originally Posted by ATaylorRacing

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.

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.