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.

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!

baseline
 

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

Any change in diameter?

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!

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.

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

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?

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?

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).

Hucho?
 

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%.

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.

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

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

Actually, saying a wider tire alone decreases rolling resistance is half the story.

Any tire shape that increases the contact patch with the road decreases rolling resistance.

The main reason why?

The larger contact patch allows the tire carcass to flex less, which in turn takes less energy to roll.

Pumping a tire to a higher pressure accomplishes the same thing, but not as greatly as going to a larger contact area.

Why?

Because as you pump up the tire, what happens to the contact patch?

Right, it gets even smaller, which then in turn causes the carcass to be stressed over a smaller area.

Instead, think ATV tires or similar and you get the idea on a better approach.

However there are side effects:

1) A big balloon tire can easily upset the handling of the vehicle
2) Offers more frontal area to the wind, with an increase in drag
3) In extreme cases can raise the vehicle CG and affect handling
4) More unsprung mass, which lowers ability to respond to bumps
5) More gas to accelerate the mass of the larger tires
6) Harder on brakes when slowing down
7) Less mechanical advantage of disks/drums, making pedal pressure go up

Jim

Thanks for that. And for posting your web site - a great resource.

I learned something new in this thread.

Prius Experience Reconciliation?
 

Hi All,

What am I missing? The experience in the Prius comunity is that the bigger section tires have worse mileage. Dramatically worse. For example, people who have put on 17's (225 section?) have a hard time toping 40 mpg as reported on Prius chat, in 2nd Gen Prius. Wheras people with the 185R15's easily top 50 mpg. Both the tires have about about the same diameter.

Also, a constant load rating tires that is bigger in diameter will have a smaller section (similar enclosed air volume). One would expect the larger diameter tire to have less rolling resistance due the smaller flexing.

Like most everything there's likely a continuum.

I don't think this thread is meant to suggest that wider is automatically better.

I would assume that "wider = lower Crr" is true when you compare tires with otherwise identical design & materials.

If you switch tire design, then it's apples & oranges. Maybe that's what happened with the Prius people.

EG. I've shown that my 155/80/13 tires roll significantly farther than my 175/70/13 tires. But (A) they're different brands, and (B) very different tread designs. (I can't speak about (C) the materials in them).

Thread resurrection.

Been looking into this again. Part of the reason tire width makes such a big difference in the real world is that wheels have such a bad CD that a small change in their frontal area has a big impact on the total CD of a car.

Tires and wheels count for around 50% of the drag in a streamlined car like the Prius but are only a small part of the frontal area. Seemingly small changes in wheels should make a proportionally larger difference in total drag.

The air under a car flows outward from the centre and hits the tires at a yaw angle. Because of this all four tires are, for practical purposes, part of the frontal area. If we assume that's the case this may then double the frontal area change from the usual assumption when estimating changes in FE from wider tires. (I didn't do this in the following calculation)

At highway speeds these changes would far outweigh changes in RR.

This would seem to back up the assertions of many Prius owners who have seen a big reduction in their MPG from a increase in tire width.

Plugging some numbers into the calculator also backed up my thoughts on the subject.

I got about a 4% reduction in mpg from wider tires using the following:

Data from http://www.energy.ca.gov/transportat...%20Testing.pdf(thanks to Capriracer for finding that.)
In that study the CRR dropped .002 going from a 175 to a 235 which closely matches the change in width in the Hucho study Darin posted.
CD + .026 (Hucho)
Frontal area + .4 square feet. (calculated)

Results at 60 mph Narrow tire: 45.67 mpg wide: 43.74 mpg: ~4% difference
@ 75 mph 5% difference
At 40 mph it's a wash.

There are no hard numbers showing a strong correlation between tire width and RR. There does seem to be a slight reduction in CRR for wider tires of the same dia. but there would need to be a huge reduction for it to wash out the hard CD numbers we have from Hucho.

That's my 2 cents based on the limited data and using the calculator.

When going to a larger diameter _wheel_, weight increases rapidly. An aftermarket alloy 14" wheel weighs 14.6lbs, but the 17" version of the same wheel weighs 22.5lbs. An extra 32 pounds of unsprung rotating weight. If the same width tire is used, the tire may be slightly lighter (less sidewall), but nowhere near enough to make up for the increased wheel weight.

Big wheels are for styling, unless you need room for big brakes. Performance tests have shown smaller wheels generally perform better, with the possible exception of slight improvement in braking (possibly due to better brake cooling).

What goes to prius there are good points shown, but I say few more:

- Prius aerodynamics and add on parts is designed to work best on that car which gives that 0.25 cd. So if you put wider tires air dont go smoothly pass the tires as they are optimised. That about one inch more wider tires dont seem much but if look that at you 3D computer screen you immediately see that who has designed something this much wrong when designing tire spats etc.

- Larger wheels and tires weight more, that has huge effect on city driving and in P&G "mode"
- rims might have less aero look. Prius original rims are optimised for aero vs looks to prius. Aftermarket rims only for looks.
- low profile tires are not best for rolling resistance

New Michelin Tires Designed For EV's and Hybrids

Michelin thinks 175/70 R10 would be best bang for buck when it comes to fuel economy tire. So 175 tire can have enough load ratings vs low rolling resistance characterics. Sad that they are not yet availebly :/.

I've often wondered what the rolling resistance of the skinny antique car tires were. The wooden/steel banded horse-drawn wagon wheels have a very low rolling resistance, which they have to be, considering their limited output equine power source. The wheel/tire technology of the early cars was derived from the earlier horse-drawn wheel/tire technology and so would be expected to also have low rolling resistance to make the most of the limited power output of the early automotive engines. A Ford Model A weighs about 2200 lbs and runs on 3" wide tires mounted on a 19" wheel. These wheels and tires are still in productuon. It would be pricy, but it would be interesting to see what kind of mileage and handling/braking a set of these would produce if mounted on a modern car.

and big brakes are (relatively speaking) not needed in the ecomodder world, since we try to minimize the usage of them

Not to be too picky.....
 

According to Tire Guides, Model T's came with 21" wheels (tire size 4.40-21) and those carried over into the Model A until 1930 when the wheels became 19" (tire size 4.75-19).

-a "T" forum

I got my vw in the fall It needed new tires so I went for 155/80x13 instead of the stock 175/70x13, it had 215/60x13 on it by a prior owner.

My new tires are taller than the last ones on it. The new tires are winter tires rather that "all (3) season" tires. I figure that narrow would be good for aero and for winter traction.

I am going to run them all year. I figure that the winter tread will cost some mileage but not getting stuck is worth it. Hoping to see better mileage once the weather gets warmer. I am still running some humongous mud flaps behind them. Going to take them off when I get a chance.

Thanks for sharing some real data on the cd.

I always figured that the size of the tire has nothing to do with tire patch size. It in my mind is a function of tire pressure. You have 500 pounds on a tire and there is 50 psi in the tire. you will have 10 sq inches of contact patch. a wide tire will have a wider patch and a narrow tire will have a longer patch but both have 10 sq inches.

Urban legend- it's not so. Yes contact patch area will vary with psi but not with that math.

I wasn't aware that gas laws of charles and boyle don't apply to car tires.

Well... they don't.

Go measure it. I did.

Prior to reading capri racers conclusion that wider and taller tires will improve FE, I was planning on mounting a set of 15x4 alloy rims with 165/80-15 VW Beetle tires in the efforts of getting the ultimate FE tire/wheel combination for my 'stang.

hmm... i think i have an opportunity to do an ABA test of the effects of larger tires on my mustang ragtop.

I have another mustang with the same exact 14" polycast wheels as whats on the white rag now, except the tires mounted on it are 215/70-14 radials instead of the 195/75-14 tires i am using now.

I'll weigh the both wheel/tire combos and then do my 92.6 mile FE run back to back and see if the 215's will yield better FE numbers.

Word of caution. A-B-A testing means everything else remains the same.

One of the things that clouds this issue is the differences between tires.

http://www.energy.ca.gov/transportat...%20Testing.pdf


This says there can be as much as a 60% difference between tires of the same size. The effect that is important - the difference in F/E caused by tire width alone - is pretty small - 0.1%. This means any test has got to eliminate differences in tires or the effect will be unobservable.

Zonker, if you are going to do this test you need to make sure you have otherwise identical tires - same make, model, and state of wear (New would be good).

And because OE tires are NOT usually identical even within a line of tires, it might be good to post what tires you are considering using BEFORE you conduct the test. I can steer you away from potentially different tires.

It would be bad if you spent all the time and effort to conduct the test only to have the results invalidated because of a poor selection of tires.

I am averaging slightly over 40 mpg this winter and spring in my Saturn (sc1) with narrower but taller tires, 165/80R15 vs 185/65R15. I have just switched the fronts to 205/70R15, the biggest tire that will fit without rubbing on the struts. Unfortunately the front skirts are not working, as the tires have cut the bungee cords and the right side ripped off, so there is some drag penalty. I will report the mileage after the next fill up, in about 3 to 4 weeks. Coast down tests have not been definitive yet, but it seems a bit slower. This may be due to the skirts though. At 62 mph (100 kph) on the speedometer the car is actually at 70 mph on the GPS and the 2 liter engine is at 2200 RPM. I have to make sure I don't speed. The car is a real cruiser.