Does tire width affect MPG?

[MCaesar]

	Quote:			Originally Posted by VanillaRT4me
	Wrong, MCaesar. Let's be clear about this. Two tires of the same construction, model and brand are compared. The only difference is the width with all other conditions being equal. So, the car weighs the same during the comparison, atmospheric conditions and tire inflation are a constant as well. What will you find? The contact patch area remains the same, but just changes in orientation and shape. I hate to break the news to you, but this are the laws of science we're talking about here and they don't change for Dodge Magnum owners. Most people have difficulty understanding this, but those of us in the aerospace industry do understand and it's essential that we do.

Obviously I was referring to two different brand or model of tires.

As for the laws of science, you might want to check yourself as I have a degree in Physics from a top school just like you so try that crap on someone else.

[VanillaRT4me]

	Quote:			Originally Posted by MCaesar
	Obviously I was referring to two different brand or model of tires. As for the laws of science, you might want to check yourself as I have a degree in Physics from a top school just like you so try that crap on someone else.

There is no need to resort to attacking fellow members on this forum. Your response is disappointing, to say the least.

If you're going to include tires from different brands, sizes, types, and different conditions in the comparison, then anything is possible. In this case, a single honest answer cannot be given, one way or another, as to the size comparison of the tire contact patch.

Have a nice day.

[Scott]

	Quote:			Originally Posted by VanillaRT4me
	Wrong, MCaesar. Let's be clear about this. Two tires of the same construction, model and brand are compared. The only difference is the width with all other conditions being equal. So, the car weighs the same during the comparison, atmospheric conditions and tire inflation are a constant as well. What will you find? The contact patch area remains the same, but just changes in orientation and shape. I hate to break the news to you, but this are the laws of science we're talking about here and they don't change for Dodge Magnum owners. Most people have difficulty understanding this, but those of us in the aerospace industry do understand and it's essential that we do.

MCaesar's response seems reasonable and measured. Before you complain further, reread your statements and put yourself in the other guy's moccassins. Hint, check the dictionary definition for condescending.

Whatever physical principles you are appealing to, they don't necessarily trump observations. The principles you cite may not be applicable because they don't take some real-world variables into account. So, what about the observations of contact patch of different width tires on glass?

Scott

[VanillaRT4me]

	Quote:			Originally Posted by MCaesar
	Of course the contact patch changes. It will change even for different tires that are the same size. The contact patch will change depending upon the weight of the car and the construction of the tire and firmness of the rubber/sidewalls. One car mag did a great test of this 2 years ago by parking different tires on a glass plate and taking pics from underneath.

Scott, can you honestly say I was the only one being condescending here? Read the first paragraph in response to my posting. Tell me that was polite, if you honestly can.

Sure, let's go with the highly scientific "tires on glass" study. I give up. You win.

[rappstr]

It seems to me that narrower tires are often designed to operate with considerably greater tire pressures which would in-turn give a smaller rolling footprint and greater mpg.

[Hal'sMag]

255/55/18's fit fine. The control arm clearance doesn't change. The tire is just 1/4" taller in radius and 1/2" diameter, and 1.2" wider.
The extra 3" of tire width has no aerodimanic impact on a shoe box the size of a Magnum.
Reset your Tire diameter for your speedo.
RPM's drop slightly at the same speed. See tire size calculator.

I've found no difference in MPG, just in looks.

If the stock tires you're replacing are Coni self sealing, anything will be a lighter tire. That 1/2" band off goop on the inside is heavy!

I'd pay more attention to load range of the tire and traction rating.
I bought XL tires with a fairly high tread wear rating, and they ride significantly stiffer than the coni's did.

[TxSwimDad]

Generally i agree that tire width doesn't really make much diff, but I will say that the tread of my tires are seen much more than the stockers were originally.

I have been very pleased w/ the Kumho's I got the 255/55/18 on the stock RT rims. Price is good for a tire that I get 20k miles out of with my spitited driving habit.

[2005rtmag]

I just went through this. I replaced 235/55x18 Michelin MXV4 Energy tires with 255/55x18 Michelin MXV4 Energy tires. I adjusted my speedometer/odometer with my superchips to keep the it correct. My mileage went from 24.0mpg to 21.6mpg running the same trip on the interstate that I make every weekend.

Oh, the tire pressure in both sizes was 35 psig. I then checked the 255's with 45 psig just to see if that would make a difference. It didn't make any difference.

[jg]

	Quote:			Originally Posted by Hal'sMag
	The extra 3" of tire width has no aerodynamic impact on a shoe box the size of a Magnum.

Not true. Frontal area and coefficient of drag (Cd) are THE drivers in aero effiency. Cd is a HUGE determining factor in highway fuel economy. Rolling resistance and the actual shape of the section of the tire are also factors.

If you run wider tires, how can it not effect fuel economy? I'd bet this is the reason the tires on most Chrysler produced vehicles are so narrow. Honestly, a 225 width tire on a premium/luxury car? What other reason is there? ...aside from weight. They're trying to make up for powertrain/chassis inefficiency.

The findings in the post above prove this.

[srt8_hmi]

Man this is painful. I will send you the extra money to cover the Lost MPG's if this thread is locked! We have to professors of science arguing over tires!

[TxSwimDad]

I would suggest comparing the frontal view of the Magnum w/ stock width tires versus the 255. I know I see alot more tire sticking out w/ the 255's. I just added the Challenger fender extensions to the Mag to cover some of it. Looks better not seeing so much tread.

[jg]

	Quote:			Originally Posted by srt8_hmi
	Man this is painful. I will send you the extra money to cover the Lost MPG's if this thread is locked! We have to professors of science arguing over tires!

Here's some more reading. Wish I got paid like a scientist!
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).