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Old 06-17-2008, 01:28 PM   #1
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Tire size and aero

the previous owner of the car put on what seems to be wider tires on the car and they stick out alittle from the wheel well. i wanted to make a rear wheel skirts but if i did the expensive cardboard skirts would gride up against the tires. my question is are the these tires hurting my aero and in turn my mpg?
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Old 06-17-2008, 01:36 PM   #2
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To answer your question fairly. YES.
One size up will not affect your mileage much but any larger will hurt your fuel mileage. Wider means more area for air to pass by and more road friction.

If stock is 225 wide and you put on 235's on it will be a minimal impact on gas mileage. Put 255's on and you will see a hit with the gas mileage.
Being the tires are extending past the wheel wells they are much too wide.
Tires excessively wide will also hurt the handling of the car also.

In reading the other thread you started you spoke of a larger tires on 15" vs the stock 14" rims. Depending on what tires they put on it could affect you speedo and MPG calculations.
Just to show and example 185/14/65 is stock. The previous owner swapped those out for 195/15/60's. The speedometer could be off as much as 2 to 3 mph.
Speedometer says 65 mph when your really traveling 67-68 mph.
Your traveling farther than your speedometer is showing. Thus it is going to show lower gas mileage then what your really getting. Plus at highway speeds your car is running faster and thus burning more fuel then it would at speedometer registered speed.

That is why many compare their speedometers to GPS units. My car is pretty close to dead on. The speedometer, Scan Gage, and the GPS all match at any given speed.
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Old 06-17-2008, 01:36 PM   #3
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I would say that wider tires would definitely have a higher rolling resistance than a narrower tire. I don't know about the aero aspect, but I would assume it slightly changes the aero aspect as well.
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Old 06-17-2008, 05:33 PM   #4
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Jay, you are probably wrong about rolling resistance. Check the link in my sig. All other things being equal, the wider tire will almost certainly have less rolling resistance.

As for aero (and weight), the wider tire would almost certainly be worse.

ALS, why do you say that wider tires would hurt the handling?
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Old 06-17-2008, 06:04 PM   #5
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I'm not sure I believe more rubber on the road is less resistant if the tires are the proper size for the vehicle and properly inflated.
A taller tire would serve to better fill in the open space in the wheel well and a narrower tire would bring the tire inside the fender and out of the air-flow. If fender skirts are up for consideration, having the tire more to the inside would allow a lower skirt.
On handling I found a narrower tire easier to turn.
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Old 06-17-2008, 06:18 PM   #6
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Quote:
Originally Posted by theholycow View Post
Jay, you are probably wrong about rolling resistance. Check the link in my sig. All other things being equal, the wider tire will almost certainly have less rolling resistance.

As for aero (and weight), the wider tire would almost certainly be worse.

ALS, why do you say that wider tires would hurt the handling?
My logic is that the narrower tire has less surface area in contact with the road surface for rolling resistance to act upon. The wider tire has more traction and more surface area in contact with the ground. This seems to me to be a larger surface area for rolling resistance to act upon. Of course I haven't been a Physics classroom since the early 90's. I could be wrong. I would suppose that there is some sort of coefficient for rolling resistance that could be broken down to the square inch, and then be multiplied by the size of the tire's contact patch in square inches. I would figure that's why overinflating tires reduces rolling resistance. The tire gets harder (It does not flex as much), and the contact patch gets smaller.

-Jay
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Old 06-17-2008, 06:40 PM   #7
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Quote:
Originally Posted by Jay2TheRescue View Post
My logic is that the narrower tire has less surface area in contact with the road surface for rolling resistance to act upon.
You didn't read the link in my sig, did you?

Rolling resistance doesn't come from tire surface in contact with the road surface. It comes from the sidewall deformation for the length of the contact patch. Given the same pressure and weight, the contact patch will be the same size; so a wider tire will have a contact patch for less length, deforming less length of sidewall.

More detail is at that link.
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Old 06-17-2008, 07:01 PM   #8
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Quote:
Originally Posted by theholycow View Post
You didn't read the link in my sig, did you?

Rolling resistance doesn't come from tire surface in contact with the road surface. It comes from the sidewall deformation for the length of the contact patch. Given the same pressure and weight, the contact patch will be the same size; so a wider tire will have a contact patch for less length, deforming less length of sidewall.

More detail is at that link.
No, honestly I did read that, but the tire size calculator did not really say much about tire width. It had everything else though.
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Old 06-18-2008, 05:11 AM   #9
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Now that I've had a decent night's sleep, I've started thinking about this, and whether or not I'm doing the right thing... When I bought The Beast the dealer had just put a brand new set of P265/75/R16 Cooper Discoverer H/T tires, (Treadwear 520, Traction A, Temperature B). The OE size is 245/75/R16. Do you think I really have anything to gain by changing the tire size?

No matter which size I choose I will most likely buy a high quality Michelin tire, either the XC LT4, or the LTX A/T2. My brother in law manages the service dept in a local dealer and can get me a really good deal on them.


http://www.michelinman.com/action/ti...+Cab+Short+Box
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Old 06-18-2008, 08:20 AM   #10
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Ah, I see the problem. I failed to be specific enough...I should have said to read the "tire width" canned post, but you went to the size calculator.
http://www.gassavers.org/showthread.php?t=7713
That's the in-depth analyzation of rolling resistance.

The tires that you have now are taller as well as wider than the OE size. They probably gain FE for taller gearing and less RR, but they probably lose a little due to aero concerns. If your speedometer and odometer weren't adjusted, your mileage calculations and speed are slightly inaccurate.

What pressure are you running in them? I may have already asked you that.
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