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Discussion Starter · #1 ·
In my continued quest for aftermarket wheels, I took a short hiatus to look more into the tires. Using the tire calculator at http://www.discounttire.com/dtcs/infoTireMath.dos I was playing around with the various sizes to see how it would vary from stock. In a 20" wheel, the plus sizing I had in mind was a 285/50/20. However seeing this size tire on an actual vehicle was not appealing to me and does not do the aftermaket wheels any justice IMO. I for one prefer a smaller tire sidewall exposed. I should note the the OEM 20" tire size is a 255/55/20. This does bring the sidewall height down by an inch...its progress!

Here are some results I got when using the calculator. Stock numbers listed are actual, the rest are shown as their relation to the stock numbers.

Stock
255/65/18 255/55/20 255/50/20 275/45/20 275/50/20 285/50/20 265/50/20
Sidewall height 6.53 -1.01 -1.51 -1.66 -1.12 -.92 -1.31
Section Width 10.04 .00 .00 .79 .79 1.18 .39
Overall Diameter 31.05 -.1 -1.01 -1.31 -.22 .17 -.62
Circumference 97.55 -.2 -3.18 -4.11 -.70 .53 -1.94
Revs per mile 669.89 .21 22.52 29.50 4.78 -3.65 13.65
Speed at 65 65 64.98 62.89 62.26 64.54 65.36 63.70

Everything seemed to be pretty clear to me except the Revs per mile. What impact does the revolutions per mile have to the vehicle/tire itself? I'm assuming possibly less treadwear life ???. I see that with the 255/50/20, 275/45/20, and 265/50/20 I am able to acheive a more desirable sidewall hieght, but at the expense of both the revs per mile and speedo reading inaccurately. I can deal with the speedo thing as long as it is reading slower than I am actually going.

Anything else to be concerned with besides the speedo and revs per mile with any of the calculations above?
 

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The revs per mile could be real important because some tires have a softer formulation and those don't last as long. So you really also want to factor in the warranties for those other tires. A m/s rated tire will normally wear longer because it is stiffer. Once you get a feeling for how long the tire will last then you could multiply the revs per mile by an estimated milage and get a feeling for how long the tire will last. Then its a matter of price vs looks. Of course, if you are in a cold or rainy climate, it might not matter cause of safety reasons.
 

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I would imagine us in the north, would get longer mileage tire wear due to cooler yr round climate? Heat on rubber is worst enemy.
 

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Discussion Starter · #4 ·
Well I found this online today.

Speedometer Change Calculator
[/color]

"Whenever you change tire diameters, you also impact the accuracy of the vehicle’s speedometer. A tire with a larger diameter revolves fewer times per mile than a tire with a smaller diameter. Change tire diameter and you effectively change the revolution speed of the vehicle's drivetrain, to which the speedometer is connected.

To determine a tire’s revolutions per mile, divide 20,800 by the tire's overall diameter. Example: 20,800/28.9 = 719.72 revs per mile.

With that information, you can now measure the impact a tire change will have on speedometer accuracy:

Determine the revs per mile for both the current and recommended tires. Use either the tire maker’s databook or the calculation method above.

Divide the figure for the current tire by the figure for the tire you’re recommending. Example: If the current tire measures 842 revs per mile and the recommended tire measures 800 revs per mile, the formula is 842/800 = 1.0525.

Multiply the result by a speedometer reading in miles per hour. Use 60 mph as a baseline number. Example: 1.0525 X 60 = 63.15. So a 42 revs per mile change means that when the driver’s speedometer reads 60 mph, his true speed is 63.15 mph.

Remember that the difference grows exponentially as speed increases. If you calculate against 80 mph, for example, the difference is 4.2 mph. At 100 mph, the difference is 5.25 mph."[/color]

So it ties back to the speedometer reading.I recall someone (I think the owner of the red jewel Acadia with 20" wheels), had commented that the dealership could recalibrate the speedometer. Has anyone had this done that can confirm this?
 

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The other thing that is a benefit to going with a 255/50/20 is that your unsprung weight will be roughly 5lbs lighter than the 285/50/20. That will help offset the 20" rim weight penalty which looks to be about 15lbs from the standard AL wheel. So now you are only adding 40lbs unsprung which equals about 60 sprung weight to the car.

The other benefit is your car will be quicker as it is like adding 3:25 gears to the rear instead of 3:16's. Of course, top speed and hwy mpg will go down.

So theoretically your mileage should drop about 2 mpg but you'll be a little quicker off the line. Your speedo will read fast but that will save you $$ in not getting speeding tickets. ;D
 

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imgntht said:
Here are some results I got when using the calculator. Stock numbers listed are actual, the rest are shown as their relation to the stock numbers.

Stock 255/65/18 255/55/20 255/50/20 275/45/20 275/50/20 285/50/20 265/50/20
Sidewall height 6.53 -1.01 -1.51 -1.66 -1.12 -.92 -1.31
Section Width 10.04 .00 .00 .79 .79 1.18 .39
Overall Diameter 31.05 -.1 -1.01 -1.31 -.22 .17 -.62
Circumference 97.55 -.2 -3.18 -4.11 -.70 .53 -1.94
Revs per mile 669.89 .21 22.52 [/color] 29.50 [/color] 4.78 -3.65 13.65 [/color]
Speed at 65 65 64.98 62.89 62.26 64.54 65.36 63.70

:confused: :confused: :confused: :confused: :confused: :confused: :confused: :confused: :confused: :confused:
WOW, My head hurts trying to read this. HAHA, way above me.
 

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Yeah, and you definately don't want to start thinking about those 20's and the effects that the added inertia will have on acceration and braking. YIKES! All that extra weight is roughly at the outside circumference of the rim. So imagine an extra 10-15lbs per wheel swinging around and how much that will impact performance.

No wonder nobody posts on here regarding mpg and handling after they have plus sized 2 or more. ;)
 

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Discussion Starter · #8 ·
bonk1313 said:
Trust me, I didn't figure this out on my own. The tire calculator did all the work, I just typed it into the thread. :thumb:

Also the 255/50/20 will be not as wide as the 285's so you have better traction in the winter. I've gotten it narrowed down to 3 sizes and 2 tire choices. Still working on a rim to put inside them, but we all know that story :banghead:
 

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IMGNTHT- I think you should just go with 22's. 285/40/22's and it looks great. I'm starting to think I won't be happy with the look of 20's.
 

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Discussion Starter · #10 ·
Rod S said:
IMGNTHT- I think you should just go with 22's. 285/40/22's and it looks great. I'm starting to think I won't be happy with the look of 20's.
Oh trust me I have been down that route. Of course it will look better, but then there are less choices in tires of the "All-Season" variety. Or I'd have to swap them back to the factory 18's in the winter. I'm not opposed to it, but my wife is really hell-bent on the ride quality(or should I say her perceived lack thereof)... :beatdown:
 

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Not to hijack the thread here but how much difference in circumference is supposedly allowed before there are issues? Right now there are no winter tires available in the OEM size and I am trying to figure out how far I could vary before having issues, if none were available next year. There is a 255/55-19 but not the OEM 255/60-19 for my Enclave.

Thanks for any education.
 

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Rod S said:
The other thing that is a benefit to going with a 255/50/20 is that your unsprung weight will be roughly 5lbs lighter than the 285/50/20. That will help offset the 20" rim weight penalty which looks to be about 15lbs from the standard AL wheel. So now you are only adding 40lbs unsprung which equals about 60 sprung weight to the car.
The other benefit is your car will be quicker as it is like adding 3:25 gears to the rear instead of 3:16's. Of course, top speed and hwy mpg will go down.
So theoretically your mileage should drop about 2 mpg but you'll be a little quicker off the line. Your speedo will read fast but that will save you $$ in not getting speeding tickets. ;D
Is that an actual 2 MPG or will the odometer read differently with the larger tires thus making it a bogus 2 MPG? Just trying to clarify. :thumb:
 
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