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I bought a set of 4 LTZ 18's and I was wondering whether or not I should spend 150 at the dealership to get the speedometer calibrated or if the change is so minimal that I can avoid it. I haven't put them on yet. I'm driving a 2008.
 

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I bought a set of 4 LTZ 18's and I was wondering whether or not I should spend 150 at the dealership to get the speedometer calibrated or if the change is so minimal that I can avoid it. I haven't put them on yet. I'm driving a 2008.
Changing wheel diameter doesn't affect speedometer calibration. Changing the overall circumference (pi * tire diameter) of the tires does. The speedometer is calibrated to how far the tires travel with each revolution.

Compare the overall circumference of your original tires to the new ones to determine the difference that you'll see reported by your speedometer.
 

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Changing wheel diameter doesn't affect speedometer calibration. Changing the overall circumference (pi * tire diameter) of the tires does. The speedometer is calibrated to how far the tires travel with each revolution.

Compare the overall circumference of your original tires to the new ones to determine the difference that you'll see reported by your speedometer.
I agree. Changing the rims changes the inside diameter of the tires. But the outside diameter is determined by the other tire parameters.

I'm not sure how to cross the tire size to get from one rim to the other. For example (out of thin air) suppose you have a P235-70R16, the 18" cross isn't just swapping 18 where the 16 is. One or both of the other numbers will change as well in order to maintain the same outside diameter.

The numbers involve ratios rather than exact dimensions, and I always have to look them up. Whoever came up with system of tire designations was just a little different, I think :)

That said, given the actual designation for the stock 16" tires, it shouldn't be too difficult crossing them to an equivalent 18".

Doug

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I agree. Changing the rims changes the inside diameter of the tires. But the outside diameter is determined by the other tire parameters.

I'm not sure how to cross the tire size to get from one rim to the other. For example (out of thin air) suppose you have a P235-70R16, the 18" cross isn't just swapping 18 where the 16 is. One or both of the other numbers will change as well in order to maintain the same outside diameter.

The numbers involve ratios rather than exact dimensions, and I always have to look them up. Whoever came up with system of tire designations was just a little different, I think :)

That said, given the actual designation for the stock 16" tires, it shouldn't be too difficult crossing them to an equivalent 18".

Doug

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Determining the overall diameter of a tire can be confusing. It requires a little math. I'll run through it for a 235/50-18 tire, but the same calculations would apply for any size tire ...

As we know, the first number, 235, is the section width expressed in millimeters, and the second is the aspect ratio multiplied by 100. The last number is the diameter of the mounting wheel. Let's call them A, B, and C, in the order described.

First divide B by 100 to calculate the actual aspect ratio.
50 / 100 = .5

Next, multiply A * B to obtain the sidewall height in mm.
235 * .5 = 117.5 mm

Now, divide the result by 25.4 (mm/in) to obtain the sidewall height in inches.
117.5 / 25.4 = 4.626 inches approximately

Next, multiply that number by 2 to account for both the top and bottom sidewalls.
4.626 * 2 = 9.25 inches approximately

Add that number to C (the wheel diameter) to obtain the tire diameter. This is the overall height of the tire.

9.25 + 18 = 27.25 inches

It is also the overall diameter of the tire. Tire retailers will typically round the number up to 27.3 inches, but 27.25 is more accurate.

:)

 

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I've done the exact same thing and the easiest way to check is to use a gps devise, easiest is your phone on waze or something similar that shows your speed and compare it to the cars speedo, you'll see just how far off you are if any...
 

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I've done the exact same thing and the easiest way to check is to use a gps devise, easiest is your phone on waze or something similar that shows your speed and compare it to the cars speedo, you'll see just how far off you are if any...
Us old school guys would set the cruise control on 60mph and time the distance between the mile markers :)

Doug

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