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Tire sizes are commonly stated as tread width/aspect ratio/wheel diameter. So if you have a tire that's a 265/70-R17, it works out like this.

Tread Width = 265mm
Aspect Ratio = 70% of Tread Width
Wheel Diameter = 17"

So; 265mm x .70 = 185.5mm (This equals the distance from the face of the tread to the wheel bead.)

185.5mm / 25.4mm = 7.303" (This converts the above value from mm to inches.)

7.303" x 2 = 14.606" (This represents the distance from the face of the tread to the wheel bead at both the top and the bottom of the tire.)

14.606" + 17" = 31.6"

So the total outside diameter of the tire = 31.6"

To determine the circumference of any tire, take the total outside diameter and multiply by pi (3.1416).

For the above tire, you would multiply 31.6" by pi. Hence:

31.6" x 3.1416 = 99.29" circumference

To determine the revolutions per mile for the tire:

There are 5280' per mile. First, you need to convert feet to inches.

5280' x 12" = 63,360" per mile

Then divide the inches per mile by the circumference of the tire to find the revolutions per mile.

63,360" / 99.29" = 638.13 revolutions per mile

Keep in mind that these are the "calculated values" and that actual individual tires by various manufacturers will vary from the calculated values. However, this will get you very close.

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'04 Ram QC SLT 4x4 Hemi / Flame Red / 3.92s / 4-Wheel ABS / AR Vanguards / 285/60-18 Cooper HT+ / Bilsteins / Hellwig Sway Bar / Westin Ovals / BedRug / Bed Xtender / LoRider Lid / Infinity +XM / 17769 DynoMax / '03 Tailpipe / Mopar Tip / aFe Hi-Breather

I have a problem with the math using your formula and going from a 265-70-17 your formula equals 31.6, by changing the width to a 245-70-17 I come out with 30.5! This is the same tire overall diameter should be the same only difference is width? Where have I miscalculated?

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2004 WHITE RAM 1500 STX package 4.7 REGULAR CAB SHORT BED,RUBBER BED MAT,TINTED REAR WINDOW

It is not the same overall diameter. The tires diameter is determined by the aspect ratio X the diameter-every time on every p rated radial tire. If the width changes from 265 to 245 then the tires diameter will go down UNLESS the aspect ratio comes up. If you had gone from a 265/70 to a 245/80 then things would be different.

I have a problem with the math using your formula and going from a 265-70-17 your formula equals 31.6, by changing the width to a 245-70-17 I come out with 30.5! This is the same tire overall diameter should be the same only difference is width? Where have I miscalculated?

My truck came from the factory with 245/70-17 tires. I purchased new 18" chrome American Racing wheels and wrapped them in 285/60-18 Cooper Zeon XST tires. I then had the computer reset for the revolutions per mile for the Cooper tires, so my speedometer would remain correct.

{[(245 x .70) x 2] / 25.4} + 17 = 30.5" OD

{[(285 x .60) x 2] / 25.4} + 18 = 31.46" ~ 31.5" OD

I'm going to run the factory wheels in the winter, but I don't want to keep having the computer reset for the change in tire diameter. So I picked up a set of 265/70-17 tires, with an OD of 31.6"; close enough to the OD of the 18s I run in the summer.

The only constant in determining tire diameter is the wheel size; 16", 17", 18", 20", etc. The tire width times the aspect ratio, determines the meat of the tire between the pavement and the bead. A 245/70 tire will have the same amount of "meat" whether it's a 16" tire, or a 17" tire. The diameter of the wheel has no affect on the "meat".

If you need some more examples, just give a yell.

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'04 Ram QC SLT 4x4 Hemi / Flame Red / 3.92s / 4-Wheel ABS / AR Vanguards / 285/60-18 Cooper HT+ / Bilsteins / Hellwig Sway Bar / Westin Ovals / BedRug / Bed Xtender / LoRider Lid / Infinity +XM / 17769 DynoMax / '03 Tailpipe / Mopar Tip / aFe Hi-Breather