random tire size information

I just had to search all over for this information, so I thought it might be handy to have all in one place.

To calculate a tire's diameter (found at tirerack.com):
Tire size example: 195 75 R14
1st we need to get all of the numbers in the same measurement - the first part of the tire size is in millimeters and the last part is in inches. You can use a converter or multiply inches by 25.4 to get millimeters, or divide millimeters by 25.4 to get inches. This works either way, but I'll be using millimeters for the example.

we'll say the 1st number of the size is s (section width)
2nd number is a (aspect ratio)
skip the letter - it is the style of tire, and doesn't play a part in this
3rd number is w (wheel size)
    Aside - if you want to know more about tires, hit up the wiki page

2(s * .a) + w = d

working with our example:

2(195 * .75) + (14*25.4) =
2(146.25) + 355.6 =
292.5 + 355.6 = 648.1mm

If you have changed tire size and want to know how to calculate your new speed (found at ehow.com):

n = new diameter
o = original diameter
s = speedometer reading

  •   If you want to determine how fast you are actually going now, compared to what your speedometer reads:

n * s / o

we'll use 733.5 for our new diameter

733.5 * 55 / 648.1 = 62.25
So if your speedometer says you are traveling at 55 mph, with the new tires you would actually be traveling 62 mph

  •   If you want to know what your speedometer should read in order for you to be traveling at a certain speed switch your diameters:

o * s / n

648.1 * 55 / 733.5 = 48.6
So if you want to go 55 mph, you would need to be traveling 48 mph according to your speedometer.

To determine your actual traveled miles you will need a trip meter. You will need to divide the new diameter by the original diameter and then multiply your metered miles by that number:

733.5 / 648.1 = 1.132
we'll say the trip meter reads 203 miles traveled:
203 * 1.132 = 229.8 actual miles traveled

To figure your mpg, just use the new number of traveled miles instead of your trip meter reading:
229.8 miles / 15 gallons = 15.32 mpg

I know this isn't something I would normally post about, but it needed to be all in one place. I would create handy dandy calculators, but I don't know how to do that.
Good Luck!

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