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Question about tire size

Lazydog

Lazydog

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I know many have thought about it, but no one has asked about it.
So here we go.

Stock tires are 245/75/16

245 Tread width
75 Wall height
16 Rim size

My question is this, buy changing the tire width how does this affect the speed?
I could understand this if I were to change the wall height but I don't understand why when change the tire width.

Looking at the sticky for the Tire Calculator is what started me thinking about this.

By placing 265/75/16 on my truck the calculator says my speedo is off by 3.9%. Tire hight is the same so how can this be?

I've been :banghead: trying to figure this one out.
 
Width wont change the speedo or the gear ratio_Only height can do that.Isn't a 265 taller than a 245 by a little more than an inch?
 
okay here is how tire size works, the 245 is 245mm of tread width. the second number say 75 like your example is aspect ratio or percentage of the tread width. so it is 75% of the 245mm. so when you add 20mm and bring it to 265mm 75% is higher, thus taller tires..
 
here it is from tirerack.com. better explanation


Section Width
Following the letter(s) that identify the type of vehicle and/or type of service for which the tire was designed, the three-digit numeric portion identifies the tire's "Section Width" (cross section) in millimeters.

P225/50R16 91S

The 225 indicates this tire is 225 millimeters across from the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified width wheel. This measurement is also referred to as the tire's section width. Because many people think of measurements in inches, the 225mm can be converted to inches by dividing the section width in millimeters by 25.4 (the number of millimeters per inch).

225mm / 25.4 = 8.86"

Sidewall Aspect Ratio
Typically following the three digits identifying the tire's Section Width in millimeters is a two-digit number that identifies the tire's profile or aspect ratio.

P225/50R16 91S

The 50 indicates that this tire size's sidewall height (from rim to tread) is 50% of its section width. The measurement is the tire's section height, and also referred to as the tire's series, profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the lower the sidewall. We know that this tire size's section width is 225mm and that its section height is 50% of 225mm. By converting the 225mm to inches (225 / 25.4 = 8.86") and multiplying it by 50% (.50) we confirm that this tire size results in a tire section height of 4.43". If this tire were a P225/70R16 size, our calculation would confirm that the size would result in a section height of 6.20", approximately a 1.8-inch taller sidewall.
 
I would like to thank everyone for clearing this up for me.
Now I will be able top sleep better at night.

Have one on me. :cheers2:
 
The section width is not neccessarily the tread width.

The section width is the sidewall-to-sidewall dimension. The tread width is the part of the tire that touches the ground, generally narrower than the section width.

(See picture below)​

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