Im replacing my tires, i currently have the factory installed 275/65/18 BFG Rugged Trail's white wall, and i have seen 275/70/18's same tire style, and they are cheaper? I also looked at the specs and the only real difference other than the size is the tread depth is only 10/32 as compared to 13/32 on the originals(275/65/18). I know thats not a lot, but there must be another reason the tire is cheaper from the same place. To be honest i really dont know what this middle # effects? I know its a bigger tire, which it said the diameter was at almost 33" versus 31.5?" on the originals...so its taller obviously, but it also said the wheel widths ranged from 7-9, and the originals claim 7.5-9.5? Maybe im just dumb, but it wasn't making sense on the wheel widths...I guess im just trying to see if this tire is going to wear out quicker with the lesser tread depth, and if its going to look right, not sure wqhat that middle # reflects....Need some help wandell...thanks

 

The first number is the width in mm. The second number is the aspect ratio which is the ratio of height to width.

For example a 275/70/18 is 275 mm wide, 70% of that 275 is the height and it fits and 18 inch rim.
A 275/65/18 is the same width but 5% shorter.
A 265/70/18 should be just a hair narrower and shorter than a 275/70/18. Shorter because since the second number is a ratio, if the width is less, the height will be less if all of the numbers are the same.
www.4lo.com has a metric tire calculator that will give you the tire size in inches if you prefer that.

Is the 275/70/18 that you a P series tire? Thats a light duty passenger based tire and not a truck based tire. Your load carrying ability would be much less.

Edit: Noticed that I am only 4 months late in reply. Didnt notice the date.