Just wanted to post again for folks looking to go with different size tires. The size marked on the sidewall is not the exact width of the tire. It just provides nominal guidance as to size. Rim width can also impact actual width once installed. You should understand the manufacturers actual published section width for a given rim width and how to estimate the impact if you have a different width rim (the claimed industry rule of thumb is that every + or - 1/2" change in rim width affects section width by a corresponding + or - 1/5").
Here is a thread from another forum with a good example of two marked 255 width tires mounted to the same width rim. There is ~ 1/2" difference in the actual width of the tires.
255 tires - Not all created equal! - AudiWorld Forums
Two tires of different brands marked as 255 width, mounted to same width rim, yet actual width is physically ~ 1/2" different.
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Some manufacturers publish that their width and height variances are up to 3-4% or more. Online tire calculators will only tell you so much and they shouldn't be relied on for anything more than anecdotal/directional estimating. If you want to know for sure, you will need to mount and measure (or compare to your current mounted tire), and test fit the tires on your vehicle.
It is theoretically possible to find a 255 marked tire that is almost the same installed width as a 275 marked tire (if the 255 was on the high side of variance and the 275 on the low side). Stop relying on an online calculator that says warning variance exceeds 3% when it may not really be the case. Do your homework...
Also, for those concerned about their AWD systems getting messed up by having different size tires on the same axle or different size tires on FR vs RR axle, this is also why you must verify that replacement tires are actually the same size. If you were to put 2 different brand tires marked with the same size on the same axle, they may have a size/circumference difference. Same thing if you have one brand on the front axle and a different brand of the same marked size on the rear axle - they may not be the same actual size/circumference and could cause damage to your driveline if the variance is large enough. I know no one here runs mismatched tires, so feel free to disregard.