AKRON—How can tires of the same nominal size designation vary in diameter by as much as an inch or more? One reason is that the Tire & Rim Association's voluntary guidelines, which most manufacturers follow, afford designers considerable latitude in determining tire section width and outer diameter—provided the sum of these two dimensions meets or exceeds the ``minimum size factor'' established by the TRA.
Put another way, when a tire's section width is added to its outer diameter, the total must exceed the number listed for that size under the ``minimum size factor'' column in the TRA yearbook.
In the case of the ill-matched General Ameri. way and Michelin XH4 tires cited in the accompanying article, the inflated Ameri.way's section width measured 7.77 inches, which, when added to the tire's 27.11-inch outer diameter, totals 34.88 inches—a sum comfortably above the 34.55-inch minimum size factor established for size P205/75R15.
The XH4's 8.30-inch section width, when added together with its 26.49-inch diameter, yields a total of 34.79—which likewise exceeds the TRA's minimum size factor for that designation.
Thus, the dimensions of both tires were well within industry guidelines even if they varied enough in circumference to damage a four-wheel-drive vehicle's transfer case.
At the extreme, a tire designer could come up with a P205/75R15 tire as large as 27.3 inches in diameter or as small as 26.22, said a tire engineer who asked not to be identified.
That amounts to more than an inch of difference when the tires are new. It doesn't take into account the additional size difference that will accrue if one tire is brand new and the other is worn.
``That's why vehicle and tire manufacturers are constantly making recommendations to replace a set of tires as opposed to replacing individual ones,'' he said.