Ever notice how a tire is round except for the part that's resting on the ground? That part is flat and flanked by short segments of bulging sidewalls because the tire is being pressed into the pavement by the weight of the vehicle on which it's mounted. As the tire rolls, that flattened section of tire pops back into its natural round shape, and the next section of tire is pressed flat, until the entire tire revolves and every part of it has been flexed flat and back again.
This constant flexing creates a tiny amount of heat, which adds up with time. A typical tire revolves roughly 750 times as it travels a mile. At 60 mph, that's 750 revolutions per minute. As the miles roll by, the tire gets hotter, which is why the tire pressure increases by about four pounds per square inch (psi) after a tire has been driven for a while.
If a tire's air pressure is too low, it will get even hotter because reduced pressure results in more flexing. Taken to the limit, maximum tire flex occurs when pressure is zero and the tire is flat. Increased load on the tire has the same effect because it causes the tire to flatten more at the contact patch. Higher speed also makes the tire hotter because it flexes more often. Finally, the outdoor temperature matters. Whatever the mix of tire pressure, load, and speed, a tire will get hotter when it's 90 degrees outside than when it's 30 degrees.
How hot does the tire get? Typical tires experience a temperature increase of roughly 50 degrees after running on the highway for a half-hour. If it's a 70-degree day, this suggests a tire temperature of about 120 degrees. If it's a hotter day, or you're driving faster, or you're hauling a heavier load, or your tires don't have the proper air pressures, they could get hotter. To limit these temperatures, some automakers recommend increasing tire pressures for heavier loads or sustained high speeds.
But if it gets hot enough, any tire will come apart, usually by shedding its tread in large strips from the tire carcass.
