There's only 10 mm difference in width between a P245 and P255 tire. That's less than a 1/2-inch, so one could think it's close enough make the difference insignificant. Actually, there are ways to use these sizes at the same time and ways to avoid.
If you have a pair of P245 tires and a pair of P255 tires to put on your car, put one size on the front wheels, the other size on the rear wheels. The larger, P255 tires should go on the rear of rear-wheel drive vehicles or on the front of front-wheel drive cars.
Putting mismatched sizes of tires on the front of your car may result in the car steering or pulling one way or the other as you drive. This won't be noticeable if the mismatched tires are on the rear wheels, but in either case, expect the tires to wear out significantly sooner than if you had a matched set.
The P245 and P255 numbers only measure the width of the tire. The other numbers showing a tire's size include the rim diameter on which the tire will fit and the height of the tire's sidewall expressed as a percentage of the width. A P255/70-16 tire, for example, has a tread width of 255 mm, the tire's sidewall height is 70 percent of 255 mm and the tire will fit on a 16 inch rim. These numbers are important when choosing and matching tires for your automobile.
Never use mismatched tires on vehicles with 4-wheel drive (4WD). The wheels on a 4WD vehicle have to turn at identical rates; with mismatched tires, some wheels will turn slightly slower or faster than others.
