Metric sizes: 265/70R17 115T
265 is the tire's width in millimeters, sidewall to sidewall.
70 is the aspect ratio: the sidewall height as a percentage of the width. Here the sidewall is 70% of 265 mm, about 185 mm or 7.3 inches.
R means radial construction, which is nearly every tire sold today.
17 is the wheel diameter in inches that the tire fits.
115 is the load index: how much weight each tire can carry at full pressure. T is the speed rating: the maximum sustained speed, here 118 mph.
P and LT: passenger and light truck
A P in front (P265/70R17) means a passenger tire. LT (LT265/70R17) means a light-truck tire, built with stiffer construction and rated to carry more at higher pressures. If your truck came on LT tires, or you tow and haul, stay on LT.
No letter usually means a metric (Euro) passenger tire. For sizing purposes the numbers mean the same thing.
Flotation sizes: 35x12.50R20
Off-road and lifted-truck tires are often sized in inches. 35 is the overall diameter, 12.50 the width and 20 the wheel diameter. It's the most direct way to know exactly how tall a tire is, which is why lifted-truck owners use it.
Working out the real height
For a metric size, the overall diameter is the wheel diameter plus two sidewalls. A 265/70R17 has two 7.3-inch sidewalls on a 17-inch wheel: about 31.6 inches tall.
That number is what has to stay close when you change wheel size. The calculator does the arithmetic for any size, metric or flotation.
Where to find your size
On the tire sidewall, in raised letters. On the sticker inside the driver's door jamb, which also gives the recommended pressure. And in the owner's manual. If the door sticker and the tires on the truck don't match, someone changed them; the door sticker is the factory size.
Questions
Can I put a different size tire on my wheels?
Yes, within limits. The tire has to fit the wheel's diameter and width range, and the overall height should stay within about 3% of the original to keep the speedometer and safety systems accurate.
What do the letters after the load index mean?
That's the speed rating. Common ones are S (112 mph), T (118 mph), H (130 mph) and V (149 mph). Match or exceed the original.
Is 285/70R17 the same as 33 inches?
Close: a 285/70R17 works out to about 32.7 inches. Metric and flotation sizes rarely line up exactly, which is why it's worth calculating.