How much variance is allowable when changing tire size? I have a '99 F-150 with 255/70R16 tires. Can I purchase 265/75R16 or LT215/85R16 tires? If so, how would I know this? What is the maximum that the numbers can be changes from the original tire size? I know that the last number (in this case 16) can not be altered, but I am confused as to the allowable variance in the other two numbers.
Any help would be greatly appreciated!
Thanks,
Greg

 

Well, 255/70R16 means: 255 millimeters wide at the widest point. The sidewall is 70% as tall as the tire is wide, so 255 x .70 = 178.5 millimeters tall. To convert to inches divide millimeters by 25.4. So the tire is 255/25.4 = 10 inches wide, and 178.5/25.4 = 7.03 inches is the height of the sidewall. The 16 means your rim is 16" tall, and there are two sidewalls in the tire height so the tire is 16 + 7.03 + 7.03 = 30.06 inches tall, and 10 inches wide. A direct replacement would be a 245/75R16, a more common size that is quite similar, 30.46 inches tall and 9.64 inches wide. 215/85R16 is 30.39 inches tall, but only 8.46 inches wide, this tire is usually only used on dually pickups and box vans. A 265/75R16 is 31.65" by 10.43", a little taller and wider than what you have. If you replace all four tires and you haver extra room in the wheelwell, they will be alright. Basically you need to use both of the first two numbers to figure out the size, the first one is overall width (not tread width), and the second one is the sidewall height as a percentage of the tire width. Go to the tirestore and get a pamphlet of the tire you want to buy, they list the actual dimensions for the tire as they vary slightly from what they should calculate out to.