Learnin my A B C 's tire size

Here is my $0.02. If you are going to use the same wheels (that means the backspacing won't change), here is what I would do. Accurately measure the clearance you have on both sides of the tire. Let's say you have 1.25" between the tire and the rear quarter panel lip and 1.5" between the inside of the tire and the inner wheel housing (or springs, or whatever the tightest clearance is). Write down the smaller number, in this case 1.25". Decide on how much clearance you want. Let's go with 0.25". Subtract that from the 1.25" number leaving 1.0". That means you can get a tire with a cross section 2" wider than the current tire. I say 2" because half of the extra cross section would be on the inside and half on the outside. And with 25.4mm per inch, that means you can get a tire with a cross section that is 50.8mm greater than the current tire. So if the current tire is, let's say, a 225-70-15, you should be able to go with a 275-70-15. If you need to consider the height change, use this formula: (change in cross section/25.4)*(Aspect ratio/100)*2. So going from a 225-70-15 to a 275-70-15 would be: (50/100)*(70/100)*2=2.76 inches taller tire. This will help to zero in on a size without a bunch of trial and error. If you change the wheels, that means the back spacing will change, and this won't work. Hope this helps.