Tire Size Help

Backspacing -- The distance from the mounting surface of the wheel to the extreme back side of the rim.
Wheel Width -- The width of the wheel bead seats. Not overall wheel width! Most 6" rims have an overall width of 7"s
Here is an explanation of tire fitments and what all the numbers mean, and how they correlate with the old sizes.
First, lets look at metric sizes of today. Ex. 225/70R15. The first number(225) represents the cross section width of the tire at its widest point(sidewall) in millimeters. The second # (70) represents the Aspect Ratio. This a big term meaning that this tire's sidewall height is 70% of the cross section width, or 70% of 225mm. The R means simply Radial tire construction. And finally the 15 represents the rim size.
Now, if you want to go to a wider tire, but you don't want to change the overall diameter/height of the tire the rule of thumb is this:
Take our first example 225/70R15 To go wider you would do this. 235/65R15 wider yet would be: 245/60R15 and one more would be 255/55R15 You see the connection here. When doing what is called a "plus fitment", you increase the width number by 10 and decrease the Aspect Ratio number by 5. That is if you keep the same rim diameter. There are different rules for changing rim size that I won't get into here.
So, if you had a 225/70R15, and you went to a 255/60R15, then you would not have the same overall diameter/height. You would be off 5%.
On tires that came out on these early 60's cars Ex. 7.50/15, the cross section was represented in inches. It was still measured at the fatest part of the tire/sidewall. The tread on that tire was about 5 inches wide. The reason there was no Aspect Ratio number listed is because the sidewalls were always a 1 to 1 ratio. There were no percentages. The sidewall was the same as the width. Then in the late 60' and early 70's they changed to letter designations and used an aspect ratio for the first time. Ex. G-78/15. Remember those? G stood for the width which in this case was about 215mm. Then the 78 was the Aspect Ratio. and you know what the 15 meant. From what I understand the old designations roughly translated like this. F=205,G=215,H=225,L=235. That was if the 78 aspect ratio was used. So a G-78/15 was about the same as a 215/75R15. Those changed if you went to a 60 series or 70 series. The old 78 series matches closely with the 75 of today.