What's the difference between a size of 235/75-15 and 30/9.5-15?

 

Here's how to calculate size
The first number is the width in millimeters.
The second number is the aspect ratio, you need to put a decimal in front of it (or just use the number as it is using the equation below)
The third number is the wheel size.

Multiply the aspect ratio with the width (1st)*(2nd) and convert from mm to inches.
now, multiply that number by 2 and add to the wheel size. That is your outer diameter of the wheel in inches.

( (1st)*(2nd)/2540 )*2 + (3rd) = outer diameter

2540 converts millimeters into inches and converts the aspect ratio into the proper decimal.

Tire size chart

235/75-15 = 29" diamter, 9.2" wide
30/9.5-15 = 30" diamter, 9.5" wide