First number is width. Middle number is the aspect ration of the sidewall. This part is confusing because a 75 could be drastically different with width and rim size.
On a stock suspension, 33 is about as large as you'll be able to go. Figure out what type of tire you want to go with. Go to the manufacturers website and look up the specs of the tires. They will have the width and height of the tire in inches printed out to take any confusion out of the picture for you.
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