Agreeing with VagabondMatt.
I think you conflated your numbers, i.e. Diameter vs. Radius. In your example, you are adding that 2.25" to the overall diameter, when in fact that 2.25" is the additional radius, so technically the 2.25" increase in radius should be doubled (making it 4.5") before adding it to the overall diameter, which would make it 31.5" tire size.
Which means, you could conceivably put 31" tires on, but I wouldn't as you need to give enough space for tire flex from heat and load carried. 1/2" isn't enough considering that only gives each tire a 1/4" increase to its radius (1/2" overall diameter).