I bought a used B7800 at an auction thet has 12.4-16 r4 goodyear rear tires. Both are in good shape, but the diameter is 3" different causing the tractor to lean. The tractor is 4-wheel drive also. My 2 questions are: Which is the correct diameter? (they are approx 37 1/2" and 34 1/2") and Where...