Originally Posted by Ridin' Green
That would be true if you were changing the gearing in the pumps/motors because it would affect how hard the engine has to work to move the wheels/tires, but just going to larger wheels and tires will bump up the top speed slightly, though it should also take slightly longer to get to top speed. The size difference isn't enough to make that much difference in real world performance in reality.
A good example of this is with the JD HD garden tractors, the 425 vs the 445. They are virtually identical in all aspects except for rear tire size. Both use the exact same rear end and drive shaft from engine to trans, and run at the same engine RPM's (they also use the same engine with the only difference being the 445 is fuel injected and runs at higher compression to make its 2 extra HP), but the 425 comes with 23X10.50X12 tires, and the 445 comes with 26X12X12 tires. The 445 has a top speed of 8.5 mph while the 425 has a top speed of 7.5.
The only real and appreciable advantage that puppy would see with the slightly larger tires would be an even better ride.
He should pick up over 1 MPH in speed as well.