The combination of 6-12 R1 on front and 9.5-16 R1 on rear should be fine as far as front/rear ratio is concerned. The front wheels should be "pulling" the rears slightly, by anywhere from 2-5%. To check this, jack up the tractor on one side, both front and rear wheels. Rotate the rear wheel exactly once. Keep track of how many rotations, including last fractional rotation, the front wheel makes while the rear is making this one rotation. Next, calculate the distance around the rear wheel (circumference). Then, calculate the distance around the front wheel. Multiply the circumference of the front wheel by the number of rotations it made while the rear was being rotated once. Divide the answer by the circumference of the rear wheel. The quotient should be somewhere between 1.02 and 1.05, meaning the fronts are sized as they should be to maintain the acceptable ratio between front and rear wheels. If your answer is less than 1.02 or greater than 1.05, your tires are placing the geartrain at risk of unnecessary wear, not to mention additional wear to the tires. The farther away from the acceptable range, the greater the wear.
Keep in mind that Satoh originally sized tires for the Beaver based on available engine torque. Their engineers had done the calculations as to how to best match the Beaver's engine torque with optimal tire size. With larger tires installed, available torque at the ground is reduced while travel speed is increased. If you don't need the slowest travel speed possible with maximum available power, and if you don't use the tractor at high road speed much, the reduction in torque probably won't make all that much difference. Vvm.
