I have been an ag mechanic for a long time. I want to give you some more information about tire sizes and differentials and 4 wheel drive tractors.
First, the tire and wheel sizes on a 4 wheel drive tractor are picked by the manufacturer to be compatible, ie., one driveshaft revolution moves over the same amount of ground on the front and rear axles. (actually front tires might cover 1 to 2% more to help turning) This is done by tire and wheel size selection and front and rear end pinion gear ratio selection. If you change tire sizes, and a 3 to 4 inch diameter change on rear tires(which are big) is a very big difference in ground covered, you will make a big change in the distance covered by the front and rear wheels and that will put a lot of pressure on all the driveline components. The problem will be worst when you drive on pavement, but even on soft ground, the tires on one axle will always be turning faster than the other. The damages in order of severity will be: excess tire wear, drive shaft damage and u joint damage, excess wear to ring and pinion gears in front and rear axles, and most expensive, damage to internal transmission components which will require splitting the tractor to access the transmission gears supplying power to the driveshafts and possibly the transmission gears themselves. You really don't want to risk this.
A quick way to tell if there is pressure on driveline components is to engage 4 wheel drive, move 100 feet, and see if you can disengage 4 wd easily. If it is hard to move the lever, or there is a pop when you disengage 4 wd, there is pressure on the driveline and you need to be sure the tire and wheel sizes are compatible. The best solution is to make sure the tire diameters are exactly the same as the OEM tire and wheel set. (All wheel drive cars, which always drive all 4 wheels on pavement, use a special differential to split the drive to front and rear driveshafts and allow them to turn at different speeds for a short time. Even with the differential, car manufacturers caution that all 4 tires must be the same brand and have the same amount of wear to prevent damage to the transmission. Replacing just two tires will cause damage to the transmission.)
A differential is put into the front and rear axle to allow the right and left wheels on each axle to turn at different speeds for a short time while going over uneven ground or around a corner. Two different sets of spider gears on shafts to do that. The spider gears and shafts are not built and lubricated to run for long periods of time, so running two different tire sizes on the same axle will turn the spider gears into metal powder quickly. Tire sizes on the same axle must be the same to avoid damage.
There is no differential between the front and rear driveshafts on a 4 wd tractor so any speed difference between front and rear driveshafts will cause almost instant damage unless the tires can slip on a soft muddy surface to relieve the pressure.
If you want to change the tire sizes on your tractor, best to check with tire manufacturers and your tractor manufacturer to make sure that the sizes will work together, otherwise damage can happen before you even notice a problem.
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