As already suggested check with the dealer or look at machinefinder.com and see what tires other people are using. Also make sure you have the correct size rims. Here is something you may also find of interest.
Tire Ratio
On tractors with MFWD the need exists to have the front and rear tires cover about the same amount of ground, while going in a straight line. So when in MFWD the rear tires are not pushing the front tires or the front tires are not pulling the rear. This avoids putting stress on the drive train.
This is achieved by matching the front and rear tire sizes to the front-to-rear gear ratio. To determine if your tractor has the correct tires on it one needs to determine the rolling circumference (RC), this is the distance a tire travels in one revolution. It is very important to understand rolling circumference when trying to determine the proper tire setup for your tractor. Since both the front and the rear tire on MFWD tractors are engaged, the front tire requires more revolutions to cover the same distance as the rear tire. It is necessary to know the gear ratio of the tractor in order to select the tires that will adequately match the tractors front-to-rear gear ratio. The gear ratio on the typical MFWD ranges from 1.2 to 1.5; therefore, the proper setup should allow for front tire over-speed up to 5% (consult with the tractor manufacturer for the preferred lead percentage).
On my 5210 rear tires are 14.9D28 the RC is 166, front tires are 9.5D24 the RC is 124 and the front to rear gear ratio is .751. If you take 166 * .741 = 123, indicating these are the correct tires. This could also be expressed as 124 * 1.34 = 166.
To compute RC the formula is = 2 * 3.1416 (pi) * Radius. Radius is defined as = sidewall height + rim radius.
A tire size of 14.9D28 (14.9 width of tire in inches, 28 diameter of rim in inches) would be 378/85D28. The first of three numbers in a size (378/85D28) is the tire's section width in millimeters, measured from sidewall to sidewall. Millimeters can be converted into inches by dividing it by 25.4. The second number (378/85R28) is the tire's aspect ratio or profile. The number is related in a percentage. This means the tire痴 sidewall is 85 percent tall as is wide. This is a ratio of sidewall height (rim to tread) to section width. The section height's measurement can be calculated by multiplying the section width by the aspect ratio. The answer will be the sidewall height.
Rear tires: 378 mm x 0.85 = 321.3 mm + 14" x 25.4 = 355.6 mm
RC = 2 * 3.1416 * (321.3mm + 355.6mm) = 4255.5 mm / 25.4 = 167.54 in
Front tires: 241.3 mm x 0.80 = 193 mm + 12" x 25.4 = 304.8 mm
RC = 2 * 3.1416 * (193mm+304.8mm) = 3128 mm / 25.4 = 123.15 in
