The numbers on my tractor tires are 12-16.5NHS (front) and 19.5LR24 (rear). Front first: the 12 means the section width of the tire in inches. Sometimes this is the same as the tread width, usually it's a little wider. The 16.5 means the rim size is 16.5 inches in diameter. You also need to have the correct rim width, but you'd need the manufacturer's data sheet for the tire to find that out. The NHS means Non Highway Service, which means low speed only and sometimes, depending on the manufacturer, means skid steer use only, meaning the tires don't have the sidewall strength to allow use on a turning axle. Now for the rear: The 19.5, again, is the section width of the tire in inches. The L means low profile, meaning the ratio of height to width is lower than would be the case on a typical ag tire. The R means radial, as opposed to bias ply. The 24, again, is the rim diameter in inches.
Does this help? There are several types of dimensions on tires - if the ones you're speaking of are different, give us an example.
Mark