Sean, please help out a newbie, what does the 315 number stand for? Tire width?
I think RJ did a great job explaining what part of the measurement "315" is, and everyone not already familiar with tire sizing should go to the sites he linked to familiarize themselves.
That said, let me just clarify that while "315/80R22.5" is a newer-style nomenclature, and "12R22.5" is an older-style nomenclature, these are two different sizes, not a newer/older name for the same size.
If the 12R22.5 tire was "renamed' into the newer style nomenclature, it would be roughly a 305/83R22.5, however, you won't find a tire with that labeling.
Both the 315/80R22.5 and the 12R22.5 turn nearly the same number of RPM and have nearly the same rolling radius; in fact, there is more RPM and radius variability between makes and models within either of those sizes. And either tire will fit 22.5" x 8.25" rims, which were standard on most MC9s (9" rims were optional), although 9" rims are preferred for the 315. However, there the similarity ends.
Because the 315 is a wider tire intended for heavier loads, it generally has a higher Load Range or Ply Rating, and heavier tread, plies, and sidewalls. For this reason, many 315/80R22.5 tires are speed-limited (that's a lot of weight out there ~22" from the hub), and ones rated to 75mph for motor coach use are extremely robust.
In addition to making the tire itself much more expensive to build (and heavier) than a 12R22.5 model, the extra load rating earns the tire more Federal Excise Tax, which is based on the load the tire can carry. That all adds up fast, and a 315/80R22.5 tire can cost 30% more than a similar model in 12R22.5.
On top of all that, the extra footprint of the 315 makes low-speed steering harder, and on some coaches can cause the tire to rub the wheel well or other components on full-lock turns. Properly inflated for the same load, the 315 will also lower fuel economy compared to the 12R.
The 315/80R22.5 offers two advantages over the 12R22.5, neither of which will be needed by the average conversion operator:
1. The tire has a much higher load limit. If your steer or tag axle is carrying more than 15,000 lbs, or your drive axle is carrying more than 24,000 lbs, you may need to go to the 315/80R22.5 just to handle the extra weight.
2. At any given load within the range of the 12R22.5 tire, a 315/80R22.5 tire will require less air pressure to carry that load. For example, a 14,000 lb steer axle with 12R22.5 tires would need those tires inflated to 110psi, whereas that same load on 315/80R22.5 tires would require only 95psi. This will give a softer ride (although, frankly, we hardly noticed the difference when we had the 315s on), and if you spend a lot of time on soft surfaces like mud or sand it might help you to sink in less.
Anecdotally, I can tell you that we switched to 9" rims and the 315/80R22.5 tires briefly because of this last issue (better float/traction off road). We switched back to 12R22.5 because they wear better, make the coach easier to handle at low speeds, improve fuel economy, and cost a LOT less money. We have an on-board air compressor and an inflator hose. If we need the off-road edge provided by lower air pressure, we'll just let air out of the tires until we are out of the soft stuff and then inflate them back to highway pressure. Tire pressure can be safely reduced significantly at very low operating speeds.
Likewise I think it would be hard for most bus nuts to justify the extra expense of 315s on coaches of average conversion weight. The exception would be the late-model factory Prevosts with dual slides and very heavy conversions, although most of those are now equipped with even bigger "super-single" wheels on the steer and tag axles, relegating the 315s to the drivers only.
One caveat here, though: Some 315 tires are not approved for fitment on 8.25" rims. So if your coach is so equipped, and you really want (or need) the 315s, make sure you select ones approved for these rims.