Another perspective, and this only applies to GMC coaches:
GMC designed the powertrain around tires that turn 495 revs per mile. On the 4106 model, the results were 60 mph @ 1650 rpm with the stock 4-spd transmission and 4.125:1 rear axle.
GM powertrains sit side-saddle across the back of the coach (except for the Scenicruiser), consequently there is a bevel gearset that directs the prop shaft in the proper direction to the rear axle. Fourth gear is 1:1 in the stick, and the bevel ratio is 0.808, yielding a 3.333:1 overall final drive. (1:1 x 0.808 x 4.125:1)
However, many folk have converted their coaches over to the V-730 automatic, which has a 0.875 bevel gearset. The math ends up giving you an overall ratio of 3.609:1, roughly a 5% engine rpm increase at any particular road speed, with subsequent drop in top speed and fuel mileage.
Since the 4.1:1 rear axle is the "tallest" available for the V-drive layout, the ONLY way a GMC owner can pick up some of that lost 5% is by going to tires that turn LESS than 495 revs per mile. In many cases, this requires changing from 20 or 22.5 wheels to the larger 24.5 rims.
Lots of 22.5 tires turn right around or slightly more than the magic 495 rpm for GMCs, but very, very few turn less. OTOH, looking thru the 24.5 sizes, one can find several that turn less - I've even found one 24.5 Bridgestone model that turns 470 rpm, which almost, almost
matches the OEM road speeds for those who have opted for the automatic. Obviously, if one has 22.5s on the coach now, and wants to bring back that OEM performance, they'd have to switch to 24.5 rims.
Enough about GMCs and their unique requirements. . .
Two-stroke's right, however. The tire market is definitely going to the 22.5 as more popular. They're also switching to metric sizing for the HD tires, something we've had in the auto side of things for the last what, 15 years or so?
MCIs and Prevosts have normally come from the factory with 12R22.5 tires. Stock "bus size" tire for the last couple of decades. Not always easy to find at your local truck stop at Oh Dark Thirty when you need one - the challenge then takes on your wallet big time. Depending on the situation, and if you don't have a spare hidden away behind the front bumper, sometimes you can limp on by using a tag tire or outside dual for a short distance.
As I said earlier, metric sizing is starting to make inroads with the HD tires now. The familiar 12R22.5 crosses over to a 315/80R22.5, but there's a caveat here, also: Highway vs City.
Yup, there are different bus tires for transit vs highway use. All HD tires are speed rated (passenger cars are too, for that matter), with the majority of transit bus models rated at 50 or 55 mph. The major difference between highway and city tires is the sidewalls. Transit tires have much heavier sidewalls, because those drivers thoroughly believe that the cement curb is part of the braking system, so they're scrubbed often. That and in the hotter SW climates, transit brakes generate a tremendous amount of heat as they drive a block and stop, over and over. So the tires are designed for the heavier heat load in the bead area, and the heavier sidewalls to resist curb scrubbing. Ride quality is not a major concern for this market either, for that matter.
Obviously, then, highway tires are built to different criteria - a comfortable ride at 70+ mph, with lots of airflow to help keep temps reasonable (although inside duals run hotter due to less airflow). You also get your choice of steer, drive or all-position tires, just to keep the matrix interesting.
Point here, tho, is if/when shopping for tires for your coach, make sure you're getting highway rated, not city-transit rated. May look the same, but they aren't. Do your homework.
New Alcoas or Accurides run about $350 or so each, new steel Accurides are about $100. Sometimes you can score a good deal on a set from a truck boneyard, many have.
FWIW & HTH. . .