Google is only as good as the info it finds.
Diameter = distance across a circle
Radius = diameter divided by 2
Circumference = 2 x 3.14 (pi) x radius.
shows a 245/75-22.5 to be 37 inches tall (PSI / load dependent). 11 inches width is 279 mm, but 11R22.5 does not indicate aspect ratio (sidewall height), a critical element in calculating MPH / RPM, unless these have a standard sidewall height I'm not aware of. But going by 75% AR, the same calculator shows 280-75R22.5 would be 39 inches tall. Bridgestone's site shows the height as 42.2 as you state, but it just doesn't sound right to me. That's like a 90% AR. If there is something I don't know here, let me know.
shows a 39" tire with a 6.17:1 rear and 0.74:1 trans top gear pushing 2500ish RPM at 63 mph. Any way you slice it, I think you'll be playing with fire pushing your bus much past 58-60 with that low of a rear.
If you're looking to cruise at 70 mph, you'll have to take your chances unlocking 6th (may or may not drop RPM sufficiently, even if you can get it done) or go to a higher rear (numerically lower), say, 5.29, which would still turn 2350ish at 70 running the same tire without unlocking 6th. Does anyone even make a 4.63 for these differentials? It's not a ratio I've heard of, but it would turn 2100ish at 70 with that trans and 39" tires. However, if your tires are truly 42.2", a 5.29 would be at 2200ish at 70 mph without unlocking the 6th gear. A bit more RPM than I would run a diesel at, but somewhat reasonable at least.
Most of these engines are meant to run around 1600-2000 - that is where most of them are most efficient. They may governed for more RPM, but just because you can doesn't mean you should. Diesels don't like RPM the way gassers do. Even if it is safe at a given RPM, it definitely takes a toll on long term durability.
Also, for some applications, your engine could have been built a year or more before the bus was built. So your 2002 bus could have 2000-2001 parts.