Not sure if you are aware of this, but TomC sells trucks, medium & heavy-duty Class 8 rigs. In so doing, he is constantly putting together powertrain combinations to fit particular applications for his customers, and has lots of tools available to do so. So he's a great resource to help guide you to obtain the best bang for the buck. Might see if you can come up with the revs per mile for the 315 or 365 tires you're contemplating, to help him give you more accurate suggestions.
Are you planning any jaunts to the West Coast? If so, do NOT expect your bus to climb the grades we've got out here like your car does. Unless you've got over 800 hp pushing you along, figure you'll be pulling grades between 35 - 45 mph with most standard bus powertrains. Lots of busnuts are shocked to find out their bus will not go up the mountain at 70+ mph like their Chevy, Ford, Honda or Toyota will. It's called mass!
An example of our grades: I-80 between Sacramento CA and Reno NV has over 70 MILES of 4, 5 & 6% grades, climbing to 7300 feet. And, of course, you've got the infamous Eisenhower Tunnel on I-70 outside Vail, CO that's at the 11,000 ft level - wheeze city, even for the turbos! So even if you've got it set up properly for decent fuel economy rolling across the midwest, it's going to fall flat on it's face going over RockyTop West.
Just a little reality check for you.
FWIW & HTH. . .