You dont want to run different overall diameter tires on the same axle. You can change the tire size, as long as you change all of the tires on the axle. On the back this would mean 4 new tires. (Ouch! $$) Front tires are called steering tires and usually have a tread pattern and design specifically for steering. The rear tires, called drive tires, are designed for use on the rear, under power. In the end it is usually cheaper to replace the tire with the same size that the bus was designed to use, unless you find a really good deal on a set of 4 rear ones. You will also have to check with the tire shop to ensure you could fit wider tires on the back without rubbing each other as well, otherwise you would need to change rims as well.