Depending on the coach engine / transmission combination and the rear end gearing it is theoretically possible to see almost any top end speed.

As a practical matter however the lower the rear end gear ratio (as Joe points out is required for higher top end speeds) the less capable the engine is in overcoming the effects of the wind at high speeds. It takes a huge amount of our engine's power just to push the bus into the wind. That is why the slower speeds pay big dividends in fuel mileage.

I had my coach up to a fairly high speed on the way to Santa Fe at one point because we were driving around 70 or 72 and we encountered a long and reasonably steep downhill section. With the retarder off and my foot off the gas pedal the coach coasted up to about 84 momentarily. To maintain that speed however would have required my foot to be on the accelerator pedal almost all the way when on level ground, and the wind noise at that speed confirms just how hard the engine is working.

All of this totally ignores practical considerations such as how long the tires would last because I am sure the 75 MPH speed limit is established for heat dissipation, and I wonder if the engine cooling system can get rid of the heat generated by an engine runnning at maximum HP for an extended period.