It depends on how good you are at jetting it, and your riding style.
The Fireball with the TM32 on it, in proper tune, gets 70mpg at 70mph in highway cruising. That's US gallons. In Imperial gallons, it would be 85mpg.
With more normal cylinder heads, you might expect somewhat less, perhaps around 60mpg. If you are very heavy with the throttle, it might be lower yet.