MPG?
MPH?
MPDay?
All depends on the kind of riding you're doing and your endurance capabilities. If you are doing nothing more than eating miles on the superslabs, figure on anywhere upwards of 40 mpg depending on windshield height (yes, it makes a difference) and average speed (buzzing along at 75-80 will drink more gallons than 65-70). For MPH, If you're going to keep it around 10 mph over the speed limit and you take somewhat-leisurely stops for gas and food, figure on somewhere around 60-62 mph for a long (1000+) day. For MPD, 1000 is doable without too much pushing, especially if you have a lot of time on the highways, however, don't let your pride get the best of you. If your body is telling you to stop, then, by all means, stop.
Set realistic goals for yourself. What works best for me is to figure an end time and number of miles and then figure out what speed you will need to average to hit your target.
Your mileage WILL vary