I usally carry an extra 7 1/2 gals with me. Most I've ever poured in while riding is 3 1/4 if I remember right. I did 50 miles (by my GPS) that day. Was in four wheel drive most the day & plwoing thru snow & work'in the bike pretty good.
But on average, I'd say it uses a little less than twice what it does in the summer with tires .... Now, having said that .... one time out riding, becasue of the mud & snow, we were in four low most the day (with tires), only did about 20 miles & the bike used a lot of gas that day.
My 400HO (455CC)burns about 30% more on tracks but I seldom travel over 20 mph where I ride. I've never burned more than 3.5 Can Gal. on any ride and most rides are around 50 mi. Like anything else, if the pedals to the metal, GULP, GULP, GULP.