Well, I happen to have found out today, first-hand, how many miles per gallon a 2008 Spyder, fully broken-in (1050 miles) goes on a full tank of gas. I use 87 octane in California. (BTW, I called BRP and asked which was the correct fuel tank capacity: they said that the manual was correct.)
Well, I say that neither the manual nor the spec sheet is correct. I had put in 5 gallons of fuel (the most I'd ever been able to put in after getting NO bars on the dash), so I had a FULL tank. It went 147 miles when suddenly it started dying in the car pool lane. Fortunately, there's a huge car pool lane shoulder to the left and I was able to get off safely and stay safe till a good Samaritan in the guise of a U.S. Air Force sergeant on a motorcycle stopped and advised me against trying to push the roadster across a four-lane freeway to get to the far right lane and the next exit.

Anyway, he went and got me 2 gallons and charged me nothing. God bless him and his family.
Anyway, what got me into the predicament was that I figured after no bars I should have at least a gallon and a half of fuel (based on the 5 gallons I pumped and the 6.6 gallons total). That 1.6 gallons should take me AT LEAST 40 more miles. My home was about 24 miles away. I stopped short about maybe 6 or 7 miles at most. So 147 divided by 6.6 (a full tank) is 22.27 miles per gallon. That is awful!!
If that's the case, I'm probably going to sell the Spyder. Even though I bought it because it was way, way cool, I also bought it thinking about the price of gas. At this rate, my 1994 Camry does better by far: at least 30 miles per gallon, freeway or street.
I'm thinking the 6.6 gallons is wrong unless there is a reserve tank switch somewhere I don't know about and somewhere the manual doesn't mention.
I'm sure that BRP got all kinds of bad press with the thousands of cars that saw me on the side of the road. That, or thousands of people thought, "Who's the idiot with the three-wheeled yellow thingy wearing a yellow helmet in 100 degree (Farenheit) weather?"
Yeah.