You do not list what model of Spyder you own. The RS and RT have different power bands which would make a difference in your "best rpms". While it is new, my RT (SE5) gets 28-30 @ 5,000-5,500 rpm. I suspect the SM5 vs SE5 may also make a difference. Your riding style will play a part too. Hard acceleration will take a toll on fuel mileage. Keep records and be aware of your riding style....
Your mileage won't be great regardless of what you do. If you get 28 or better...be happy.
BigRed pretty much hit the nail on the head. The Spyders do have a pretty large frontal surface to push down the road so mileage is not comparable to what you'd see on a two-wheeler...
I run my RT like an old woman; I keep the revs low and upshift about as early as the computer will allow. Consequently I get fairly good mileage; 34 to 37mpg.
Your GS has the powerband about 750rpm or so up the rev scale from the RTs, so you'll just have to try different ryding styles to see what works the best... Good luck!
My SE5 gets 28-30mpg as well, but I refuse to use granny mode.
My wife recently retired. We carpooled. My commute is down from 100 mi to 80 mi/day. Love the bike and have watched the hype grow the last couple of years.
We sold our 2007 Pontiac Solstice GXP intercooled turbo with direct fuel injection so that I could buy this bike. The Pontiac weighed 2800lbs, made 260HP, would do 160 mph and we got 27 MPG in anti-granny mode. The wife usually drove faster than I did!
What surprises me the most is the lack of efficiency with these Rotax engines. It's no wonder everyone is disappointed with the fuel mileage.
The bike has a 4:1 weight advantage to go along with the poor mileage, but does have more leg room then the Pontiac did.
Not complaining, but we are stuck with what the canucks gave us.