Since I have a daily very regular route sans traffic, I decided to burn through two tanks of gas with the utmost fuel efficiency.
--Clean and waxed bike and helmet.
--No going above 62 mph (indicated) highway.
--I estimate about 40 mph on twisties, no brakes, tiny amount of throttle, 6th gear.
--Granny starts from lights.
--Engine off at long lights.
--I weight roughly 250 pounds with equipment, tank bag, tail bag, Aerostich suit, boots, gloves, etc...
--Used Google Maps to go exactly from one destination to another every day. Hence, this estimation is the worst (ie shortest) possible.
Result: about 58 mpg, so just missed 60 mpg.
It's pretty funny having a long line of cars backed up behind me because I was going exactly the speed limit, and having every single driver beat me from stoplights. On the last day I even had a full-rig Harley follow me before passing me with a WTF gesture. Of course, I was scrunched up behind the windshield, so yes I looked ridiculous.
58 mpg is not very efficient for a motorcycle under the best case driving but I don't think this would surprise anyone. 1000cc engines will never be particularly efficient engines.
Some notes: to my surprise, the GSXS is quite difficult to ride at such low throttle openings with low rpms. I think the bike stalled in 6th gear @40 mph not just a few times but many times. Every Japanese bike, including this one, I get the impression the engines are imperturbable sewing machines but apparently low rpms at tiny throttle openings seems pure misery for the engine. I'm wondering if those cut-offs and stumblings might have negatively impacted fuel economy.
Interestingly, I next got about 48-52 mpg riding "normally", or doing about 70 mph on the highway and between 40 to 100+ mph on twisties, and relatively high rpms with fairly large throttle openings out of turns. But still not "sport bike riding".
What does all this mean? I'm not absolutely sure, but it seems you have to go way slower with way less acceleration to eek out just 15% more fuel efficiency. Or maybe sport bike engines are just really efficient at high rpms. Or maybe there's some hard line in fuel efficiency whereby when the bike is turned on then it will use a minimal amount of fuel.
Regardless, riding a sport bike as a granny bike is quite painful. I do this all the time with my car and it's just fine to get ludicrous gas mileage but trundling along on a bike at or below the speed limit is quite boring. And also, the aforementioned throttle issues made it surprisingly difficult and challenging to do. So you're going slow but it requires a lot of concentration to keep the bike at a precise speed.