It's all about what revs the engine & driveline are most efficient at!!
Run with your revs too low & the engine isn't running as efficiently as it could be AND the gearing/driveline means the Spyder is not getting as many tire revolutions/distance travelled from each engine rev as it could..... Similarly, run with your revs too high & you go to the other end of the scale. But if you spend most of your time with the engine revving in its 'most efficient' rev range & your gear selection giving you the best number of revolutions of the drive tire it can at that speed/in that efficient engine rev range, then you'll get better mpg than you would anywhere outside that most efficient engine rev range/gear selection for a given road speed.
Running a fuel injected engine slowly doesn't necessarily mean it uses less fuel like it generally does on a carb fueled engine, nor does it necessarily mean the engine is running efficiently; low engine revs don't always give you the best power/torque output for the fuel that's being burnt nor does it mean that all the fuel delivered into the cylinders is being used at its greatest efficiency; compounding that with an incorrect gear selection for your chosen road speed can load up the engine in a way that makes the fuel usage even worse - eg if you get the engine lugging due to too low engine revs &/or too high a gear selection, the computer can try to inject more fuel into the cylinders in an effort to increase the power/torque output so the engine won't stall, but you still don't necessarily get to go any further for each rev of the engine even tho you are burning more fuel..... And similarly if you rev the engine beyond its efficient power/torque output range, you can end up burning more fuel than you need to burn in order to get the most efficient revs of the drive tire/distanced travelled for each rev of the engine...
Keeping your engine revs in the efficient range (which on small petrol engines is usually within a thousand revs or so either side of the revs where the power & torque delivery curves meet) and selecting the gear that gives you your chosen road speed while keeping your revs in that 'most efficient' range will give you better mpg than riding outside any of those parameters. Of course, there are a whole range of other things that can have an impact on the actual mpg numbers too: things like running the right tire pressures to give you the least rolling resistance on that road surface while still providing sufficient traction that you aren't spinning your drive tire to maintain speed; or running engine revs sufficiently high to make sure the centrifugal clutch is not slipping & wasting 'drive' & therefore fuel; and a whole heap more....
The 998 V-Twin motors run at their 'most efficient' by delivering their optimum balance of power & torque around 3800-5500 rpm, while many other motors, including the 1330 ACE motor, deliver more torque earlier in their rev range, so
their 'most efficient' revs & rev range is correspondingly lower. Add a 'less slippy' clutch & another 'over-drive' gear too, & that means the 1330 can deliver their best mpg without revving as hard as the 998, which needs to be revved harder to deliver their best. Clear as, isn't it?!

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