I have a similar situation with the 23L tank on my CCM - the last 3 litres are below the level of the carb inlet. In practice I've found that if I'm just using the bike for short trips - round town etc - those 3 litres are unusable. The bike just splutters and stops with fuel still sloshing around in the tank, but if I'm on the open road it somehow manages to suck more of it up and it will run the tank down to about 1L left. I can use a bit of the (round town) residual fuel once the bike has stopped by using a bit of a complicated workaround so it becomes a last ditch reserve. The tank has twin taps - one either side - so I shut down one of them and tilt the bike to get all of the fuel into the other side. You then have to open the float chamber drain screw and blow down the tank cap vent pipe to pressurise the tank and get fuel flowing into the carb. Shut the drain screw and fire up the engine. Good for about 15 miles and about half of the fuel.
Re higher or lower gearing for fuel economy, both my 600 singles have given their best fuel mileage with lower gearing. Sounds counter intuitive and the engine revving its head off on the open road doesn't make for a peaceful ride but that's what I've found.
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