??? 23 Liters is 6.076 gallons. (actually 6.07596, but I rounded it off) Math, it's important.
I very much doubt the tanks themselves vary much. Manufacturing tolerances and methods make it very rare for this type of assembly to be off by any significant amount. (I spent 30 years working in manufacturing as a machinist and have seen a lot of processes and quality control methods.)
However, the amount of each bike's reserve
does vary quite a bit. Most are around 1.2 to 1.6 gallons. The reason for the variations is that the float arm can easily be bent when the fuel pump is installed into the tank. It's a tight fit. My reserve was pretty close to 1.6 gallons of useable fuel before I removed my fuel pump during the install of a bulkhead fitting for my fuel cell. I intentionally bent the float arm a bit to reduce my reserve and got it down to just over 1 gallon. I found it annoying to have the reserve light start flashing and only get 4.5 gallons in if I filled up immediately. Hardly a concern when I still had some 60+ miles left. I often ride tank to tank, even with 11 gallons available now.
I once coasted into a gas station after 163 miles of hard riding, but usually get ~40-42 mpg. I'm always running panniers, fuel cell, 1 gallon hydration jug, etc so not very aerodynamic. 80 mph speed limits here too, when I'm local.
The calculated mpg is pretty silly to watch. I see 16 to 99.9 mpg depending on throttle position and conditions. Re-setting the average is a smart ideal for getting a better idea of how far you can go. Most of us can get at least 40 miles on reserve if we are traveling at steady speeds and not into a wind or heavily loaded. You should start getting nervous around 35 if you've never actually tested how far you can get on reserve.
If you have managed to get over 6.1 gallons into your tank on more than one occasion, ask yourself if it was at the same gas station. If so, you probably don't want to buy gas there any more. Pumps are not as accurate as you think, and not tested as often as you would believe. It's common for states with testing programs to only test 10-20 pumps a month. That leaves a lot of pumps un-tested. Typically they only test the stations where they get complaints, and most people have no idea what their cars are taking in terms of gallons, so wouldn't complain. Some testing organizations only test octane, not volume. Hard to believe, but true. They pump a liter for octane testing, but the pumps are in gallons in the US, so it's not as simple as you would think for them to notice.
