I'm joining a rally on a 1500 mile ride from Lands End to John O'Groats in September and it would be useful to know other owners experience with how far you can get on a tankful. I never really trust fuel gauges on bikes, although any comments on it's accuracy would be welcome. I usually start looking for a petrol station once I pass 100 miles on the trip but I'm sure the bike can do a fair bit more on a tankful. My other two bikes are early 90's carbed models and don't have fuel gauges but have a reserve supply. If the bike splutters to a halt I flip the tap over to reserve, knowing I've got around 25 miles before it runs dry.
I'll watch this with interest as I've often wondered how far can actually be squeezed out of a Street Twin (or in my case Street Scrambler) tank. Can I suggest though the only way to be sure is to get one of these https://www.motorama.it/en/givi-tank-2-5-lt-P18397.htm and go find out on your bike. Enjoy the trip, I've done it a few times (though have more often taken the 870 mile route in a day rather than the 1500 mile route)
I easily get 60 mpg on mine. On paper that is 180 miles on a full tank. But I also don't ride too aggressively. To be safe maybe look for gas at 130-140 miles.
I think the lady in MCN doing a long-term test was getting over 150 miles per tank. I'm cautious like you, and tend to take the next filling opportunity after each 100 miles.
I always carry spare fuel in one of these.... https://www.ebay.co.uk/itm/Primus-F...757456&hash=item567063189b:g:ZRgAAOSwBahU1MxN .....when out on the dirt bike, ever since running out on a track over open moorland, it fits nicely into a small backpack. What a nightmare that was. I haven't bothered doing so on the Street Scrambler as I find the fuel gauge pretty accurate. I've done over 15m with the fuel light on a few times without drama. Interestingly, Triumph quote a slightly higher MPG figure for the ST vs the SS.
Obviously will depend on your riding style, speed etc. My street twin gets typically around 3.6 l/100 km giving a potential range of 320 km or so (198 miles). I generally fill at 280 - 290 km. This is riding a mix of local rural country roads and commute to work.
I got a new one recently, been keeping an eye to see what range I'm getting. Most I've done on a full tank is 125 miles, the light was on for the last 10 of those.