I average 64 mpg (US) on my 2016 Street Twin, and 55 mpg with my 2018 T120.
Both bikes are great fun and excellent at what they do. IMO, the street twin is the better bike for shorter trips (say < 100 miles) or time in the twisties. The T120 is the go to bike for longer rides, especially when time on the highway is in the plans.
Both bikes provide almost identical range on a full tank of gas. The street twinís tank is smaller, but itís better mpg combines to provide the same range as the T120ís larger tank w/ lower mpg. In the end, the street twin is cheaper to operate, but only by about $3 per fill-up.
Intended: 2016 or 2018 Street Twin