2 MPG figures from my big bikes.
Triumph Rocket 111 2.3 litres. 42 ish mpg
Triumph Thunderbird LT 1.7 litres. 50 mpg
Both weigh in at 750 lbs
I find the R111 is very consistent and about the same with or without pillion.
I think its a fair consumption for the engine size but surprising my 2 litre petrol car does a lot more mpg than the bikes