Hi I would think more like 20 /22 if you pump up the tyres and drive slow

anything with a big bump on it's head is going to struggle to beat this figure from a hard worked 1.9

You may achive your optomistic figure on the m/way at arourd 45/50 mph

(with the wind up it's back)if the wind is in your face it may drop as low as 17/18 mpg
be interesting if you keep brim to brim milage figures
terry