I got 19.7mpg with my leadfoot and 100% city driving last week. So you're doing better than I!
It was also about 25 degrees colder outside when I filled up that time than the previous, so it may have partly been denser fuel led to more fitting in the tank....Either way, I only got about 20-21 tops.
__________________
-diesel is not just a fuel, its a way of life- 
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)
|