If there's a lot of traffic and traffic lights I do, but at 40ish and above I'll put it in 4th. And at night when the lights are blinking yellow and there's no traffic I will go to 4th even at 25mph and just growl along. Does fine. I can accelerate uphill in 4th gear from 18-19mph without a prob.

__________________
-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)