When I had my car loaded up to probably around 5000lbs traveling through pennsylvania I had to keep the tranny in 3rd gear (S) 95% of the time both for the engine braking as well as the climbs....which were difficult even in third. Some of the grades were so steep that the engine/car would quickly accelerate down them even in 3rd  I had to keep shaving off mph's with the brakes.....it was quite an experience, I did that for about 65 miles nonstop. 
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-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)
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