I can't drive 55 (Sammy Hagar)

[JC2]

I am thankful that I have/use common sense to determine the safest speed to operate our 32000# mh when pulling our 4500# towed. I have not been able to find any justification for driving these big vehicles at some of the speeds mentioned or noticed on the roads.

[rtspence]

Originally Posted by MTK46

If I drove my ISM rig at that RPM I would be passing everything in site

That rpm works out to 68-72 mph for my DSDP . Very comfortable at that speed.

[JumboJet]

Originally Posted by rtspence

Based on information from Camp Freightliner the ISL9 sweet spot is 1800- 1900 rpm for power & economy. Your speed will be determined by gear ratio an tire diameter.

I have the 300HP Cummins ISB. At 65 MPH, the Allison transmission downshifts for smaller grades. At 67-70, the Allison will hang in there longer on the grades. 67 MPH is right at 1,800 RPM. I try for 67-70 MPH if the traffic permits the use of cruise control.