Quote:
Originally Posted by Captdru
Rpms really jump up at 65-70 mph but I hear towing a heavy load in OD will burn up tranny.
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You heard wrong. What will burn up your 4R100 tranny is allowing it to get too hot. And getting too hot is almost always caused by towing heavy at less than 37 MPH. Such as backing a trailer up a steep grade. Or towing us a slow crooked mountain trail where you can't maintain over 40 MPH.
The basic rule is to let the truck decide which gear it needs. If it needs to downshift out of OD, it will. If it doesn't need to downshift, it won't.
But there are two exceptions.
When coming down the mountain with a heavy trailer pushing on you, kill the OD until you get off the mountain.
And when in hills or climbing a mountain where the tranny often downshifts and upshifts, then kill the OD until you get out of the ups and downs.
My rule is when I see a grade ahead that I know will cause a downshift out of OD, I'll kill the cruise, slow down to the speed I want to climb the grade, then manually kill the OD before the computer does it for me. (That prevents the hard jerk if the computer downshifts at my normal 62 MPH.) Then I set the cruise on 55 or 60 MPH and take the mountain. My rig will tow my 8,000 pound 5er up any interstate mountain pass at 55 to 60 MPH without overheating anything. Depending on grade, I can usually cruise at 60 MPH with exhaust gas temp (EGT) of about 1,200º. Only on the steepest interstate grades will I have to back out a hair to keep from exceeding 1,250º EGT.
If you are spending lots of diesel towing at 70 MPH, with stock tires and calibrated speedometer that's about 2,600 RPM in direct drive. You could tow all day at 2,600 RPM and not hurt a thing except your wallet for a lot more diesel.
With mine, grossing 16k with a mid-profile 5er, at 62 MPH and 1,800 RPM, I get over 12 MPG. At 66 MPH and 1,900 RPM I get about 11 MPG. At 70 and 2,000 RPM I get less than 10. And at the normal speed of 73 MPH I get less than 9. So I usually tow at 62 MPH.
I tried one experiment. I towed my trailer about 350 miles at 62 MPH (over 2,300 RPM) with OD killed. I got about 9 MPG. Then I came back the same road with a headwind and gained 1,500 feet altutude, but using OD at 62 MPH, and I got over 12 MPG. So I definitely tow in overdrive except in the rare case of going up or down a mountain pass, or crossing the Texas Hill Country northwest of Austin.
