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Originally Posted by thunderpuppy1
I'm towing an 8,000 lb 5th wheel RV with my F350 FX4 SRW. I have the 6.4, 20" wheels, Torqshift, and 3.73 rear.
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The 8,000 pound load is nothing to worry about.
Your problem is you selected 20" wheels and tires, so the tires are almost the same diameter as 35s. So compared to others who have 32" diameter tires, you have the equivalent of about a 3.41 rear axle ratio compared to their 3.73. Long legs.
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I'm towing at about 60 mph on mostly flat terrain.
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In overdrive, at 60 MPH, you're not turning enough RPM to get high enough up on the HP curve to climb more than a baby grade. In direct drive, you're only a little more RPM than you need to generate adequate power to tow 8,000 pounds without downshifting for every little bump in the road.
Your torque peak is 2,000 RPM. So for max economy, you want to cruise at right around 2,000 RPM if you have enough horses under the hood to tote the mail.
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In addition, if I am in 5th and cruising at 60, the slightest hill or headwind will cause a downshift to 4th. The RPM at 60 in 5th is about 1600 and in 4th is about 2100.
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In OD, you left too many horses in the barn. So when climbing a grade, you must take some more horses out of the barn and put them under the hood where they can help tow the load.
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Am I better off towing a bit faster to get a little higher in the torque curve?
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Depends on your definition of "better off". Since your tranny does not have an OD off button, here's what I would do on you next long towing trip:
1] Be sure you know how accurate is your speedo and tripmeter. Use a good GPS or the interstate highway mile marker test or whatever. Calculate the percentage of tripmeter error.
2] Then with the RV tied on, fill up with diesel (to the cap, through the foam, which might require 5 to 10 minutes after the automatic pump shuts off). Reset the tripmeter to zero.
3] Drive at least 300 miles with the tranny in tow/haul mode and your speed at 60 MPH. Then fill up (to the cap) with diesel and determine your MPG and reset your tripmeter to zero. Adjust your MPG calculation by the percentage of tripmeter error.
4] Drive at least 300 miles with the tranny in tow/haul mode and your speed at 65 MPH. Then fill up (to the cap) with diesel and determine your MPG (considering tripmeter error) and reset your tripmeter to zero.
Now you will know whether towing at 60 MPH saves any fuel compared to towing at 65 MPH.
If you're still a long way from home, then repete the test, but without using tow-haul mode.
Now you'll know not only the best speed for your rig, but whether to use tow/haul mode for your relatively light trailer.
For most of us, speed is the MPG killer, and we get better MPH at 60 MPH than at 65 MPH. But the long legs on your truck, combined with the higher RPM torque peak of your engine, might offset the increased aerodynamic drag at higher towing speed.
So you need to do a good test to find out.
Then come back here and report your conclusions.
