Ford V10

40 MPH with a dinghy is typical of what we just did in the mountainous section of I-70 in CO. I was trying to keep the RPM at 4200 but based on posts maybe 4700 would have been better. Worst grade was this trip I-70 from Dillon to Eisenhower Tunnel, 12 miles of 6%. Don’t like straining the tranny like that as it was making some strange whining sounds the last mile or so (5 speed Torqshift). May unhitch the dinghy if we ever do that grade again.

That engine makes its peak torque at 3,250 RPM. It makes peak HP at 4,750. To climb a hill in a hurry you need more HP. So, by staying below 4,000 RPM you are giving up HP that the engine could be making if it was spinning faster at 4,750. However, many people say the "best" RPM to pull a grade is at peak torque. At that RPM the engine is most efficient at hill climbing but not necessarily the fastest. So, take your pick, you can have speed with RPM or you can have efficiency and possibly cooler temps at peak torque but you can't have both.

You gotta go above 4 grand. It likes it there.

That is where I try to keep mine at on rpm and 40 mph going up and in a lower gear.

You have to keep the RPM up. 5000 RPM is not a problem.