Towing and gear selection

[Dave Sherman]

Got back from a week long trip to Maine on our first long trip in our camper. Up until now, it's always been within an hour or two, but it was a good two day drive.

Observation: The truck does an admirable job towing our little camper. It does have a tow/haul mode, so the transmission stays out of 7th and 8th gear. On the highway, it rarely goes into 6th unless it's going downhill. Usually stays in 5th, unless there's a hill, then it drops into 4th. So, here's what I'm pondering. The Pentastar 3.6 has a variable displacement oil pump. I can see that when I'm at 65 in 5th, it's just under 3000 RPM. It seems once it gets over 3000 RPM, then it shifts to the higher displacement and goes from about 32 PSI to about 75 PSI. It really got me wondering, which gear is better? It does about 3800 RPM in 4th at 65. So which would be more desirable, 5th gear with the lower oil pressure and better gas mileage, or 4th gear with the higher RPM, higher oil pressure, but lousy gas mileage? Main reason I ask is if I have the cruise control on, it insists on holding the lower gear after a hill. If I leave it off it will happily go back into high gear after a hill.

If I'm really overthinking this, feel free to say so.

 

[tnt_motorsports]

Personally I avoid towing with the cruise on.

I don’t know that I have a definite answer for you but I always pulled my camper with my truck in the torque curve. Granted I have a Ram 2500 with a Cummins but I always tried to have it between 1600 and 2400 RPM. I didn’t worry too much about fuel mileage. I worried more about vehicle wear and tear.

 

[AZjeff]

The owner's manuals in the last few trucks I've had said that tow/haul is for towing heavy loads. You may not need to use it, what does your manual say?

 

[joegreen]

I think 5th would be better. 32psi still meets the old fashioned 10psi per 1000rpms. If everything is riding on an oil film i dont think 75 vs 32 psi would differ as technically the oil film is separating everything.

 

[Speedbird001]

I'm on a much smaller scale (Tacoma, that has done a few 1-2,500 lb tows). Much smaller, lighter, lower powered rig, so YMMV

I opted for higher RPM 5th vs 6th gear (Manual Trans) Cost me ~1mpg I think, but kept response up for the highway traffic slinky and in my head spinning was better than lugging the drive train. I have/did use cc a-lot.

 

[fantastic]

what camper do you tow with?

 

[Kira]

I like tnt_'s torque curve consideration.
Once you learn the tow vehicle's sound.......

And, what does this camper weigh?

 

[IndyIan]

What does the oil temp do when the pump switches to high volume mode? Seems like that would be putting quite a bit of heat into the oil?
If I was ignorant of the big change in oil pressure(like I am with my cars) I'd think anything around 2500-3000 rpm would be a happy medium of rpm (coolant flow, oil flow) and throttle position. IMO I don't think most gas engines really like large throttle openings down near 2000rpm for hours.

 

[eljefino]

Best thing you could do would be to drive slower, but who wants that?

The CC holding a gear was probably because it doesn't have the logic to apply brakes. I'd turn it off and let the truck find fifth on the downhills.

How much of the load do you think was trailer weight vs wind resistance? One gets you on the hills, one gets you all the time.

 

[ls1mike]

All the gasser trucks I have owned had cruise control. I will set is and let it eat unless I am in a rather hilly spot. Like going over Snoqualmie pass or any of the bigger grades out west or I am not a major highway.
I have an 8000lbs travel trailer, a 6000lbs float and 9000lbs parade float.
The old 02 was a 6.0 4L80E and it see 4th gear on the flats pretty often. 2017 6.0 6L90E would see 5th and 6th on the flats.
This new 10 speed the engine rarely see over 3500 rpm and will hold 9th and10th if the truck is in the correct terrain.
I get what mpg I get, it is never a consideration when towing.

 

[meep]

As long as the gear ratio is not an OD gear, “I’m good.” Fluid heating and driven surface fatigue in OD gears is where most of my concern would be, with respect to the trans. Now, the trans has the luxury of active cooling, as does the engine. The rear diff, however, does not, and that sucker can get HOT, especially if it has shorter gearing. But this isn’t what you are asking. I think I would make the decision not by oil pressure but more by trans and engine temperature. If things are running hot, I’d go for a lower gear and potentially lower speed.

Are you reading the oil pressure on a gauge or going by a book that says what it does? I could see it that tow/haul could change the oil pressure bias.

My old Chevy 283 at a 27psi relief - i rarely clocked over 30psi.

Does the pentastar have piston squirters?

 

[D60]

I'm no help but never realized the oil pump was 2-stage like this. I verified in my '19 JL yesterday -- fascinating. Oooh gotta run, I see a shiny object over there.....

 

[BrendanC]

put it in drive and tow haul and let it eat. remember that you’re in severe duty and change your fluids accordingly. rear differential fluid is overlooked by many and is extremely important to replace.