5th Wheel Diesel Towing at What RPMs?

rkymntsno · 05-14-2019, 03:52 PM

We're new to towing something this big and heavy and was wondering, at what rpm do you tow at and what gear? The 5th wheel was right at 13,500 lbs. Our TV as you can see below is the CTD 6.7, 6 speed, 3.42 rear axle. At 65 mph, 6th gear, we're at about 1800 rpms. It would downshift to 5 occasionally, but not unnecessarily. It felt like it was "lugging" in 6th, and getting around 10 mpg. I downshifted to 5th gear and left it there the remainder of the trip. It stayed around 2,000 rpms and seemed to like that better. Gas mileage on the "lie-o-meter" continued to say around 10 mpg in 5th, same as 6th gear. All the numbers on the truck remained just about the same, oil pressure, trans temp, oil temp, etc. So am I right in towing around 2000 rpms in 5th gear?

NewBlackDak · 05-14-2019, 05:10 PM

For that load, you should be engaging Tow/Haul which locks out 6th gear. You should be fine.

oldmanAZ · 05-14-2019, 05:11 PM

I'm not sure what others will say, but don't I think it's possible to really 'lug' an automatic transmission diesel pickup.

The down side of down shifting would be the transmission oil temp. I don't know about your Ram's factory gauges, but my Ford OEM dash gauges are abysmal - they may be accurate (even though I don't trust them) but they respond too slowly, IMO! I have a ScanGaugeII with a digital readout that I use to display engine oil temp, engine coolant temp, trans oil temp, and the engine fan RPM.

Long story short, I was driving into a VERY stiff headwind and could stay in 'high gear' (torque converter lockup) at about 60mph ONLY when behind a big truck or on a downgrade. When heading uphill or the truck ahead of me got too far ahead, the trans would downshift and I could watch the trans temp climb. That made me nervous. I have no idea how high the trans temp might go, but when the trans shifted back into high gear (torque converter lockup) I could watch the trans temp drop.

So if you can keep an eye on your transmission temp, then downshifting is probably fine.

Mopar_Earl · 05-14-2019, 05:13 PM

Quote:

Originally Posted by NewBlackDak

For that load, you should be engaging Tow/Haul which locks out 6th gear. You should be fine.

6th doesn't get locked out. Only increases road speed needed for it to go into 6th. It's not an overdrive lock out. It changes shift points. 5th and 6th in that trans are both overdriven.

Earl

Mopar_Earl · 05-14-2019, 05:24 PM

I run in tow/haul with the exhaust brake on at all times. I let the "auto"matic do it's thing. I don't lock a gear out unless it started hunting. Or if I'm in a hilly curvy area and wanted to hold some revs when I come in and out of the throttle. 95% of the time I let the trans do it's thing.
But yes, the 3.42s and the Aisin can lug at times with a good load. I'd rather of had 3.73s but not an option from the factory.

Earl

Mopar_Earl · 05-14-2019, 06:02 PM

Quote:

Originally Posted by oldmanAZ

I'm not sure what others will say, but don't I think it's possible to really 'lug' an automatic transmission diesel pickup.

The down side of down shifting would be the transmission oil temp. I don't know about your Ram's factory gauges, but my Ford OEM dash gauges are abysmal - they may be accurate (even though I don't trust them) but they respond too slowly, IMO! I have a ScanGaugeII with a digital readout that I use to display engine oil temp, engine coolant temp, trans oil temp, and the engine fan RPM.

Long story short, I was driving into a VERY stiff headwind and could stay in 'high gear' (torque converter lockup) at about 60mph ONLY when behind a big truck or on a downgrade. When heading uphill or the truck ahead of me got too far ahead, the trans would downshift and I could watch the trans temp climb. That made me nervous. I have no idea how high the trans temp might go, but when the trans shifted back into high gear (torque converter lockup) I could watch the trans temp drop.

So if you can keep an eye on your transmission temp, then downshifting is probably fine.

His trans will lockup in 2 through 6 gears. The Aisin has dual coolers as well.

Earl

Big1 · 05-14-2019, 06:07 PM

Yep, I would and do put it in Tow/Haul more and let it do it's thing.

rkymntsno · 05-14-2019, 07:31 PM

Quote:

Originally Posted by NewBlackDak

For that load, you should be engaging Tow/Haul which locks out 6th gear. You should be fine.

Yes, we always engage tow haul mode but it does not lock out 6th gear. You have to manually drop it to 5th. I think we're fine too, just wanted to hear from others. Thanks.

Cannon · 05-14-2019, 08:26 PM

Quote:

Originally Posted by rkymntsno

We're new to towing something this big and heavy and was wondering, at what rpm do you tow at and what gear? The 5th wheel was right at 13,500 lbs. Our TV as you can see below is the CTD 6.7, 6 speed, 3.42 rear axle. At 65 mph, 6th gear, we're at about 1800 rpms. It would downshift to 5 occasionally, but not unnecessarily. It felt like it was "lugging" in 6th, and getting around 10 mpg. I downshifted to 5th gear and left it there the remainder of the trip. It stayed around 2,000 rpms and seemed to like that better. Gas mileage on the "lie-o-meter" continued to say around 10 mpg in 5th, same as 6th gear. All the numbers on the truck remained just about the same, oil pressure, trans temp, oil temp, etc. So am I right in towing around 2000 rpms in 5th gear?

I'm surprised at the 10 mpg with a 3.42 gear. I have an older F-350 with a 5sp and 4.36 gear towing 16,000 lbs at 70 mph and get 11.2 and 9.5 in mountains.

rkymntsno · 05-14-2019, 08:32 PM

Quote:

Originally Posted by Cannon

I'm surprised at the 10 mpg with a 3.42 gear. I have an older F-350 with a 5sp and 4.36 gear towing 16,000 lbs at 70 mph and get 11.2 and 9.5 in mountains.

I know. It was windy all four days of travel, from crosswinds to headwinds. But coming from a Ram 1500 hemi towing a 34' TT weighing 6400 lbs getting 7 and 8, I'll take 10 anytime.

NewBlackDak · 05-14-2019, 08:33 PM

Quote:

Originally Posted by Mopar_Earl

6th doesn't get locked out. Only increases road speed needed for it to go into 6th. It's not an overdrive lock out. It changes shift points. 5th and 6th in that trans are both overdriven.

Earl

Ahh. Didn’t know it’s Aisin. Can’t see the signatures from Tapatalk.

Mopar_Earl · 05-14-2019, 08:37 PM

Quote:

Originally Posted by Cannon

I'm surprised at the 10 mpg with a 3.42 gear. I have an older F-350 with a 5sp and 4.36 gear towing 16,000 lbs at 70 mph and get 11.2 and 9.5 in mountains.

Older as no emissions? Emissions trucks suffer on fuel mileage big time. Also a manual trans truck will always get better fuel economy.
I'm surprised you didn't run out of rpm to hit 70 with 4.36 gears.

Earl

Cannon · 05-14-2019, 09:18 PM

Quote:

Originally Posted by Mopar_Earl

Older as no emissions? Emissions trucks suffer on fuel mileage big time. Also a manual trans truck will always get better fuel economy.
I'm surprised you didn't run out of rpm to hit 70 with 4.36 gears.

Earl

It's automatic and the RPM's are not as bad as you would think. I'll have to look next week when I'm towing but I think it's around 2200 can't remember for sure. It tows great like it does have a 4.36 gear but I sometime think it maybe has a higher gear because of the mpg and rpm's. I purchased it used and the window sticker was in the dash and show's it came with a 4.36. but if you look at specs for the 2005 F-350 it does not show that gear as an option.

**norty1** · 05-15-2019, 04:05 AM

I tow a 38.5' fiver. I just put it in tow haul and let the engine/transmission do what it's designed for.

When I am cruising under 60 and on a flat surface, I will disengage tow haul and let it up-shift. I just have to remember to put in t/h for braking.

I avg 11 mpg towing.

Mopar_Earl · 05-15-2019, 04:13 AM

Quote:

Originally Posted by NewBlackDak

Ahh. Didn’t know it’s Aisin. Can’t see the signatures from Tapatalk.

I can't either but 3.42 gears are only on Aisin trucks. Makes it a safe call. Lol

Earl

Mopar_Earl · 05-15-2019, 04:16 AM

Quote:

Originally Posted by Cannon

It's automatic and the RPM's are not as bad as you would think. I'll have to look next week when I'm towing but I think it's around 2200 can't remember for sure. It tows great like it does have a 4.36 gear but I sometime think it maybe has a higher gear because of the mpg and rpm's. I purchased it used and the window sticker was in the dash and show's it came with a 4.36. but if you look at specs for the 2005 F-350 it does not show that gear as an option.

2005, so no DPF system. Think what mpg you'd get if it was an emissions truck. Lol might of been a commercial order to get that gear ratio? Sometimes you can spec items that aren't generally on the build option list when ordering too.

Earl

rkymntsno · 05-15-2019, 06:30 AM

Quote:

Originally Posted by Mopar_Earl

I can't either but 3.42 gears are only on Aisin trucks. Makes it a safe call. Lol

Earl

No, my 3.42 has a 68RFE transmission.

rkymntsno · 05-15-2019, 06:39 AM

And I have to add, I think the best mod I've done to the Ram is the RDS 60 gallon auxiliary fuel tank. I learned with the hemi after only being able to get about 200 miles out of a tank I needed to do something with the 3500. Luckily I have the payload to handle the weight of the tank and the 60 gallons of diesel, along with everything else.

Mopar_Earl · 05-15-2019, 06:44 AM

Quote:

Originally Posted by rkymntsno

No, my 3.42 has a 68RFE transmission.

Your 3500 truck with 3.42 gears has the Chrysler trans, not the Aisin? Did a quick look. The 3.42 was an option on the 68 trans. First I've heard of it. Only seen 3.73. Aisin you only have the 3.42, no other option unless you go dually with Max tow group.

So you have the lesser rated engine with the 68 trans. Does that change your gvwr and gcwr compared to the Aisin?

Earl

rkymntsno · 05-15-2019, 06:51 AM

Quote:

Originally Posted by Mopar_Earl

Your 3500 truck with 3.42 gears has the Chrysler trans, not the Aisin?

Earl

Correct. 2018 Ram 3500 SRW. The Aisin is an option I believe. You look at dealers inventory and a lot of them have the 68RFE transmission. I didn't have any choice other than to keep looking if I didn't want this one. SRWs were scarce at the time I was looking.

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