5th or 6th gear?

[14ram]

Its mostly flat around here I was wondering what gear you all run your truck in while towing.
With so many variables at play keeping it simple I am talking about the 3.42's, 68RFE and a 5th wheel at 12200 gross.

I can run 65-66mph in 6th at a hair under 1500rpm which typically gives us 11-12mpg.
Now if I run in 5th and slow down to say 62-63mph I typically get 10mpg rpms around 1700 if I remember correctly.

Any hills, strong winds etc kicks me out of 6th so I lock it out now if I run speed upto 68-70mph it will mostly hold 6th all day long. 70mph obviously is too fast on these trailer tires so I only ramp it up passing slow traffic.

Other than increased cabin noise the truck much prefers 5th over 6th at towing speeds but the 2mpg loss can add up over time also. Also see this is a "double" overdrive transmission 4th been direct drive is any concern in longevity towing in 5 vs 6?

Regards look forward to your experience and thoughts

 

[shorthorn]

I can tell you my truck seems to also prefer 5th gear when towing our trailer... I have been told the 68RFE tran's trucks with 3.42's that 5th gear is the same gear ratio as the Aisin trucks with 4.10's in 6th.

 

[SnoKing]

My Aisin 3500 tows in 5th gear in tow haul, and is getting pretty good mileage. There is a difference however.

The Aisin has a .77 5th gear where as the 68RFE has a .82 5th gear.

6th gear in both is .63.

68RFE GEAR RATIOS
Gear:Ratio:
1 3.23 : 1
2 1.84 : 1
3 1.41 : 1
4 1.00 : 1
5 0.82 : 1
6 0.63 : 1
R 4.44 : 1

AISIN AS69RC RATIOS
Gear:Ratio:
1 3.75 : 1
2 2.00 : 1
3 1.34 : 1
4 1.00 : 1
5 0.77 : 1
6 0.63 : 1
R 3.54 : 1


The only place the 68RFE wins is with the lower reverse gear.

SNOKING

 

[Tuesdak]

Run the proper gear for the situation. Automatics may not always select the proper gear. However high gear is ok when the load is light enough and the situation allows it. Monitor temps and how the truck feels with the biggest clue being how much throttle you are using to hold top gear. Towing MPG esp. recreational vs.commercial is not the big factor. Consider the expensive damage like replacing an engine that can buy more than one tanker truck of cheap diesel that I am about to outline. 2 MPG difference becomes irrelevant at that point. Commercial MPG is your bottom line, but, that's a different discussion where lugging the engine isn't an answer.

it needs to be said because I find lots of people want to lug an engine to death or are flat scared stiff to wind any engine up. It's ok to wind the diesel engine up and leave it there. Remember they power gensets at 1800 or 3600 RPM all day everyday. Unlike automotive gas engines that blow stuff when revved up and left there... Diesels don't blow up from being run at higher RPM.

Experience from the GM side is the automatics (2008 Allison) like to hold the double overdrive high gear too long. This lugging at high throttle would cause the ECT to get high 235+. (This was with a OBDII port monitor as the GM OEM gauge lies.) So if you see any mild grades or stiff headwinds you need to lock out the top gear till the conditions change. GM noted for horrible and obsolete technology cooling systems vs. the Dodge Ram trucks... Ever see washer fluid boil in the tank under the hood? Regardless pulling it out of 6th to 5th for specific grades that didn't need full throttle resulted in lower ECT's.

So what if it's in too high a gear logging the engine towing?
Not enough water pump RPM.
Not enough fan RPM. You bet you need the fan at 65 MPH!
Transmission temps can rise. (Noted for MT trucks especially.)
ECT rises.
IAT rises... Not enough fan RPM means the CAC can't cool the IAT with the turbo wound up from full load.
High summer temps with the AC on just adds to the heat load and lowers the life of the low bidder parts under the hood.

Specifically in my long term GM experience hoses suffer from the extreme ECT coolant as well as shorter battery life. Conventional oil reaches it's limits and starts to thicken up on UOA, but, thins out too much while hot. Thins out to the point of low oil pressure stop engine alarms. (A real fun message to see. GM's answer is to buffer/delay the low oil pressure message longer before tripping the alarm!) Shorter battery life in Arizona known for 2 year battery life is specifically the passenger side battery boiled dry and failed in 1.5 years, 88K miles.

This said IMO the PO of my 2003 Dodge truck left it in 6th too much literally "lugging" his 5th wheel around and blew the 5.9 Cummins engine up pretty bad. Head looked like craters off the moon and sleeved the #1 cylinder from the damage. Failed to mention this to me when I asked what repairs were done to the 1 owner 100K mile truck. #@$%! How do I know he worked the engine too hard? From the exhaust manifold warping so much and everyone saying the extreme internal engine problems I have had are rare for the 5.9.

 

[Cummins12V98]

It's pretty simple with the 68RFE. On flat ground no wind try 6th at 62 for example and see what your instant fuel economy is. Then try 5th. Run what gives the best economy. I could run 6th with my 11 HO dually with 3.42's and 68RFE on flat to hilly terrain but I found by experimenting I did get slightly better mileage in 5th. I had 28-29K combined weight. At 60 in 5th I was around 1,750RPM about the asme as having 4,10's running in 6th. So when I ordered my 15 Dually I knew that 4.10's were the right choice and now have 32,500# combined. I run the Aisin and really like the lower starting gears. IMHO 3.73's are useless. 3.42's up to 27K and over 4.10's.

 

[Deleted member 995608]

I don't have a large 5th wheel anymore but I live in the hills. I lock out 6th anyways when pulling hills. The transmission doesn't hunt for 5 or 6th anymore.

 

[RustyJC]

At 60 in 5th I was around 1,750RPM about the asme as having 4,10's running in 6th.

Close, but my 68RFE dually with 4.10s runs ~1625 @ 60 MPH and ~1750 @ 65 MPH in 6th. Not a lot of difference, but enough to mention for reference.

Rusty

 

[kthaxton]

IMHO 3.73's are useless.

Really? The compromise between two extremes is "useless"?

 

[RustyJC]

Really? The compromise between two extremes is "useless"?

I believe the point he's making is that one can tow with 3.42s in 5th or 4.10s in 6th at comparable highway engine RPMs. With 3.73s under the same conditions, 5th would be too low (too many revs) and 6th too high (not enough revs).

Rusty

 

[kthaxton]

This would require the assumption that only those two rpm ranges, in those two gears are usable, and nothing in between is usable for any load/grade combination.....which is nonsense.

3.73's will move a load better than 3.42's, no two ways about it. It will also be able to hold each gear, longer. I consider that plenty "useful". Sometimes a compromise is a good thing.

 

[Cummins12V98]

I believe the point he's making is that one can tow with 3.42s in 5th or 4.10s in 6th at comparable highway engine RPMs. With 3.73s under the same conditions, 5th would be too low (too many revs) and 6th too high (not enough revs).

Rusty

Thanks Rusty!

I have towed with both and see no need for 3.73's based on rpm's at 60-62 where I mostly tow.

 

[Cummins12V98]

Close, but my 68RFE dually with 4.10s runs ~1625 @ 60 MPH and ~1750 @ 65 MPH in 6th. Not a lot of difference, but enough to mention for reference.

Rusty

Here is my 15 with 4.10's Aisin and stock size MS2's.

 

[kthaxton]

Thanks Rusty!

I have towed with both and see no need for 3.73's based on rpm's at 60-62 where I mostly tow.

So, there is no use for about a couple hundred more rpm and slightly more torque multiplication? Interesting. Remember, I am not trying to justify a reason for someone to swap to to 3.73's, but to say they are useless is really, well, quite silly since they indeed, without a doubt, pull better than 3.42's.

 

[NIsaacs]

This would require the assumption that only those two rpm ranges, in those two gears are usable, and nothing in between is usable for any load/grade combination.....which is nonsense.

3.73's will move a load better than 3.42's, no two ways about it. It will also be able to hold each gear, longer. I consider that plenty "useful". Sometimes a compromise is a good thing.

I agree, and given a choice I would opt for the 3.73 over the 3.42 every time,

Nick

 

[SnoKing]

After 14 years with 4:10's I love my new 3:42's bobtail on the freeway. Chris

 

[kthaxton]

I'm plenty happy with my 3.42's as well, but I too would have chosen 3.73 if I could. To say they are useless makes no sense.

 

[Larry Willard]

Thanks Rusty!

I have towed with both and see no need for 3.73's based on rpm's at 60-62 where I mostly tow.

But not everyone is towing at 60 -62 MPH.

 

[Cummins12V98]

But not everyone is towing at 60 -62 MPH.

I understand that that is why I said "60-62 where I mostly tow". But I think most people do tow in that range.

 

[SnoKing]

I am towing in the same speed range as I did with the 2001.5 basically. The old truck with 4:10 was at is sweet spot of 2000 RPM at 63 MPH in OD. The new 2015 is in 5th gear and is very happy on the flats and small rolling hills. 4th is there for hill climbing and has a wider speed band without getting busy and nosy. Get boxed in on a hill or very windy slow hill and it will drop to 3rd gear to get speed back up. It never has to lug out to get RPMs back up after shifting.

HP and torque are not that much different between my bombed 2001.5 and this new truck, however the new one does the towing job so much better.

And boy does she like to fast dance on the open road bobtail. The 2001.5 with 4:10's was like dancing with a fat girl on the open road, you really had to push her (around).

If only the old 5.9's would have had 6 speed Aisin autos!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

SNOKING

 

[RustyJC]

If only the old 5.9's would have had 6 speed Aisin autos.

The double overdrive 6th gear in the Aisin or 68RFE (basically the same gear ratio in either) are like that 7th gear I kept wishing I had in my 2002 ETH/DEE dually equipped with the NV5600 and 4.10 axle ratio. 6th gear in the automatics makes the 4.10s a nice, usable gear for towing or running empty. My 2002 was mildly bombed for towing (347/762 at the rear wheels), so I gave up some power and torque when I went to the 2011, but the towing experience is so much nicer and more pleasant.

Rusty