Towing in O/D?

[JMichelson]

I have 98 12 valve automatic. When towing my trailer in 3rd gear (O/D) off at 60 mph the RPM is about 2400. If I’m under a light to moderate pull and engage the O/D (4th gear) the RPM drops to 2000. If I let off the throttle just a little, the transmission shifts into what seems to be 4th (O/D) and the RPM drops to 1600 which seems a little to low of RPM to tow. My question to you gear heads is what keeps the engine at 2000 RPM under load after I shift into 4th gear and how could I keep the RPM at 2000 at 60 mph which would seem to be a nice towing RPM? Is there a reason I would not want to tow under that situation? This is easier to see in the truck then it is to explain. Thanks, Jeff

 

[Snow man]

Jeff,when your truck shifts,and runs at 2000 at 60,your are in OD,but the Torque convertor is unlocked. When it locks,it drops to 1600,you dont want to run it unlocked at 2000,believe me,your trans wouldnt last long like that,it would overheat eventually. This is why I preffer the 4. 10 for heavy towing,unless you tow at 70-80 mph all the time. You either have to run it on 3rd wound out ,or OD,and go a little faster to get the RPM's up a little,so it isnt laboring so bad to be honest 1600 in OD,towing,isnt conductive to long trans life either. My 96 12V didnt like running at 1600 to much in OD with a trailer,empty it was fine,but with a trailer it liked 1850 minimum,and at 2000 ,it seemed happiest pulling.

 

[barryg41]

Towing

I was also concerned about towing in O/D. So I called Cummins and ask them what RPM could I tow ALL day. They said 2750! The transmission runs cooler first of all, the MPG drops some, but the engine sounds great.



As you can see, in my sig, I have 3. 54's and cant pull like I want in hills (in O/D). Wish I had 4. 10's. Learn and live!



I bought the PE EZ, I havent pulled with the EZ yet. But some of the guys here on TDR has said that take it easy with the EZ until I can upgrade the transmission.



I know this is a 99' but I would call Cummins or one of the TDR guru's will step in for the 12vers.

 

[Snow man]

Barry,the 12V auto starts defueling at 2300-2500 RPM,by 2700 its totally winded,and floorboarded to keep it there. The fuel mileage goes out the window too at that engine speed. The 24V can be run as high as 3200 contiuous,as far as I know. i run mine at 2900-sometimes for 10 minutes straight at 95-100 mph,it smooths right out at that speed,and seems to love it,it always runs better after a hard workout.

 

[pwknapp]

Mine is a 5spd with 4. 10's and I have run it most of it's life at 2500rpm. Lost 5th and ran 2700 miles in 4th at 2750 to get home. Does seem to take it all in stride.

 

[Boldt's Wagon]

It sounds like you have the original 245/75R16 tires. Next tire change get 235/85R16 or 265/75R16 and you will drop to 2300 rpm at 60 mph in 3rd. (4 percent tire diameter change) Also get Piers 3000 rpm spring kit and you can run 70 mph in 3rd when necessary.

 

[illflem]

If you run over 2000 rpm with a 12 valve you may as well have bought a V-10 as your fuel economy will be about the same.

 

[Boldt's Wagon]

When you have an automatic, 0. 69 OD ratio, and 3. 54 gears, you don't have many options.



When I tow the 11. 5 ft by 8. 5 ft barn door weighing 11,000 lbs I can get 11. 5 mpg in 3rd/2400 rpm/60 mph or 11. 5 mpg in OD/1800/65 mph. I choose to use 3rd and save the OD and torque converter clutches.



The other option I did not choose is a US Gear or Gear Vendors for about $2,000. That buys a lot of fuel.



The 3. 54 and 0. 69 gears sure are nice for running solo at 80+ mph. Try cruising at 80+ with your 4. 11 gears, 5 speed 12 valve.

 

[RedRamer]

Check those rpms again.

with the 3. 54 rears you should be getting 1,725-1750 rpm at 60 mph.

Up here that's 100kph (60mph). I find I'm pulling at 110kph most times = (68 mph) = 1,875rpm and that is my sweet spot.

I used to follow the traffic at 120kph (75mph)= +- 2,050 rpm, cruise on with air...

but the mpg dropped from 17 to 14... .