What Gear are You Towing In? - Diesel Forum

thunderpuppy1 · 05-05-2008, 10:25 PM

I'm towing an 8,000 lb 5th wheel RV with my F350 FX4 SRW. I have the 6.4, 20" wheels, Torqshift, and 3.73 rear.

I'm towing at about 60 mph on mostly flat terrain.

I'm finding that I'm unable to shift into 5th unless I accelerate up past 65 (in 4th) and then let it slowly back off the fuel and the truck will then shift to 5th.

In addition, if I am in 5th and cruising at 60, the slightest hill or headwind will cause a downshift to 4th. The RPM at 60 in 5th is about 1600 and in 4th is about 2100. I've tried with tow/haul both on and off and haven't seen any difference.

Are any of you noticing similar behaviour? Am I better off towing a bit faster to get a little higher in the torque curve?

Stymie · 05-05-2008, 11:58 PM

at 1600 you are under the curve, as you asked about, and the truck doesnt have the power to pull along in 5th.

thunderpuppy1 · 05-06-2008, 08:25 PM

That's kinda what I figured.

In terms of fuel economy, am I better off towing @ 60 in 4th gear @ 2100 RPM or upping my speed to ~70 and towing in 5th at 1800-1900?

I'll be pulling again in a couple of weeks so will do a test at that time.

doczenith1 · 05-07-2008, 12:56 PM

Quote:

Originally Posted by thunderpuppy1

That's kinda what I figured.

In terms of fuel economy, am I better off towing @ 60 in 4th gear @ 2100 RPM or upping my speed to ~70 and towing in 5th at 1800-1900?

I'll be pulling again in a couple of weeks so will do a test at that time.

I vote for 60mph. I know with my truck speed makes a huge impact on fuel economy because of wind resistance and the large frontal area of our trucks. On the other hand, I've heard there's a threshold for turbo charged diesels based on rpm and boost. You basically want to stay below a certain rpm so that your not creating too much boost and reducing fuel economy. I'll be curious to hear the results!

SmokeyWren · 05-07-2008, 04:18 PM

Quote:

Originally Posted by thunderpuppy1

I'm towing an 8,000 lb 5th wheel RV with my F350 FX4 SRW. I have the 6.4, 20" wheels, Torqshift, and 3.73 rear.

The 8,000 pound load is nothing to worry about.

Your problem is you selected 20" wheels and tires, so the tires are almost the same diameter as 35s. So compared to others who have 32" diameter tires, you have the equivalent of about a 3.41 rear axle ratio compared to their 3.73. Long legs.

Quote:

I'm towing at about 60 mph on mostly flat terrain.

In overdrive, at 60 MPH, you're not turning enough RPM to get high enough up on the HP curve to climb more than a baby grade. In direct drive, you're only a little more RPM than you need to generate adequate power to tow 8,000 pounds without downshifting for every little bump in the road.

Your torque peak is 2,000 RPM. So for max economy, you want to cruise at right around 2,000 RPM if you have enough horses under the hood to tote the mail.

Quote:

In addition, if I am in 5th and cruising at 60, the slightest hill or headwind will cause a downshift to 4th. The RPM at 60 in 5th is about 1600 and in 4th is about 2100.

In OD, you left too many horses in the barn. So when climbing a grade, you must take some more horses out of the barn and put them under the hood where they can help tow the load.

Quote:

Am I better off towing a bit faster to get a little higher in the torque curve?

Depends on your definition of "better off". Since your tranny does not have an OD off button, here's what I would do on you next long towing trip:

1] Be sure you know how accurate is your speedo and tripmeter. Use a good GPS or the interstate highway mile marker test or whatever. Calculate the percentage of tripmeter error.

2] Then with the RV tied on, fill up with diesel (to the cap, through the foam, which might require 5 to 10 minutes after the automatic pump shuts off). Reset the tripmeter to zero.

3] Drive at least 300 miles with the tranny in tow/haul mode and your speed at 60 MPH. Then fill up (to the cap) with diesel and determine your MPG and reset your tripmeter to zero. Adjust your MPG calculation by the percentage of tripmeter error.

4] Drive at least 300 miles with the tranny in tow/haul mode and your speed at 65 MPH. Then fill up (to the cap) with diesel and determine your MPG (considering tripmeter error) and reset your tripmeter to zero.

Now you will know whether towing at 60 MPH saves any fuel compared to towing at 65 MPH.

If you're still a long way from home, then repete the test, but without using tow-haul mode.

Now you'll know not only the best speed for your rig, but whether to use tow/haul mode for your relatively light trailer.

For most of us, speed is the MPG killer, and we get better MPH at 60 MPH than at 65 MPH. But the long legs on your truck, combined with the higher RPM torque peak of your engine, might offset the increased aerodynamic drag at higher towing speed.

So you need to do a good test to find out.

Then come back here and report your conclusions.

thunderpuppy1 · 05-07-2008, 10:19 PM

Thanks for the feedback folks. As I mentioned, I won't be towing until about 10 days from now. I'll check back in with anything I've learned.

MDarrough · 05-08-2008, 01:33 PM

Over spring break in March, we went to Beavers Bend, Ok. That's was right at 1000 miles round trip. We were mostly on interstate hiways, so I decided to do some experimenting. I set it on 60 for about 50 miles, then 65, 70, and 75. I was watching the dash read-out the whole time to monitor the est. MPG. I find the computer to be right on with the hand calc mileage.

I found that there was only .5 MPG difference all the way across the board!! I found the engine to be most "happy" at about 72 MPH, so that's what I settled on for the rest of the trip.

BTW - my 5er weighs in loaded at 13,200, and I averaged 11.5 the whole trip.

Mark

Mark Kovalsky · 05-08-2008, 01:54 PM

Did you reset the trip computer between each segment? If not it was averaging the MPG over the last 1000 miles, or since the last time it was reset.

MDarrough · 05-09-2008, 10:17 AM

Oh yea, I forgot to mention that. I did reset it for each segment. It took it a while to settle in on a number, which is why I did 50-mile segments. The 11.5 overall was from hand calc.

BTW Mark, I have a question for you please. The tranny on my ’01 had some slip, but the ’08 is even worse. I installed the Sonnax valve and spring mod on the ’01 and it responded well. Do you know if the ’08 will respond as well to this mod?

Thanks,
Mark

Mark Kovalsky · 05-09-2008, 03:41 PM

No, the same mod won't work on your '08. There's no valve body on the '08, so there is nowhere to install that mod.

MDarrough · 05-09-2008, 09:11 PM

Thanks Mark. I figured that's what you would say.

Any suggestions to firm up the tranny?

Thanks,
Mark

Juzplanekrazy · 05-15-2008, 09:41 AM

I don't have 3.73's in this truck but i did have them in my Dodge 5.9 and I just put in Tow/Haul mode and head down the road. I do that same thing in this new Ford 6.4 with 4.30's. When I'm towing my 5th wheel I hit Tow/Haul and leave it alone.

MDarrough · 05-15-2008, 12:26 PM

I'm sure that 450 with the 4.30 gears is great. My '01 had a Suncoast tranny with the heavy TC and it was a monster. Unless you've had an aftermarket tranny like that, you can't understand what I'm talking about when I say the OEM tranny is very weak and has LOTS of slip.

Looks like I may have to make the trip to Lead Hill, Ark. some time in the future!!

Mark