Best gear for towing

[RLDSL]

I've been trying to figure out which gear is best for towing at highway speed for fuel mileage. I've got 4.10 rear and zf5 behind a 7.3 w non wastegated ats turbo

I'm pulling a 5th wheel that when loaded and with fluids is around 10k pounds .
On the windy mountain roads I run in 4th and rarely have to shift, on the interstate, I can pull 5th most of the time, running 55-65 but I have to keep my foot in it and don't get much boost. If I drop to 4th I can take my foot out of it and at that rpm the turbo is pumping nicely ( especially since I put a 4" exhaust on it

I've been trying to gather some fuel mileage records , but with 3 youngun's, the mayhem surrounding fuel stops has inhibited my record keeping .

I'd appreciate any input , I don't want to burn any more fuel than I have to at these prices

Thanks------Robert

 

[towcat]

with your pulling weight, 4.10's aren't bad at all. 3.55's tend to be a bit doggy when packing some wieght. My F450's have 5.13's for in-town trucks and 4.56's for my carrier. Mileage really sucks with those two. 10 mpg at "normal" driving and I don't drive normal at all.

 

[RLDSL]

I was mostly wondering if I'll get better fuel mileage running in 4th or 5th gear, although I
have been considering swapping the gears in the rear end if I can't squeak a little better mileage out of the thing.

-----------Robert

 

[towcat]

robert-
my rule of thumb is to run whatever gear suits you the best. technically, the best rpm for feul economy is from 2k to 2500. mileage falls off over 2500. Personally, I can never drive in that range. It's just my habit. On a long run, I like to drive wherever I am comfortable at and most importantly, where I have the best choices of control of my vehicle. When running around 3k, I have the engine braking power at the best to start slowing things down and then the aux trailer brake knob if I have time to get to it.

 

[Agnem]

I'd say the best gear is the one that makes your truck the happiest. Lowest EGT's, water temp, and RPM generally add up to longer life and better milage, but the 3 are not always in sync with what your asking from it. I always tow in overdrive or double OD if the truck stays happy.

 

[RLDSL]

On the interstate, it stays cool. Water temp only gets up to about 200-210 when the fan clutch kicks in and it'll drop right back to 190. EGT were running about 900 when pulling steady, but with the new 4" exhaust they're running about 600-750, less than that when cruising.

I still haven't turned the pump up after putting the turbo on ( I wanted to take it easy on the new engine until I got it broken in ) so I guess I need to get that dialed in. I don't start seeing much boost till around 2200 rpm

I guess what i was mostly wondering was if running in OD with my right foot flat on the floor was actually saving me any fuel over running in 4th . I guess I need to pay more attention to what rpm range it's staying in

Last trip I took across Texas and Oklahoma before putting the new exhaust on, I was only getting about 11mpg and I wasn't too impressed with that, but I was having to stand on it a lot harder and wasn't getting hardly any boost with the old 2 1/4" pipe and clogged muffler ( as in clogged and rusted out so bad that when I cut it off, I couldn't lift the thing .

-------Robert

 

[Mr_Roboto]

On an IDI, the amount of fuel injected is directly related to the accelerator position. So whichever gear (4th in your case) uses less throttle should net you better mileage.

Should you regear? Only if you are loaded heavy most of the time and want to be able to use that 5th gear all of the time. There is no harm in running in 4th.

 

[RLDSL]

I just had to take a trip to town and jotted down the speed/rpm relationship in a nutshell
4th gear at 55mph= 2600 rpm
5th gear at 55=2100
5th gear at 65= ~2350
5th gear at 70= 2500 RPM

Exactly what that'll tell me at the moment

yes, the foot position is what i was wondering about, but in 5th over 4th at 55, it's turning 500rpm slower, meaning ( uh oh, high math time, i'm in trouble 2000 less injection strokes per minute out of the pump, but more fuel volume in each of those strokes.

My brain's startin' to hurt here

I suppose i could get off my dead backside and run some comparison runs, but it just seems like the thing has been getting better fuel mileage running through the Ozarks on the back roads than it does on the interstate. Could just be an illusion since I'm not traveling near as far in the same amount of time.

--------Robert

 

[Agnem]

One thing I'm certain of, is that staying below 2000 keeps you in your torque curve better. That's really where most of your power is. I know when Lady Moose is towing the camper, If I run at 23-2500 and 5 pounds of boost, she will heat up a whole lot faster than at 1700 with my foot all the way in it, and only a couple of pounds stuffed in. Slower road speeds pushing all that air with the camper also contribute to lower temps. I also wonder if maybe the water isn't flowing so fast through the system, that it doesn't spend enough time in the radiator to get cooled off.

 

[RLDSL]

I haven't had any problem with too much heat ( that may change when I get the pump turned up , It actually runs at a civilized temp most of the time ( only time it's started to climb was on a long pull about 14-20% and that hill is torture on anything , I ran that one with the trailer shortly after break in, I figured if it would pull that thing, it would pull over anything . and this is with my 3 row radiator, which , unfortunately, has a pinhole in the middle somewhere that i hadn't been able to pinpoint the location , so Yesterday I decided to order a custom built all aluminum radiator from Rodney Red, 5 row equivilant . I shouldn't ever need to worry about the coolant temp with that sucker in there ( besides, I run Evans waterless coolant, so I don't really worry about the temp when it climbs anyway ) .

------Robert

 

[zigg]

I agree with Mel

I think the best place to get good fuel mileage is when you're at the peak of the torque curve. Everything I have seen so far(Dyno stats) suggest that the peak torque curve for the 6.9 and 7.3 is around 1800rpm. Max HP is higher(2600) but it's the torque that does the work.

On my '86, with 4sp and splitter, and 3.55's it got amazing fuel mileage. I ran 60mph at about 1900rpm, and it'd run like that all day, and get consistently over 22(Canadian) mpg. Before I put the splitter in, it was turning more like 2500rpm, and got only around 18mpg. With the splitter, I was always able to keep the truck pulling in the 1700-2.000rpm range.

My present truck, a '93 with the E4OD runs around the same, 2,000rpm at 65 but it has a 4.10 rear, and pulls way better. I haven't done a mileage check yet, but I figure it is just a bit lower than the '86 was.

So, if you want the best fuel mileage, you got to get the rpms down below 2,000, if you're running it at 2500 or higher, don't expect to see great mileage.

just my .02

Zigg