Sarge1124

I was on a hunting trip this weekend heading up I76 towards Nebraska which is almost completely flat and was pulling my 8k lb travel trailer, the first thing that I noticed was at 80mph you had the go pedel completely to the floor and RPM's were at about 2200. the average miles for the whole trip 99.9% highway about 460 miles was 8.5mpg this thing is a pig, granted a pretty nice looking pig though. I guess I was expecting more, on my 02 CTD that I traded in for this one averaged almost 14 or so on this same same stretch of highway pulling the same trailer and when you stomped on the go pedel on the 02 it would put you back in your seat. Do you guy's think that there might be something wrong with my 04 or am I just used to my slightly bombed 02? Mike

spots

Put some mileage on it and let it break in properly. Then if your mileage is down you can complain. Did you try it out of overdrive??

bulabula

Is something wrong? I dunno. But 2200 rpm, with a 48RE OD/3.73's does translate to about 80 mph. But it was floored to maintain that 80, and not accelerating? Maybe you were in a lugging situation and shoulda unlocked it and/or used direct drive rather than OD?

Sorry I couldn't be of more help, but something doesn't sound right to me.

On edit, with about 17k of weight running 80, your mpg will improve if you back off a bit.

RobG

Sounds to me like you had a major headwind and didn't realize it.

I just got back a little while ago from my first "test" tow with my new '03. I towed my 24' enclosed car trailer (which is for sale -- see the Classifieds) with two 4x4 quads in it, total weight of about 5500#, from Reno up to Donner Summit and back. That's a pretty good climb and is about 65 miles each way. I wanted to do this not only to see how the new truck handles with the trailer, but also to give it some "working" break-in miles. I'll probably do the trip again on Wednesday, but with a third quad in it, to bring the total trailer weight up to around 6200-6300#.

For those who know the area, there's a stretch of I-80 between Reno and the California border that usually has lots of winds coming down it. This was definitely the case today. It was all I could do to maintain 65 mph (and I have 4.10s) until I got into the mountains and the winds diminished.

Once past that area, I could do 65-70 pretty easily up the grade, though if I had to slow down for another car (e.g. if the left lane was blocked) then I'd lose my momentum and would end up at 55-60 for a while until I could get over, kick it into 3rd and then pass people.

I used about 1/4 tank of fuel for this little trip, and I filled up when I got back into town. 298.7 miles and 26.234 gallons to fill it up. So figure 300 miles total and 130 miles worth of towing. Or almost 50/50 towing and empty. That worked out to 11.4 mpg. I don't think that's bad AT ALL for a truck with just over 600 miles on it, and considering that I intentionally kept the throttle planted the whole way. I wanted to keep combustion temps up to aid in break-in. I don't want it to take 28k miles to break in this truck like it did on my '01.

As for your 8.5 mpg issue, try backing off the throttle a bit. WOT when it isn't necessary isn't going to help. Case in point... I rented a POS U-Haul (with the Ford 7.3L nonturbo diesel V8) back in 1995 when I moved from Texas to California. I had this POS full (a 14' model) and towing my old Subaru on one of their car trailers.

What I noticed was that floored, it blew tons of black smoke and went maybe 65 mph. But if I backed off the throttle just a little bit... maybe 3/4"... it didn't blow much smoke at all, and still went maybe 65 mph. The difference was 2-3 mpg improvement using less throttle.

Rob

Sarge1124

I am not complaining it is just a question, I like this truck allot but was wondering if anyone else has noticed this. As for trying it out of OD yes I did this is my first auto and I am still learning how to use it and bucking a head wind it ran exactly the same in both directions NE and SW so I do not think that this was the case. I understand that I was going a little faster than normal but I was trying to compare this truck to my old truck. Again, I really love this CTD and I am not complaining in any way shape or form, this truck is great especially after a few mods, I am forcasting a expensive future. Thanks for the reply's, Mike

[quote author=spots link=board=20;threadid=19346;start=0#msg181709 date=1062972598]
Put some mileage on it and let it break in properly. Then if your mileage is down you can complain. Did you try it out of overdrive??
[/quote]

Gunship Guy

I have the same auto and ratio on my '03 and tow a 10,000 lbs trailer. My mileage is much better than what you reported. I'm getting around 14 mpg. I tow at 65 mph and feel I have a lot of peddle left. Can the mpg really drop off that much between 65 and 80 mph?

dealseeker

[quote author=Gunship Guy link=board=20;threadid=19346;start=0#msg181769 date=1062985281]
I Can the mpg really drop off that much between 65 and 80 mph?
[/quote]

Quick answer: Yes!

doug

dern right your mph will drop if you go 80 instead of 65mph. in the sweet spot thread we've been discussing the fact that required HP goes up as the cube of vehicle speed. Here's what that means in practical terms:

1. it takes 8 times as much HP to go 80 as it does 40. So if you're using 50 HP to go 40, theres no way you'll be able to do 80 unless your 305 HP HO can make 400 HP.

2. the difference between 65 and 80 mph is closing in on double the HP, which means at least double the fuel consumption rate. for example, the CTD consumes 50 lbs of #2 diesel per hour running at 148 HP (lowest rpm possible at peak torque). It consumes 114 lbs per hour developing 305 HP (HP peak/2900 rpm). so thats a 206% increase in HP that uses 228% more fuel to go 123% further. not good for mpg.

Here's another look at fuel economy: suppose you are blasting away at WOT and 305 flywheel HP, pulling your 5ver up a hill going 72 mph (the 6 speed will pull 2930 rpms in 5th gear and stock tires at this speed). According to cummings, you're consuming 114 lbs per hour of fuel. according to this site:

http://www.vermontfuel.com/server/energy_specs.html

Diesel is 7.034 lbs per gallon so during that hour of 305HP WOT you burn (114lbs)/(7.034lbs/gal) = 16.2 gallons of fuel. thats 72 miles / 16.2 gallons = 4.4 miles per gallon.

Now drop your speed to 57 mph. this requries only 148 HP and the cummins consumes only 50 lbs of fuel per hour. So if you drive 57 mph for one hour you'll burn (50 lbs)/(7.034 lbs/gal) = 7.1 gallons of fuel. Thats 57 miles / 7.1 gallons = 8 mpg.

SuperGewl

You should slow down and keep it just below 2krpm's, that usually the sweet spot, also go ahead and let the cruise control run it. You will find out the the computer and run the auto much better than you ever could. Most people are getting about 11-14 mpg when towing much bigger loads. Just my .02 worth.

kayjh

[quote author=Doug link=board=20;threadid=19346;start=0#msg181978 date=1063038891]
dern right your mph will drop if you go 80 instead of 65mph. in the sweet spot thread we've been discussing the fact that required HP goes up as the cube of vehicle speed. Here's what that means in practical terms:

1. it takes 8 times as much HP to go 80 as it does 40. So if you're using 50 HP to go 40, theres no way you'll be able to do 80 unless your 305 HP HO can make 400 HP.

2. the difference between 65 and 80 mph is closing in on double the HP, which means at least double the fuel consumption rate. for example, the CTD consumes 50 lbs of #2 diesel per hour running at 148 HP (lowest rpm possible at peak torque). It consumes 114 lbs per hour developing 305 HP (HP peak/2900 rpm). so thats a 206% increase in HP that uses 228% more fuel to go 123% further. not good for mpg.

Here's another look at fuel economy: suppose you are blasting away at WOT and 305 flywheel HP, pulling your 5ver up a hill going 72 mph (the 6 speed will pull 2930 rpms in 5th gear and stock tires at this speed). According to cummings, you're consuming 114 lbs per hour of fuel. according to this site:

http://www.vermontfuel.com/server/energy_specs.html

Diesel is 7.034 lbs per gallon so during that hour of 305HP WOT you burn (114lbs)/(7.034lbs/gal) = 16.2 gallons of fuel. thats 72 miles / 16.2 gallons = 4.4 miles per gallon.

Now drop your speed to 57 mph. this requries only 148 HP and the cummins consumes only 50 lbs of fuel per hour. So if you drive 57 mph for one hour you'll burn (50 lbs)/(7.034 lbs/gal) = 7.1 gallons of fuel. Thats 57 miles / 7.1 gallons = 8 mpg.


[/quote]

I'm confused. So you're saying that at 57MPH the Cummins will get 8MPG? What about guys that are reporting cruising at 60MPG empty and getting 18MPG+ ? Doesn't the load on the engine and the work required to maintain 57MPH figure into the equation? It must take more gas to maintain 57MPH with a 5,000# trailer than without one doesn't it? I'm sure you are correct about the exponential increase in fuel consumption with the increase in speed, but your last calculation confuses me.

doug

WOT. look again at my calculations and the conditions I assumed.

phox_mulder

WOT = Wide Open Throttle,

in other words, flooring it, something you'll rarely have occasion to try, unless you're drag racing.


phox

Lightman

Doug, while that seems to make some sense, I know from my TDI, that 60mph equals about 54mpg, and 80mph equals about 47mpg. Not nearly twice the fuel consumption. Is this a specific cummins phenomenon?

phox_mulder

According to the overhead MPG readout, here's my facts.

80 mph+=10 mpg on the overhead. (very seldom)
75 mph = 11-12 mpg on the overhead.
70 mph = 13-14 mpg on the overhead.
65 mph = 22 mpg on the overhead.
60 mph = 23 mpg on the overhead.
55 mph = 23-24 mpg on the overhead.
any slower than 55 and mpg starts going down as well.

WOT at any speed = 4-8mpg on the overhead.

All noted on flat road.

I am currently averaging 16.3 mpg, hand calculated, filling to same point in filler neck, odometer/speedometer corrected with Superlift Truespeed.
Overhead MPG average sits right at 13.5, reset on every fillup.

2002 24V, 3.55 rear, 35 inch tires, automatic tranny.


phox

doug

hey guys, note that in my post I'm describing very specific examples, all of which are running the CTD at WOT -- peak output. So yes, Lightman, these are CTD specific examples. also, note that the 60mph to 80mph thing corresponds to (nearly) double the HP requirement -- thats just the cubic rule at work, unrelated to any particular engine. For the cummins that's at least twice the fuel consumption rate according to Cummins published data. Maybe some engines run more efficiently at higher output levels, who knows.

I deliberately chose arbitrary highway speeds and engine rpms that the CTD trucks can achieve in real driving conditions, but also chose them specifically to correspond to the particular published peak output levels, rpms, and fuel consumption rates for the engine -- conditions rarely encountered but relevant to the towing situation because it shows the folly of flooring it while on that 9% grade when you can back off and save some fuel.

basically I tried to fit real driving conditions to the published Cummins data to bring some meaning to life so you guys could say "don't that beat all. some numbers I can relate to..."

The drop from 72 mph to 57 was chosen via the cubic rule to correspond to (again) a published HP level where we know the fuel consumption rate. so again, I manipulated the numbers so that I could draw from known and published performance data.

and kayjh, guess I didn't explain WOT. sorry about that. when I said "blasting away at WOT" that means what phox said. foot on the floor and engine running like a bat out of you know where!