Maximizing LLY fuel mileage

[Kennedy]

I can't get 20's at 70 mph, but at 60-ish mph I can run solidly into the 20's. It will be interesting to see what Uncle Wally reports as I just sent him a tune for his dually. The DIC shows encouraging results so far, just need to hand check to verify...

[Uncle Wally]

Just to give a comparison as to what my fuel mileage was before John's tune.

05 LLY Crew Cab Dually - stock tires.

Loaded - light to moderate

Modifications at the time were John's 4" exhaust, Uni Air Filter, Mega Filter and Lift Pump/Fuel Pump Pickup mods. I had EFI Live at the time, but chose to leave the stock tune in it for a while.

I was pulling a two place snowmobile trailer with two snowmobiles on it and I had one snowmobile in the bed. There were three of us and our gear in the truck. We added two honda three wheelers to the trailer on the way back.

I drove from Minneapolis to Island Park Idaho in March this year. Temperatures were in the mid 40's for most of the trip out and in the low 60's for most of the trip back.

My best fuel mileage per tank - hand calculated - was 12.5. That was treating the throttle like an egg, using cruise at 60 and never letting the tack go over 2000 rpm.

My worst fuel mileage per tank - hand calculated - was 10. My personal belief is that I got a batch of marginal fuel. I didn't change my driving style much, so I don't know of any other explanation.

Unloaded - with tonneau cover - EFI Live now installed - only changes to the tune were to EGR controls. No added fuel, no altered timing.

I get my best mileage at 50-52 mph. Treating the throttle like an egg, never letting the tack over 2000 rpm even on acceleration. I can get between 16 and 18 mpg. I drove this entire tank of fuel - no wife variables on this one.

I usually don't drive that way day to day, nor does my wife. Normal driving yields a pretty consistant 14-15 mpg.

The only issue is that I spoiled my wife with the new truck, and she won't drive our gas miser car anymore. My wife drives totally different than I do even with me giving her constant grief. Her fuel miser skills have gotten better, but are still not at my level. She only understands two pedal postions: foot completely off the pedal, and the pedal all the way to floor (for both foot feed and brake pedals). With that said, it's hard to get a good fuel economy run when my wife drives. I will work on that when we head to Pittsburgh in July. That way I can actually show her what I am talking about.

Sorry for the long post, but I thought it would be good for a comparison. I installed John's tune yesterday and drove about 45 miles on it. Fuel mileage on the DIC was astonishing. I won't give numbers at this point as I need to hand calc it. You'll here from me after a couple tanks of fuel.

Waldo

[Kennedy]

Just FYI, the changes made should not have had a big impact on DIC readings, in fact, should be none at all...

[REDTRUCK05]

Are you happy getting 14-15 under normal driving conditions? U say your best milage is at 50-60 mph? That makes sense, its logic. People just dont drive that slow on the interstate in Mpls. I like to drive the speed limit 70 mph. If I do this I get between 9 and 11 mpg, pulling 5600 lbs. If I treat the pedal " like an egg shell" unloaded I can get 14 - 15 at 70 mph. If i slow it down to 60 mph ( only when I have to ) I can get 16 - 17 mpg. Do you guys really think these numbers are ok? I think not...

[REDTRUCK05]

I can't get 20's at 70 mph, but at 60-ish mph I can run solidly into the 20's. It will be interesting to see what Uncle Wally reports as I just sent him a tune for his dually. The DIC shows encouraging results so far, just need to hand check to verify...

Let us all know how ur new tunes do. Why is your 05 for sale?

[Kennedy]

Let us all know how ur new tunes do. Why is your 05 for sale?

I'm waiting for more feedback from my testers. I started another tester at the Rendezvous.

I'm not really pressing to sell my 2005, but I put it up in anticipation of acquiring a 2006 LBZ.

I expect 19-20+ mpg from a single rear wheel truck driven at approx 60mph. My MPG drops at 70, but seems to drop less with tuning.

[ToddMeister]

I'm the economy tune tester from the Rendezvous John was talking about above. Took a long trip this past week and averaged 20 mpg with expressway 70-75 mph, some highway at 55-60 mph, and a bit of city driving mixed in. Outside temps were 75-80 degrees.

Truck is a 2005 GMC Crew Cab 4x4 Duramax LLY/Allison. Presently 13,000 miles on the truck.

I want to take a trip of primarily 55-60mph highway driving to see what the tune is capable of.

I am also running Michelin LTX M/S 265-75-16 tires, which will lower your mileage a tad bit from the stock 245's.

At 70-75 mph EGT runs 600-650 degrees, and boost is about 3.5 lbs (on flat roads). At 55-60 mph EGT is 550-600 degrees and boost is less than 2 lbs. These readings are taken pre-turbo.

More to come....

Todd

[Uncle Wally]

John - thanks for the reminder. I'm still out of state so there is more to come.

Initial reports thus far:

I weighed or entire rig at the local feed lot as we were leaving: 20500 lbs.

1st tank w/ DIC (no hand calcs at this time) - 13.6 mpg (the wife drove)
2nd tank w/ DIC - 15.8 mpg (I drove)
3rd tank w/DIC - 14.5 mpg (we both drove) This tank also had some some really steep grade coming through Wheeling, EV and then into PA south of Pittsburgh (all hills).

To come:
Hand calcs to compare against DIC (although I did notice less of a discrepancy with the 'fuel used' portion of the DIC).
Empty tank mileage with only me driving
Complete rundown of performance and mileage when I get back home

Just a quick note. Thanks.

Waldo

[Uncle Wally]

OK. Sorry for the wait. Taking 3 weeks off makes for alot of catching up to do.

Hand calcs are as follows:
Tank #1 - 13.55 mpg - DIC showed 13.6
Tank #2 - 13.65 mpg - DIC showed 15.8
Tank #3 - 12.84 mpg - DIC showed 14.5
Tank #4 - 22.36 mpg - DIC showed 23.6
Tank #5 - 16.16 mpg - DIC showed 18.3
Tank #6 - 13.15 mpg - DIC showed 13.9
Tank #7 - 12.29 mog - DIC showed 13.3
Tank #8 - 11.58 mpg - DIC showed 12.2
Tank #9 - 12.04 mpg - DIC showed 13.1

Again - I fill the truck the same way every time: fuel is at the lip and almost ready to spill out with the truck sitting as level as possible - engine not running.

I compared these figures to hand calcs from date of purchase. They are as follows:

Bone Stock - average of 13.91 mpg over ~10k

With JK's exhaust, Uni Filter and Mega filter upgrades - average of 13.72 over ~7k

Add EFI Live to above (only EGR fooled with - no added fuel or timing) - average of 15.9 over ~3k

Add JK's timing tune to above - average of 14.9 over ~5k (this is skewed as a majority of this is towing our horse trailer - only about 1500 miles without the trailer)

The shining star thus far: I hand calced a tank of fuel at 22.36 on a trip from Pittsburgh to Baltimore and back. This was my one chance to use an entire tank without my wife's heavy footed influence (which ever peddle she has her foot on, it goes to the floor). I only stopped once and that was to pull into my buddies driveway. I drove like a grandpa (no offense intended) and treated the throttle like an egg, never going above 55 mph, and coasting to stop signs and lights whenever possible. I know everyone doesn't drive this way normally, but I wanted to know what the truck was capable of. I usually drive this way when I just filled up the tank and am pissed about how much it just cost me. After a few hours, I'm back to my normal style that yields about 18.5-19.5 on the DIC. I usually get impatient with drivers that shouldn't be on the road, so there are a few full throttle runs and the occasional 80+ mph flights. With that said, I'm tickled to death that a 4dr dually can get this kind of mileage and have the power to play around every now and again.

Had I to do it over again, I would have done the same mods within the first 100 miles of ownership, and I still have a couple to go as of yet.

I keep some pretty decent records, so I would expect an update by the end of summer.

Waldo

[Albee]

I had a 02 LB7 2500HD W/Allison. Took a trip back to Missouri from Wyoming. From Cheyenne to Lincoln Ne. the truck got 20.7 MPG (hand calculated) running 75 the whole way. On the way back to Wy. the truck got 18.3. Here we go with uphill and downhill and wind. The wind always blows here and alway from West to east. I was very happy. The truck got consistent 15.5 to 16.5 running my day to day routes in town during the summer months. During the winter months it would get right at 14-15 MPG.

The new 06 LLY 3500 Dually, Allison does not do as well. I have not taken the trip back east since had this truck. But running my standard routes here in town the truck gets 13-14 in the summer and in the winter it drops to 12. I now pull a 18K fifth wheel with the truck and pull at 60-65 MPH average. Pull 175 miles to camp over three passes going as high as 9800ft. elevation and the average for the trip is 9.3 there and back. I use much more going than coming home. These were all bone stock truck numbers. I have now added 4" catback exhaust. But do not see much difference, maybe .5 MPG improvement at most. This truck does not do as well as the other one. But as stated before it is heavier and has the extra wheels. But she sure do run good!!

This is a great forum!!!! Keep up the good work guys.

[rob from bc canada]

I'm waiting for more feedback from my testers. I started another tester at the Rendezvous.
...
I expect 19-20+ mpg from a single rear wheel truck driven at approx 60mph. My MPG drops at 70, but seems to drop less with tuning.

This "Economy Tune" for sale yet? What is involved and how much?

Need any Canadian testers???

I've got about 17,000 km on my truck now, and I've kept records for every fillup so far.

[Kennedy]

The tunes are available any time. There is a review by Todd Smith on this site.