Truck was bine stock for the first 180,000, and my mileage still sucked. Now F it, - heavy *** 35's, 4.88's. Sitting up taller probably doesn't help it out much either. But I'll still be buried in that thing.

 

I had an 04 RCSB 5.3. It was never great or poor on fuel

 

2003 1500HD here and it gets 7 mpg towing a 26' boat and 9-10 MPG towing my Camaro on an open trailer. Unloaded it will average 12-13 MPG. I have had it tweaked via HPTuners as well and it definitely picked up power and response but MPG is the same.

The 4L80E combined with a heavy truck hurts the MPG big time.

 

The cathedral 6.0s use the same tb, intake, and injectors as the 4.8 and 5.3 also.
Remove the fan clutch and watch the mpg increase quite a bit and towing power as well. I run em on all 3 of my 6.0 trucks in separate iterations and love them and routinely haul at least 200lbs extra in the truck and haul cars etc that weigh 2k to 17k for some heavy equipment.
4.10s in all 3. Id like 3.73s actually as 2500rpm at 70+ drinks gas anyway but does keep it in a nice torque range.
Pedal modulation and not using cruise control helps as well as a good tune, regular maintenance, and a nice cai....and def efans.

 

Ouch!! Talk about hurting. I hope the lake was pretty close to the house and the drag strip is too. It's just hard to believe a modern fuel injected motor would do so terrible. That's worse than a old school big block which displaces almost a 100 cubic inch difference.

 

Whoa. TOTALLY forgot that trucks had belt-driven fans! HUGE power sponge, AND fuel sucker.

 

^^^^^Put e fans in my truck - noticed absolutely nothing.

It's not just a modern fuel injected motor, it's a modern fuel injected motor pushing a S ton of weight. Empty my pickup weighs 5,500lbs. My 68 Suburban is 4,000. Burb has a mikd 396, both get about the same mileage. Burb has a much better power curve for general cruising, gutless when romped. Truck is exact opposite.

 

When I ordered my 2001 Chev 2500 (NOT HD) I specified a 5 sp manual and a 3.73 gears. With now over 200,000 miles I still average 18 to 19 mph @70 mph. I have seen 20.6 once due to road construction that kept my speed to 60 mph. BTW with the topper it weighs over 6200 lb. It is an ext cab swb 4X4. When pulling my Wilderness 22 ft camper it drops down to 12 to 14 mpg. On level ground it will cruise at 65 mph in 5th gear. Unfortunately level ground is scarce in Wyoming.

 

Ive not just done it to mine but at least 30 others. both 99-04 1500s and nbs and nnbs 2500s. Every single one saw a gain in performance and a little better mpg.

What fans did you use and how did you control them?

I can tell you you should have noticed a cooler ac, less engine bay clutter, better throttle response, less water pump issues, less low speed fan roar when its very hot outside.
Maybe next tim i get one done i can do a before and after situation to show actual gains.

 

It's not just the 6.0. If you tow, your mileage goes down. My 5.3 gets about 12 towing a 3000# trailer. My Ecoboost gets 12-13 towing a 3000# trailer.

 

Anybody here owns a 8.1 motor? I heard they get 10 to 12 mpg on average. Neck to neck with a smaller 6.0 am but there is a major torque difference. The 8.1 motors are rare and quite different. I wonder is the fuel injection the same style or does it have a direct port injection setup......

I also wonder if the altitude have a lot to do with the gas milage.

 

What liter is the ecoboost? Sound like you pull up and down hills a lot.

 

8.1s arent that rare.
Gm makes an 8.0l now also.
The 8.1 uses ls coils and ls computer to run it. They were all dbw afaik. Used the c5 vette tb.
They have a very odd intake and had alot of oil consumption and pcv issues.
They def got a bum rap but did drink gas. They had cast iron heads but used an ls style cathedral port. They are fairly weak stock and limit lift but after some work they arent bad.

 

They don't burn that much more fuel than a 6.0 but yes have more power

 

What's the budget here?

 

Not quite sure what I want for a truck yet but im not going under the bus for a 60k diesel. Anyway
I'm trying to figure out the problem here with these 6.0 motors vs a 5.3 which some guys are claiming 20 mpg and a 6.2 gets better milage than both and has more power than both. It's a real head scratcher.... I wonder if GM cammed the factory motors wrong ?

 

04 - 07 1/2 lq4 got the 01-02 ls1 cam which was markedly better than the cam that was in previous models.
The 4l80 trans has tall gears. For towing and hauling the 4l60 gear would be better and it even has a slightly better overdrive even has better reverse. The trans is weaker of course in stock form but lighter. Built up like a lvl4 + from rpm trans swap from 4.10s to some 3.73s would be pretty good. Thats as far down as you can go on the 10.5" ring gear....from motive/Richmond at least. not sure on aam or yukon etc.
The crusing rpm makes a big difference.

 

This particular trip was a vacation and the lake is 6 hours away. Our local lake is only 20 minutes away though. The dragstrip is about an hour and a half drive.

These are big, heavy trucks with power robbing drivetrains in them. I knew going in that it wasn't going to get great fuel mileage. I had an AWD Buick Rainier with a 5.3 in it and it would only manage 16-17 MPG unloaded. My old 02 Avalanche would get 14-15 MPG. Some of the guys claiming they get upper teens with these older trucks must set the cruise at 60 MPH on flat ground.

I work from home 2 days a week and the other 3 days I drive a 2002 Saturn SL2 that gets over 30 MPG. The truck just sits until I actually need to use it. A diesel wasn't in the cards for me either because I didn't want those potential problems and didn't want to spend a bunch of money for a lower mileage one.

I plan on doing E-Fans on my 1500HD in the next couple of months. I already have a set of Fbody fans so all I need is to order the harness.

 

If you sit down and figure out what a couple miles per gallon saves you every year it ain't much.

As for why are the 6.0's so much harder on fuel well it's a plethora of reasons and not just one culprit.

If this truck is a daily my vote is for a 6.2 truck with the max trailering package. I've had 5.3, 6.0's, 6.2's, 454 vortec, etc. Of course when it comes to towing a Dmax is king but unnecessary.

The big block trucks (454, 8.1L) aren't as cool as they sound. But they do tow well

 

If money was not a factor, I would have a Duramax, but I am happy with the 6.0/6L90 despite the 12 mpg.

I was getting 11.5-12 hauling a Grand Wagoneer on a Uhaul trailer and running 75-80.

Edit:

My 2010 2500HD is heavy, right about 7,200 on the scales at the landfill.

The computer indicates 12.1 average, 5,564 gallons of fuel over 113,203 miles.

My Dad's 2013 1500 crew cab shorbed 4x2 5.3/6L90? Has AFM, not sure about VVT.

The computer indicates 18.8 average, 1,868 gallons of fuel over 108,846 miles.