Taming the Beast - Making a GMC 2500HD... Less Terrible
 

Hi all,

Not so much an introduction for me, as a resurrection.

Old timers on the site may fire a random neuron or two when they see my user name, as I used to contribute a little bit around here. Back in 2005 I picked up my 2003 Jetta TDI and modded it from approx 46 hwy up to ~56 mpg hwy, and from 90 BHP stock to more like 140. It was a GREAT car, fun to drive with nothing but scheduled maintenance and like 6 or 7 cents cents per mile avg lifetime. But she was starting to age, I was bored with it, and with maintenance looming I decided to jump out while she still had some reasonable market value.

My build/mod thread on here.

Farewell, lil' TDI.... I hope your new owner is as meticulous as he promised.

In her prime:
http://i287.photobucket.com/albums/l...z/IMG_3848.jpg

Anyway. I sold the TDI and my old beater pickup truck to focus on first obtaining a dependable, capable work truck, before again finding an economical daily driver when my finances allow.

Enter, THE BEAST (needs a more original name... forthcoming, I hope. Suggestions accepted).

http://i287.photobucket.com/albums/l...s/IMG_1719.jpg

Basically the exact opposite vehicle to daily drive vs my prior TDI. oof.
Actually, I'm trying as hard as I can not to drive this thing. I can drive my wife's passat wagon @ 30+ mpg whenever she doesn't expect to stray far from home during the day. But I'll probably still drive the truck 2-3 days a week or so, so the mpg simply HAS to improve. I have lots of ideas/plans, but not a lot of free time - we're still finishing building our new home and getting life established out on the land (for which having a dependable pickup is pretty crucial).

Anyway, I can justify owning this truck a number of ways, but the bottom line is I went from 50 mpg to 13. Ouch. It's nice that gas is cheap, but I simply cannot accept such poor mileage. My current goal is 15mpg in winter, 18 in summer. 20 would be amazing.... possible? we'll see.

Typical weekend doing chores (we heat with firewood):
http://i287.photobucket.com/albums/l...SizeRender.jpg

Modification posts coming as I've already done some stuff.

Step one: pick the low hanging fruit.

There's no denying that I will be able to do very little about the gross weight of this beast. Simply not much to be done about it. Even removing 100-200 lbs from a 5500 truck will not be very noticeable in terms of mpg. But my commute is mostly back road cruising, and the aerodynamics of this thing can be vastly improved.

For example:

The truck came with this front wind deflector / bug shield. GONE.

http://i287.photobucket.com/albums/l...dbcddff685.jpg

I mostly listen to local radio stations. Don't need the stock 3 foot antenna. I also don't ever plan to renew the OnStar service. Antenna swap!

http://i287.photobucket.com/albums/l...s/IMG_1936.jpg

Capped the OnStar antenna stud:

http://i287.photobucket.com/albums/l...s/IMG_1937.jpg

And put the shortly on the hood:

http://i287.photobucket.com/albums/l...s/IMG_1955.jpg

I think it looks pretty sharp there, actually.

The front end of this truck is a total aerodynamic mess. I plan to seal up all the gaps eventually. Found that some rubber hose filled between the bumper and grill ok:

http://i287.photobucket.com/albums/l...s/IMG_1949.jpg

Vs:

http://i287.photobucket.com/albums/l...s/IMG_1948.jpg

But then it was time to spend a little money.

Some tonneau cover manufacturers claim as much as 10% mpg improvement from a simple cover. Bull sh!t, but, it can't hurt. I wanted one to protect my cargo anyway, so it made sense to buy and install one more or less right away after I bought the truck. Installed it at work as soon as it arrived:

http://i287.photobucket.com/albums/l...338536c054.jpg

One thing I always do to my vehicles is install a badge-less front grill. I dunno why, I just really love a stealthier look. It also makes fabrication of a partial grill-block that much easier.

First, pull the grill shell / front clip and cut out the stock GMC part.

http://i287.photobucket.com/albums/l...s/IMG_1945.jpg

Then fit the replacement:

http://i287.photobucket.com/albums/l...s/IMG_1947.jpg

I didn't get pics of making the grill block itself. I cut some abs plastic rather meticulously until it fit nicely and just zip-tied it in behind the new grill. I estimate the block-off of the main upper grill opening at around 70%. Plenty of airflow hitting the radiator still; I may block the lower openings if I can figure out a clean way to do it (no duct tape for me).

http://i287.photobucket.com/albums/l...s/IMG_1950.jpg

Mods: request move this thread to ecomodder central forum? Seems more appropriate from what I am seeing. thanks.

Does your Chevy have an electric fan or one with a clutch?
Some wheel spats might work. I saw some on a new GMC.

Mechanical fan w/ clutch. Electric fan swap is definitely in the cards this summer. I've heard tales of +1 mpg right there alone.

Wheels spats.... like as in some flat panels directly in front of the wheels/tires? Planning on it. Along with a front under-tray and air dam.

More about the truck.
 

Did I really need a 2500HD? No, I'll be honest, probably not (though towing my tractor/cars/equipment rentals is going to be more comfortable). But what I really did need, was a rust-free, low-mileage, dependable work truck.

I really wanted to find a chevy/gmc 1500 with a 6-spd transmission and cylinder-deactivation, but it turns out they are still out of my price range with anything less than a ton of miles.

A 1500 truck with the 5.3L V8, even with the old 4-spd 4L60 trans, would likely return appreciably better mpg than this, but I honestly could not find a rust-free, well cared for example in many months of searching.

I searched craigslist daily until this beauty popped up. A one-owner, 2500HD SLT, but in the smallest/lightest configuration (aside from the regular cab / long bed, which doesn't work for me as I have 1 kid now and another on the way) and with the smallest available engine, the teeny tiny 6.0L V8. :p

2003 GMC Sierra 2500HD SLT
LQ4 6.0L V8 engine
2WD/4-HI/4-LO manual transfer case
4L80 HD 4-spd Trans
4.10 rear end gearing (ugh)
~5600 lb curb weight (best data I can find)
10,200 lbs towing capacity (under-rated) and ~3800 lbs payload (dayumn).


Not a great starting point for economy. I somewhat naively assumed that with my laid-back commute of gentle back-road cruising, I could still see high teens in this truck. Now it would appear that will be difficult (but I WILL get there this coming summer, or die trying).

Starting fuel economy in the fall months: 13.0 mpg

Now that we're into real winter... it tried to get worse (a LOT worse) but my initial mods are holding it steady so far.

You're not the only crazy person here, I have the same truck in 2WD. http://ecomodder.com/forum/showthrea...0hd-32970.html I'm mainly doing aeromods, though parts will be replaced with more efficient ones as they fail, if available. I was on vacation for the last month, so I haven't put many miles on it with the mods I've done so far. Stock I would get about 13MPG in mixed driving, about 14MPG doing 75MPH, and 16MPG doing about 60MPH.

If you got the time, money, tools and energy, maybe you should make a truck aerocap similar to this guys aero cap (link down below), it should yield alot better results than the cover. I know I would love to own a Ford ranger just so I could do this to it lol

http://ecomodder.com/forum/showthrea...tch-33308.html

That's pretty terrible because I was getting between 12 and 13 with my 454, 3 speed transmission, 4.10 gears with 1970s non aero dynamics in mixed driving.
All I did was tune the carb for lean burn and EOC.
I removed the fan, power steering and alt and getting 14mpg in the winter. I pulled all the accessories off the engine just as it got real cold. So MPGs could be higher in warmer weather.

Regular cab pickups are pointless, good call on getting the extended cab.

Regular cab pickups are only good for project goers. Those interested in drag racing, my welding shop teacher owns a 90's f150 regular cab short bed that does 9's in the 1/4 miles. I would own a regular cab pickup but then again I dont have a family or anykids.

Thanks for the replies all.

Phew! Thanks so much for the link! I didn't know anyone else was crazy enough to invest mental energy and time into what may seem a somewhat fruitless endeavor. Will definitely follow your progress/mods for inspiration.

Yeah... it's hard to argue with the inherent reliability of these trucks. Start changing stuff... and it may not ever be the same again. And short of new heads/cams/headers (mucho $$) there's not a lot of combustion efficiency improvement to be found on these small block engines.

Sounds like about what I am seeing in mine. 2WD vs. 4WD is probably not as big a difference in these trucks since the curb weight is so high and same poor aerodynamics & ride height.

Eh. I know those really do the best job of getting a pickup into the proper aero profile, but they really ruin so much functionality of having the truck in the first place. Not to mention I've only seen a few that aren't totally butt-ugly. If I made one, it probably would be too. :o

Ha, yeah. I mean, probably similar final OD gearing, your truck may be a bit lighter, and they didn't do much with aerodynamics yet for this generation... I promise you I can beat your mpg numbers without removing my engine accessories though! :p

Monitoring and retaining the heat.
 

Ok, so, I posted some of my mods kind of out of order here.

OBVIOUSLY, my first mod was this bad boy:

http://i287.photobucket.com/albums/l...s/IMG_1670.jpg

Took a few attempts to find the right x-gauge commands to add the transmission temp, air/fuel ratio, and spark-retard. But it's nice to have such diverse instrumentation available.

One of the reasons I really wanted to add the trans temp reading is that I was starting to have issues with no torque-converter lockup when it was really cold out. While I'm still not totally sure that there's nothing wrong with my TC, it did seem correlated to ambient weather temperature. I guess there is some programming logic to keep the TC unlocked to help warm up the transmission, but it was costing me some serious fuel.

The previous owner of my truck was meticulous and dedicated to maintenance, thankfully. So of course he still had the original window sticker. I didn't read it over until after taking the truck home, but much to my delight I saw he optioned the engine block heater! I dug around in the front bumper and sure enough, there was the plug.

Now, if I'm going to go to the trouble of plugging my truck in on a timer to warm the block/coolant, I realized I might as well help the transmission and oil out a little also. So I quickly ordered two of these 150W pad heaters:

Amazon.com: Kat's 24150 150 Watt 4"x 5" Universal Hot Pad Heater: Automotive

http://i287.photobucket.com/albums/l...L._SL1500_.jpg

So I slapped 'em on the oil pan:

http://i287.photobucket.com/albums/l...s/IMG_1938.jpg

And transmission pan:
http://i287.photobucket.com/albums/l...s/IMG_1939.jpg

Shot of both for context:
http://i287.photobucket.com/albums/l...s/IMG_1940.jpg

Then I got both pad heaters wired in with stock block heater up to a marinco plug in the front bumper:

http://i287.photobucket.com/albums/l...s/IMG_1942.jpg

With a timer set to be on from 3 or 4am-8am I am getting anywhere from 40-60°F above ambient for the trans and coolant (no oil temp gauge yet). Whoo-hoo!

So, not quite "toasty warm", but well above the ~10 degree air we've been having lately. And I am happy to report that the T/C locks up much more readily. Probably somewhere between a 1-2mpg impact from pre-heating before my morning commute. The one time I previously tried letting the truck warm up for 10 minutes on a frigid morning before I drove to work... well, I am too embarrassed to report how bad the fuel economy was. Let's just say... single digits.

Good first mods. You reminded me about the pad heaters too... might have to pick one up.

You need one of these:
http://ecomodder.com/forum/showthrea...hlight=5%2C500
If you want toasty warm.

What transmission and rear end does that truck have?
You may not need 4.1 gears.

Mine originally weighed 5600lb, I weighed it on some scales.
The alt is going back on as soon as my bracket shows up and I have electric power steering to install. I tested it so I know it works, just have to install it.
http://ecomodder.com/forum/showthrea...ion-32895.html

4L80 trans and 4.10 gears.

I certainly don't need 4.10s, would rather have 3.73. But 4.10 was the only option in these 2500HDs. And with 4WD I have absolutely no desire to change both front and rear gears. Just a downside to this truck that I will have to live with I guess.

Your projects are impressive. Ambitious, too! Thanks for doing them and sharing.

The 4.10 gears are really overkill in these trucks when they're empty. Over 2k RPM at 60 and creeping up on 3k at 75.

Do you plan on doing any more aeromods?

Oh yeah, definitely. Will be trying to get all the low-hanging fruit taken care of, for sure. Following your work eagerly.

Kinda hard to get new mods done this time of year without a garage to work inside of though. Hoping to get my pole barn shell erected this year; might not be able to afford the concrete floor right away though....

All tucked in.
 

How the truck sleeps. Not surprised that my heaters can't get this thing too toasty given how much cold breeze is sweeping through the rig all night. Once I'm in a garage they should be a good bit more effective. Still helping a ton though!

Single digit mornings this week... brrrrr.

http://i287.photobucket.com/albums/l...s/IMG_2001.jpg

And while at work. We have some construction / parking issues currently that allow me to go rogue these days, and just park up against the building. But next winter I'll probably be stuck far away from any doors that would let me run an extension cord outside.....

http://i287.photobucket.com/albums/l...s/IMG_2003.jpg

6.0 not known for mpg.

Best to focus on mechanical, first:

1) Perfect alignment. Verify as to best, not just OEM acceptable.

2) Steering slop. From steering column outwards; eliminate.

3) KONI FSD shock absorbers.

4) Upsize front antiroll bar and install one on rear if not so equipped. Grease able polyurethane bushings.

5) Rear Panhard Rod. See if Hendersons Lineup has one for yours.

6) Zero brake drag. Verify. Get it up in air to do so, and lube all moving parts.

7) Highest quality tires, such as commercial traction Bridgestone Duravis. Wear like iron with excellent performance. On a Hunter GP-9700 balancer to Lexus/Mercedes spec. Centramatic balancers all around.

8) Get it on a Cat Scale. Driver, full fuel and normal gear aboard. For second and third scale readings have port or starboard tires off of pads in order to know all wheel position weights.

9) Synthetic fluids. Donaldson or Baldwin filters. If ten years or more in age, then new hoses and belts plus fasteners. New factory battery cables. AGM battery (Sears Diehard Platinum). All new exterior lamps (Phillips Nightguide head lamps; new housings if yellowed). Bosch Icon wiper blades after two/three coats of RainX (especially on side glass and mirrors). New windshield if sandblasted.

10) Have driveshaft removed and ujoints replaced if over 100k miles or ten years. Shop must be able to spin balance shaft. Install Balancemaster.

11) All other book maintenance 10-20% ahead of book recommendations. A new owner starts a new wear pattern on the vehicle. Get ahead of curve.

Steering pays greatest benefit after align and brakes. Shocks and bars are hugely beneficial to an EM driver. Others allow more precise control without wear issues clouding records.

The 45-mph and lower mechanical issues pay better than the 60-mph aero stuff. But, not sexy, and not clear to most as to cost/benefit. As they will in fact add to longevity, their importance must not be overlooked.

Great to see block and pan heaters!! In the diesel world we look at 90-minutes warmup as being 90% of the benefit of plugging in all night.

Every so often it's important to take it out for a 100-mile run. Burn off acids and condensation in fluids, reinvigorate greases, and get tires up to stabilized temps so oils can migrate through casing. That takes 1.5-hrs at steady state to achieve equal temps inside and out of casing.

SCHAEFFERS Oil products, also. Throughout truck.

Wow.

Great list! Thanks for taking the time to type that all out. :thumbup:

Unfortunately you just described several thousand dollars of work. :p

I will need new tires next year at some point, and I definitely planned on getting aligned at that time. New shocks would also be wise before I align. Have Michelin LTXs now, which I like, but 2WD traction is lacking. Was thinking Michelin LTX M/S next time.

And yes, my truck does need the steering column bearing replaced. It's a bit sloppy right now but tracks smooth on the road. Could still be wasting some fuel and tire wear there, no doubt.

The centramatic and balancemaster products look very interesting. I like the idea a lot but cost is an issue. Same for rear panhard bar kit.... $463! ouch.

I doubt my old coolant hoses and windshield are wasting fuel though, lol. There's something to be said for not messing with things that already work....

Thanks again. A great list for any HD truck owner.

Staying ahead of the wear curve pays more than day to day FE as shortened work life means another vehicle. Depreciation is low on yours, now, so don't make mistake of allowing deterioration as is usually done. I would be spending $2-5k on something like yours from the day of purchase. Too much experience the past forty some odd years that always argued for staying ahead.

Truck handling is not well appreciated. The test is to stay lane-centered despite all difficulties. Mines a slot car . . how's yours? Mine will have zero corrections to your many, and what it does have will, literally, be done with thumb and forefinger. On a freeway entry cloverleaf or other where we may compare speed. Etc.

Keep thinking thru the problem. You aren't there yet. Number of steering corrections and need to bleed speed to accommodate these lunkers is what separates them. Best shocks and suspension mods only downside is upfront money. A set of tires is $1300 for me. But they last over 120k. I could easily have wasted your savings by making the tires work harder. Etc.

Handling is huge.

Again I cannot disagree with you in principle, and really appreciate your comments, but perhaps you have the wrong idea about my intended vehicle use here. This is my personal daily driver for now, but is intended to be an extra, backup vehicle in the hopefully not-too-distant future. A weekend work truck and camper hauler. Even as it is right now, I'm only driving it around 100 miles per week. No commercial hauling, no plans to ever hit 250k miles on this thing. It only has 89k miles now.

It simply does not make sense for me to blow $3000 into my suspension, nor could I afford or justify doing so. The truck rides well now, steers well, tracks straight, etc. I will try to upgrade and maintain the truck as best as I can, but throwing money at parts that may not even actually need replacement is not really responsible behavior, you know what I mean? If I "get behind the wear curve" a little, I can replace parts to bring it back again. The truck is not going to collapse into a worthless pile because the shocks get soft. And while I don't really care too much about resale value or depreciation, because I hope to keep this bad boy around forever, I'm more concerned with preventing body rust, frankly. That's what actually kills trucks and takes them off the road here in Michigan.

And from a fuel economy standpoint.... with sub-$2 gasoline, my wallet doesn't really care what mpg I get. But my pride and guilt do, and my general belief that we all just have to use less of this sh!t. I can do aero-mods for almost free... no reason not to. Steering and suspension work will be as-needed, because A) I have no garage to work in myself, and B) not much spare cash at the moment. Hopefully I can fix both of those points within the next 1-1.5 years though.

Anywhoo.

Increasing engine efficiency (and power :D ) is always another modding priority of mine.

My last two daily drivers have been turbocharged, where increasing power is about as easy as cranking up the boost and putting on some bigger intake/exhaust pipes.

But now that I'm in the land of naturally aspirated operation and stoichiometric air fuel ratios... there's not as much easy modding to do.

An air intake upgrade is one legitimate option though. I know this flies in the face of a lot of readers here who would rather sacrifice some power with a warm air intake to help reducing throttling/pumping losses, but, I would like to be able to tow 10,000 lbs with confidence. So a less restrictive intake path can still help reduce pumping losses a bit.

Based upon some independent dyno-testing results on a GM 5.3L, I opted to go the cheaper route and replace only the filter and intake pipe. The factory airbox can already flow plenty of air.

BBP Independent Intake Test Results | Chevy Truck Forum | GMC Truck Forum - GmFullsize.com
https://dl.dropboxusercontent.com/u/...%20Results.pdf

I went with an AEM dryflow filter for it's high flow and washable characteristics.

http://i287.photobucket.com/albums/l...28-20129_1.jpg

The stock intake pipe has a funky "sound tube" appendage that surely doesn't help the airflow.

http://i287.photobucket.com/albums/l...s/IMG_2002.jpg

Replacement pipe still had a notch to clear the mechanical fan shroud.... GRRrrr, that fan has to go!

http://i287.photobucket.com/albums/l...s/IMG_1999.jpg

The next mod is not something I would ordinarily pay any extra money for, but, I got suckered in by some discounts and free shipping while buying other things. The only reason I actually bought the plug wire set was an attempt to fix an occasional P0300 "random misfire" code that I have been getting (more on that later).

http://i287.photobucket.com/albums/l...s/IMG_2007.jpg

Fancy red color bling, but no change in performance (duh). Was still getting the P0300 code, mainly at highway speeds. This was actually a huge problem, because any reported misfires cause the ECU to unlock the torque converter. So then you are wasting fuel into transmission heat, and eventually burning up the TC. Not cool.

Internet search to the rescue! Thankfully I have access to a professional grade GM scan-tool here at work (GM Tech II). Turns out all I had to do was a "crank position variation learn" procedure. Just select the function, rev the engine, and boom, done.

http://i287.photobucket.com/albums/l...resdefault.jpg

The ECU "re-learns" the relationship between the crank and cam angle sensors, and re-adjusts its criteria for determining misfires. So basically I wasn't actually having any misfires, but the ECU thought I was. No more! Hopped on the highway to verify, and had full TC lockup and ~+2 mpg.

Getting much more consistent TC lockup now.

12.7 mpg for last tank though. Blarg. We can do better.

Ok, getting closer to my goal of a tank-avg 15 mpg in the winter time.

Got 14.0 mpg on the entirety of my last tank. Not so awful. With a few more aero-mods, and an ECU tune, I think I can make it pretty easily. Though with a possible early spring season right around the corner, I may not truly find out until next winter as I'm not quite ready for any more mods just yet.

I can hit 15+ mpg easily on individual trips. But any short trips around town really kill the mileage. Same for days like today... 5 degrees this morning. The engine heaters can only do so much while parked out in the wind.... basically a cold start still.

ScanGauge
 

If you were to momentarily pull in behind a semi at close quarters and reset the Scangauge,you could tell very soon what completely streamlining the truck would do.Then decide if the numbers looked good enough to go into deep aero mods.
I added 5-mpg to the Dodge with only aero.
And 9-mpg to the Toyota.
These are HWY-only numbers.

^ that's a great idea, thanks! I hardly ever drive on the highway so hadn't thought of that. Will give it a go at some point.

Be sure it's a box van. Moving vans have the lowest ground clearance
If the road is clear ahead of the van, likely he'll have the cruise on.

Steering corrections per 100 miles travel is cited by big truck mfrs as FE significant. Keeping perfectly lane centered is the game. On a straight piece of Interstate I can pretty well let go of my wheel for the better part of a measured mile (my one ton Dodge or my Kenworth); really, just holding it against crown with two fingers.

If you can't, it should be top of the list to correct. Getting out of wind is a good way to isolate.

Old thread i know, but any updates? I own a 2007 2500hd classic cclb 4x4 and having hard time breaking 11mpg. Currently have headers, exhaust, volant intake, black bear tune, and e fans. I honestly haven't seen even 1 mpg increase with any of this.

My family has a 99 2500hd, same body style as yours, and it is definitely a gas hog, 12-13 mpg driving, 8-9 mpg towing. With something that inefficiently designed, small things like that won't make much of a difference until the low hanging fruit is taken care of: smaller engine, much taller geared transmission, skinnier taller tires, aerocap in the rear, and an airdam would be necessary to see any major FE gains. Other than that the #1 way to improve FE would be driving style, mazimizing coasting time in neutral or with DFCO. EOC if it was a manual.

Yeah my driving style i know hurts. I drive a company van all day at work and sometimes i just want to get on it and hear the exhaust lol. And honesty i only drive it like 5k miles a year. I need new tires coming up and am going to switch away from the all terrains to some highway treads. I would love to switch over to a 6 speed manual transmission but the cost is too much for a truck that barely gets driven right now. The one think i really want to do is convert to manual locking hubs. I could see real improvement there, but again the cost is high for the rewards.

EcoModder! Wow, blast from the past. Glad it emailed me about this thread.
Having two young kiddos makes it harder to find time to care about aeromodding and hypermiling, lol. Sorry all my photobucket pics are dead. F U, photobucket. Not redoing those. Imgur for the win.

Not too many updates from me on my 03' ext-cab short bed 2500HD. I did get new wheels and tires. The Michelin LTX M/S are amazing tires, great grip wet, dry or cold. But I did go bigger and heavier... not an mpg gain, oops. Can still get solid 14-15mpg tanks in mixed driving.

https://i.imgur.com/ZTsDsJJ.jpg

My truck is up to 107k miles now. I really don't drive it very much. Went through two cheaper VW GTIs as daily drivers, both needed a bit of work and then I flipped them for modest profits after about a year each. Got used to about 30mpg in each of them, though they both required premium gas.

And then I just recently impulsively jumped on a new VW Alltrack when I realized they weren't making or selling them anymore after the 2019 models - last chance to get a turbo, 6-spd manual-trans, AWD family wagon. Only $22k NEW, with a 6-yr, 72k mi warranty. Currently getting a steady ~30 mpg in mixed driving, might be about all she can do.

Hadn't really thought about any more aeromods for my 2500HD. With only local driving to tow and haul stuff, and ~2000 miles of driving per year now, it just doesn't make sense to prioritize spending time on. BUT! We just got a new camper to haul around, which means highway trips to camping destinations. So a couple items are back on the table that I sadly never got around to - lower air dam, front wheel spats, maybe some undertray action. Will any of that matter when pulling a huge, dirty brick behind me at ~9mpg? Dunno, but seems more worth trying now.

https://i.imgur.com/rK8fF79.jpg

Driving style is the easy one, man! coast, coast, coast.

DFCO is tough in these trucks, mine has to be above 60mph when starting a coast down or it wont even bother. My scan gauge has taught me that when cruising a back road at 55mph, it is actually beneficial to accelerate gently to ~62 first before a coming turn or stop sign, to coast down on DFCO as much as possible.

Tires are huge too. When I got my 20" wheels, I really wanted some K02 all terrains for style and off-road capability. But you know what? Michelin LTX M/S tires are just as capable with fresh tread, and then you get completely QUIET driving and definitively better mpg. My 265/60/20 LTXs were only $215/tire, too. Highly recommend.

I'm surprised your electric fans didn't help more. That job is still on my list, contemplating doing it at the same time as a cam-swap. An LT1 or LS2 cam would really wake up the 6.0L LQ4, and presumably not hurt mpg - need to further verify that part though.

I have a very heavy tool box ladder rack deal on my truck. I removed it to help a friend move some furniture and kept it off until after a 400 mile trip to my dads iver Christmas. Tried to keep it about 5 over the speed limit to keep me in the left lane and avoid constant lane changes as its mostly 2 lanes each direction. Went from 9-10 mpg to 12.8mpg on the way there and 12.5 on the way back.
Im now using it for work again so the box/rack is back on and again getting about 9.5-10 average. The traffic out here doesnt help me at all though. We'll be going 65-70 and then come to a stop on the freeway and immediately back to 65.
Being said im looking into getting a 2.8l duramax van this year. Ive read good reports 20-35 mpg depending on load and driving. If i go that route truck will go back to big load and towing only duty.

The best roof racks are easy to remove so you only put them up when needed and take them off when not needed.

Well it's too bad that photobucket stole all my pics from this thread (and many others).

But I still have the 2500HD. Probably never getting rid of this beast, it's just too good and new truck prices are astronomical, of course.

I did complete a couple more aeromods last summer, in advance of a 2000 mile camper road trip.

Most importantly, a front air dam, finally. The 4" tall lawn edging looked big in my hands, but on the truck, it's clear that 6" tall would have been better. Oh well. Combined with some front wheel spats that my front tires rub on. And one extra piece of coroplast behind the air dam to smooth any flow that sneaks under it.

https://i.imgur.com/ri6EFpP.jpg

Then a bulb went out in my center cab-clearance light atop the roof, and I decided to finally try fabricating a more low-profile setup with LEDs. Was not easy, but I think it works.

https://i.imgur.com/rbzIsno.jpg

https://i.imgur.com/TR05uvy.jpg

Unfortunately the truck had to sit outdoors all winter long. So she got some detailing love this past weekend. I don't intend to daily drive it again, but we have a lot of projects on the ranch for this summer/fall and I'll be making a lot of errand runs for materials (lumber $ = ouch).

https://i.imgur.com/smw54wN.jpg

As for mpg these days. This is about as good as long trips or full tanks get. Significantly better than 12mpg, though.

https://i.imgur.com/yilXIj9.jpg

This is just straight up wrong. I don't know where this idea that OEMs don't do aerodynamic development on trucks/SUVs/vehicles that the public perceives to be "unaerodynamic" comes from, but it isn't true.

I have several papers on the aerodynamic development of various pickup trucks, and while this generation GMC/Chevrolet trucks isn't among them, the development of the 1988 C/K is. The paper is quite long and detailed; it overviews not just the optimization of the 1988 model but has quite a bit of information on the development of the two previous generations of full-size Chevrolet trucks.

Oh please. I was referring to the 2500/3500 trucks specifically, but still. I didn't say "none", either.

No front air dam. No underbody panels whatsoever. No wheel spats. No front grill smoothing or closures. No vents to/from wheel wells. No rear wheel well liners. No cares about bulky protrusions like antennas, roof cab lights, door handles, etc. No curvature at rear of cab or bed for slimming the rear wake, etc. Front track is 3" wider than rear. I could go on; there's dozens of areas where some basic engineering effort could have been applied, but clearly wasn't. This stuff was well known in the mid/late 90s when development of this platform was underway.

Bottom line, there was no return on investment for applying this kind of effort. I don't really blame them. Gas was $1.50/gallon, no EPA ratings for HD trucks anyway, and no one cared.