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Deezler 01-11-2016 11:55 AM

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:

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).

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):

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

Deezler 01-12-2016 01:40 PM

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.

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!

Capped the OnStar antenna stud:

And put the shortly on the hood:

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:


Deezler 01-12-2016 01:52 PM

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:

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.

Then fit the replacement:

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).

Deezler 01-12-2016 01:56 PM

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

MobilOne 01-13-2016 12:59 AM

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

Deezler 01-13-2016 11:12 AM

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.

Deezler 01-13-2016 04:11 PM

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.

vskid3 01-13-2016 05:36 PM

You're not the only crazy person here, I have the same truck in 2WD. 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.

Plush1992 01-13-2016 07:30 PM

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

oil pan 4 01-13-2016 11:11 PM

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.

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