04 Tahoe 20mpg goal
 

I'm new to ecomodding anything, but my recent purchase of a 2004 AWD 5.3 Tahoe has me looking for ways to get over 20mpg per tank. It's rated at 13/14/17. I'm currently tuned for and running 93oct e10-e15.

I've changed my driving habits quite a lot... I do 50 most of the time, minimize my idle time, put the trans in neutral when coming up to stop signs and slowing down, accellerate quickly keeping rpm below 3k, load around 40% and keeping AFR as lean as the tune allows during accel(14.2).

Here's what I've done
-BlackBear Performance ECU tune (horsepower and mpg)
-OEM 05-06 Tahoe/Suburban front air dam
-Roof rack delete
-Synthetics in Front/Rear diff, trans, engine and transfer case
-OEM 05 Electric fan conversion
-75% grill block

I haven't really been trying/modding for gas mileage until very recently. That's why I joined this site to get the last 2-3mpg of my 20mpg goal.

On the very near horizon
-3" front 5" rear drop
-LED lights conversion
-Getting the ECU retuned again with better shift points and leaner cruise AFR
-Coroplast or aluminum bellypan

Ideas are welcome. Just as a heads up I'm near my limit for visual mods.

Andrewhans

Make your self a bigger air dam or alter the factory one increasing its reach.

I run a lot leaner than 14:1 during part throttle light acceleration, I petal the carb for between 16:1 and 17:1.

I was able to get about 3 more MPG out of in town driving just doing EOC and turning the engine off when it wasn't needed.

I would love to do EOC coasting but my brakes are powered by the powersteering pump and the auto transmission would be damaged due to lack of lubrication since the pump isn't spinning with the engine off. About the best I can do in that department is Engine On Coasting.

As far as the AFR's are concerned I will be sending the ECU in to get retuned for much leaner AFR's during cruise and shifting earlier under more throttle.

The air dam is currently 9" from the ground so after I lower it will be down to 6". Is it advisable to go lower than that?

If that Tahoe has an 4L60E well then my longest EOC was 17 miles and it still works fine.
I have seen plenty of 4L60E and 4L80E transmission vehicles be flat towed over the years.
That myth appears to mostly apply to foreign FWD auto transmission cars.
I know I cant flat tow our auto transmission VW or Hyundai.

My diesel also has hydro boosted brakes and I EOC and brake all the time. The only reason to depend on power brakes is if you have some physical disability limiting the amount of force you can apply to a petal with your leg.

My air dam is about 5 inches and some one else on here has an air dam on here made from a rock conveyer that is 3/4 inches off the ground, its a full size red truck and I cant remember the guys user name.
The lower part of my air dam is made from lawn edging and takes a beating but that's is by design. It is easy to replace.

Just a note on this: are you familiar with DFCO (deceleration fuel cut-off)?

Depending on whether you're actually trying to slow down or whether you're just trying to coast, staying in gear is sometimes advantageous, depending on how your Tahoe handles it.

That's good to know!

I won't be doing anything involving lawn edging as looks are still very important to me. I think I read about the red truck on this site. Pretty impressive actually. I don't want to scrape on everything which with my lowering kit happening soon I may have enough of a problem as is.

I would love to do this and I will be asking my tuner to add this to my ECU update. The Tahoe currently doesn't do it though.

The 2 things that helped me the most (thus far) have been narrow light weight tires with steel wheels. And having my stock computer programmed with the low altitude factory program.

Most aluminum wheels are actually heavier than stock steel wheels in the same size. And there was a recall on the stock computer for my van. The new program they put in made a giant difference.

My best thus far has been 26mpg highway. With my goal being 30mpg. My next mods will be new tires for summer, domed hub caps, electric fan conversion, and power pulleys. I wanna see where these leave me before messing with any aero mods. But a partial grill block, front and side skirts are in my future.

I'm surprised you are having that much trouble hitting 20. My sister-in-law had a 4wd 5.3 Suburban we borrowed and even towing a large popup camper with 7 passengers we got 15. Then in a all interstate trip not towing at 70-75 it got 18. Does your 17 include a lot of in town? If not I would think about trying w/o the chip for awhile.

Hitting 18 on the interstate is fairly easy, so long as the wind is in your favour. I've only hit 20mpg once, that was from puttering around Yellowstone at 45 mph for several days. The aerodynamics at high speed is a wall that you won't overcome in stock form.

I have the AWD model not a selectible 4wd system or even an Auto engage 4wd system. I am ALWAYS driving all 4 wheels. It does include a normal amount of city driving. All highway I was able to get 18.8 before my tune or aero mods. I haven't had a chance to run the same trip yet after the tune. The combined rating of 14 is pretty close to what I would expect most other people to get with my vehicle. I'm happy with anything over 17 on my day to day commute.

I think 20mpg on the interstae will be totally doable once I lower it 3 in the front and 5 in the back to bring the vehicle level and low. I will work on aero a little more with a bit of a belly pan to clean up the bottom some. But you're right there is only so much I can do before I hit an aero limit.

I have actually already bought a set of OEM 20" wheels for a 2015 GMC Sierra. They are much better looking than the factory wheels and I don't expect to lose more than 1mpg while running them in the summer. We will see though.

Also I'm considering a 2wd swap not for mpg reasons but because the front diffs in these aren't built well and have to be rebuilt quite often to stay quiet running. Going from AWD to 2wd would be a massive jump in highway mileage as well I'm sure.

I will be getting the ECU reprogrammed shortly for better eco shift points and leaner running in the low to mid load areas of my map.

05-06 OEM Airdam, also you can't see it but I have a 75% grill block on as well
http://i268.photobucket.com/albums/j...205_140016.jpg

My new wheels, can't wait to get these on!
http://i268.photobucket.com/albums/j...123_163529.jpg

Roofrack delete, looks way better!
http://i268.photobucket.com/albums/j...203_115614.jpg

Electric fan conversion from 05 model
http://i268.photobucket.com/albums/j...205_214227.jpg

Overall distance side profile
http://i268.photobucket.com/albums/j...217_164444.jpg

While your converting it to 2wd you could swap in a manual transmission that would make a massive difference in fuel economy and reliability

I have already been eyeballing T56 and nv4500 swaps.

That is the problem with 4wd.
Its guaranteed 4wd when things will bind and break and its guaranteed 2wd when you actually need it.

Well it's been a rough summer for gas mileage.

The wheels and lowering happened about the same time so I think one negated the other to be honest. I still need to get the ecu retuned again, just haven't had the down time to send it in for a reflash.

I work all day in the NC heat so driving back and forth to work with the AC off just isn't an option for me personally. The heat has also been severe enough that I removed the grill block and am now 100% full airflow. All of these things have led me to poor mileage.

I am starting to keep track again of fuel mileage as I was getting very discouraged when seeing 15.xx mpg overal per tank so I stopped keeping track.

The 75% upper grill block may turn into 100% once the average daily temp goes below 70. I'll just rely on the lower grill to do the trick. Should be fine. I am also going to be actually accomplishing the belly pan on the bottom too.

She looks much better than the day I got her.

http://i268.photobucket.com/albums/j...3540987776.jpg

Are you sure that your 2004 GM vehicle doesn't do DFCO? The way to tell is if you have a mpg gauge, it will say 99.9 mpg when you coast. Otherwise, I don't know how you would know.

I missed this thread the first time through...

You are doing all the things I would suggest... but you have exactly the worst version of the Tahoe to start with for mpg. What gear ratio do you have? I'm guessing that is hurting you as well.

When shopping for a Tahoe for my wife, I picked out this one specifically for mileage:
http://ecomodder.com/forum/member-da...ifes-tahoe.jpg

It was lowered a little (didn't actually realize that when we bought it), and had 22" wheels, but was otherwise stock. 2wd, 4.8L V8, 3.23 gears. Now has 20" wheels from an '08 LTZ Silverado with 45psi in the tires. It's an '05 so it already has the OEM air dam (which is plenty, I wouldn't go lower) and the e-fan. We regularly see 20-25mpg hwy. Average is quite a bit lower though, since she does mostly city driving and has a rather heavy foot.

You can check out my Silverado thread below for some ideas if you like. Pictures were reposted late in the thread: http://ecomodder.com/forum/showthrea...tml#post441133
A couple pointers for blocking the grill:
1) Do you have a tranny cooler on the drivers side, in front of the radiator? I think this is standard on all of these trucks. I would strongly recommend leaving a hole for airflow there, especially if you tow.
2) The slots in the bumper should also bring air into the radiator.
3) Blocking from the front was much more effective for me than blocking the rear.

Good luck to you, I'm curious to see how you do.

Yes, I agree completely that I have the worst version for mpg. AWD 5.3 and 3.73 gears. None of which are helping me out.

I wanted the 5.3 as they make more power and the difference in mpg is not that large at all. The part that really kills it, is the AWD.

I also am keeping 40 to 45 in the tires as well. We have the same size wheels and most likely the same tires as well. 275/55/20

Transmission cooler is in the center, right behind the Chevy emblem. On the drivers side is the powersteering cooler. I have both. I use the Torque App to both monitor all of my temps and to tune my driving. That's why I removed my 75% grill block as the trans temps were becoming to high for my liking on a day to day no load highway driving(175-185°)

The slots in the bumper have a black plastic fillers. There is no airflow through them for mine. The only airflow I would be getting would be through the lower opening. I don't think I will be blocking from the front due to appearances(I really like how it looks lol)


It turns out my ECU does to DFCO. I just haven't been watching at the right times yet. I noticed it the other day. I would like to get the tuner to have it enter DFCO sooner and hold it longer.

At 285hp for the 4.8L vs 295hp for the 5.3L, I was fine with the smaller engine. They are pretty close in performance, with a slight edge in power for the 5.3 and economy for the 4.8. In spite of the high gears, it tows our 4500+ lb travel trailer just fine.

I'm probably mistaken on the cooler then. Neither my old Silverado nor the Tahoe have the other cooler you are referring to behind the emblem and neither have tranny temp sensors. As long as you are monitoring the temps, you should be fine.

From memory, both my Silverado and Tahoe have the slots completely open, with flaps behind it that deflect air towards the radiator. I'll double check on the Tahoe tonight.

I understand not wanting to alter the looks. My wife tolerates it on my Civic but will raise hell if I touch her Tahoe.

My gauge cluster does not have a trans temp gauge on it either. I use Torque for that. Both of your vehicles do have trans temp sensors from the factory, the computer will change shift patterns based on temp as you warm up the vehicle. You just need to be able to access them and monitor them.

My vehicle has the tow package as well so I have the trans/powersteering cooler. If yours doesn't have the aux cooler directly behind the emblem your trans cooler is simply part of the radiator, as is standard on these trucks.

The only thing I wanted different on mine is 2WD vs AWD. I would easily be averageing 17.5 to 18.5 per tank instead of struggleing to get 17.5.

Once winter hits and I put my grill block back on and do the belly pan I should be hitting 18 per tank... hopefully.

You've got me second guessing everything I thought I knew about these trucks. :)

My old Silverado definitely had the bumper slots open, and a coworker's truck ('06) is the same way. Our Tahoe has them covered with thin pieces of plastic. Never noticed that before.

Neither my Silverado nor my Tahoe have factory tow packages and don't have the auxiliary cooler in front of the radiator. I use an UltraGauge to display additional gauges that are not on the OEM dash, and neither displayed tranny temp. I took this to mean the sensor wasn't there...

After looking into it further, tranny temp is not a standardized output through the OBDII port, and because the output is proprietary to GM, my basic UltraGauge can't display it, I would need the programmable variety (UltraGauge MX). You use the Torque app, does it show tranny temp?

Have you figured out how you are going to construct your belly pan?

The AWD must be costing you a lot more than you think.
I had an '06 GMC Sierra C1500 WT, so 4.3L, 4L60E, 3.23:1, and 245/70R17s. Even with lowering and a tonneau, nothing i tried ever got it over 20 mpg until, i stopped caring about MPG be ause i suddenly needed better towing. I swapped a 4.10:1 axle, got the PCM reflashed, and had no difficulty seeing 22 MPG. For MPG I would have tried a 3.73:1 after that. My similar 94 came with 235/75R15s and a 3.73:1 axle, and was the old TBI, so swirl-port heads rather than the Vortecs, but regularly did 23 MPG stock, and more than once hit 26.
You should actually have less aero drag than my 06, didn't read which engine you have, nor axle ratio, nor tire size, but with a 4.8 and 3.73s you should see 23 if you were 2wd, and you may want to try removing the front shafts until winter. With a 5.3 a 3.42 gear should also do 23.
Doing a cam isnt cheap, the cam alone is 400, plus gaskets, labor, and a tune, at minimum, and you should do valve seals and valvesprings, but would make a dramatic improvement, especially in low RPM torque.

I think the AWD is costing me a lot but I have no way to confirm it. I can't remove the front shafts because the transfer case has a viscous coupling with a 40/60 split to it. Without the front connected the transfer case would burn up within a mile or two. I have seriously thought about buying a used 4x4 transfer case, installing it and just making it stay in 2wd.

5.3 engine, 3.73 front and rear, 275/55/20 tires. The wheels are 20x9's which is a factory tire/wheel combo on newer GMC Sierra's.

The Tourque app does display trans temp for my trans. I keep a sharp eye on it all the time since it's the main killer of the 4l60e trans.

For the belly pan I want to do either coroplast or large black abs sheet. Maybe 1/16" or 1/8". If there are ideas of materials that easy to procure locally that may work better I'm open to ideas.

The big areas I want to hit are the running boards, the front area aft of the OEM air dam, and most of the center. Leaving room for exhaust heat to escape and for servicability. I will take pics as well once I get to it.

You can safely drop to 255/60R20s, without them looking stretched.
Transfer case would help, but it would still be always spinning the front shafts and front diff / gears, plus a proper transfer case gives you low range, but has no differential. If you are going off road, the low range will help on descents, and stick with 275/55s.
If you can, do the transfer case and pull the front shafts, that should get you equal to a 2wd.
If you do the tc, go ahead and upgrade to a NP241C, with a new chain and new seals, plus new synthetic lube. Then you'll never have to give it another thought.
Your tires are taller than i assumed, so the 3.73s may be about ideal.

The 3.23s are too tall for best mpg, but they do let you shift to a lower gear at higher speeds, which can help highway towing.
When it was new, and came with 3.42s standard, GM called the 4.8 " a real sweetheart " in regard to fuel efficiency, ( stock ). Back then the standard tires were 235/75R16, which was unusual. I drove a U-haul one and was impressed, it pulled away from a Vortec 5.7L C1500.
That's part of why i have bought 2 of those engines since. That, and they can be turbocharged past 1200 horses with just porting, a cam, valvesprings, ring gapping, injectors, fuel pump, and a tune. Doesn't even need forged pistons. I still have one, not turbo'ed yet.

I'm hoping people claiming 23 and 25 mpg are talking about flat road cruising at highway speeds. If that is the case I already can hover around those numbers according to the DIC. This is very easy to do right now.

The reason I feel this goal is so difficult is I'm looking for a normal day to day tank average of 20mpg. Not a none stop highway trip either.

If people are actually getting tank averages of 21-23mpg... then I have a long way to go or some major changes that needs to be made.

Or maybe the route I need to take to work each day simply will not allow that to happen based on traffic lights on the highway I can't help but hitting.

Or the AWD really is that draining with this drivetrain.

I was talking about a steady cruise. For a commuting average, you might manage it with an S-Blazer, but not a Tahoe.

I figured most were.

My current record for a one way trip from work parking lot to my driveway is 19.1 going off the trip average on my Torque app. If I go off my DIC its upwards of 21mpg for the same trip. My DIC reads inaccurate for the tank average compared to hand calculating it, pretty common really.

For my day to day driving it's always the Torque app that I pay attention to. It's far most accurate and reads lowered which makes me pay more attention to how I'm driving.

Per the owner's manual, for my Tahoe (2005) 3.42 gears aren't actually offered. They were standard on 4wd and 2wd 5.3L Suburbans could get it, but 2WD Tahoes only got 3.23 or 3.73. Mine is RPO code GU5, so I have the 3.23s

I've had tank averages in our Tahoe at 23-25mpg... but only on long highway trips. This is just setting the cruise control and letting it go down the freeway.

On my old Silverado, I had a handful of tank averages at 20-22mpg, commuting in mixed traffic. I could have done so more often, but I rarely went a whole tank w/o heavy loads/towing, especially considering a tank could get me nearly 700miles.

Again, for belly pan ideas, check the Silverado thread in my signature. I have a few pictures in there of how I built the framework it mounts to. Coroplast worked well and even had some hard impacts with road debris and was left mostly unscathed. It's pretty durable, but inexpensive and lightweight.

When the 4.8 was new, was 99, not 05, sorry for not clarifying.

New personal best
 

http://i268.photobucket.com/albums/j...4-19-49-37.png

Just achieved my current one way personal best from work to home.

50mph max, load based cruising, mirrors folded in, engine deceleration whenever possible neutral coasting up to stop lights as well, AC off, windows up, 93oct, 100% full tank of fuel.

So next on list is putting the grill block back on. 50% to start then I'll go either 100% or 75%.

After that it'll be the belly pan and then the rest of the bottom.

I like what you did for your belly pan. Looks simple and I could carry that idea almost all the way to the rear of Tahoe.

Double post

Put the 50% grill block on today.

I must be doing the pulse and glide thing wrong. My mileage went down to 17.3 on the same trip as that personal best pulsing to 60 and slowing down to 45. It was the sweetspot for the lockup converter and deceleration fuel cutoff on my engine.

During the pulse I was accelerating briskly, maybe too fast though. I'll keep trying. It seems thats how a lot of people get a boost to their numbers.

You aren't getting any "glide" with DFCO; you are getting engine braking. P&G basically isn't going to go well with an automatic.

Fair enough. I'll switch back to how I was driving before. I can only really get around 30% load with the auto before it kicks BACK down to a lower gear during acceleration.

Is that enough load to benefit from the quick acceleration and better bsfc?

In my opinion that is better than kicking down a gear no matter what the load.

I don't have any instrumentation. But being familiar with my a/t vehicles, I know the minimum speeds I can "tease" the upshifts and torque converter lockup at by slightly lifting the gas pedal. Top gear all the way, baby.

Seeing as I have a bigger engine than most, does anybody have any suggestions of when I should be shifting?
2000, 2500 more?

I have been doing around a 2200 shift point while accelerating with high load and letting off to get the trans to shift early and then I roll back into the throttle. It has been what "feels" like I'm acceleraring quickly but not wasting energy. How accurate that feeling is, I don't know.

That feeling is probably more accurate than you think. Especially since your acceleration technique sounds spot on.

There is a range of acceleration rates that all give close to the same gas mileage. If you accelerate in that range and move with traffic, you are doing about as well as can be expected.