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Old 04-06-2014, 12:21 AM   #1 (permalink)
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Tweaking a Tahoe, the tow vehicle

I've been lurking and reading for a few days and decided to register. Not too many people trying to improve the FE of an SUV, but I'm one of them.

A little background...

We bought the Tahoe to tow our 1966 Streamline Countess 26' vintage travel trailer. Last year we towed it over 3000 miles to Northern CA, and to Yellowstone and more.

At the moment we are touring the Southwest, the Grand Canyon, Moab, Bryce Canyon, etc before hitting Disneyland and heading back north.

We are lucky enough to not use the Tahoe for commuting since we work from home, so it's mostly used for Costco runs, towing, long trips, and the occasional trip in town that's too far to walk. I'd say we drive about 10k miles per year, with almost half of that on long vacation trips.

So far on this trip we are averaging 13.3 MPG, mostly towing, which I think is pretty good. But I would love to improve it, and also love to improve just the Tahoe for when we need it for long trips.

We have a 1974 Alfa Romeo Berlina and now a 2002 Saab 9-5 2.3L that both nearly double the Tahoe mileage, so we use those whenever possible as well.

Plans for the Tahoe include:

Lowering it - partly for handling, also for FE. I already have the kit, just need to find the time!
Removing the roof bars, and switching out the Z71 rack for a sleeker rack. We still use it occasionally, so I want to keep the side bars on.
Partial grill block. I'm worried about trans temps when towing, so need to be careful with this.
2" airdam on bottom of bumper
Partial belly pan. I'm not sure I have the time for a full pan, but could do the engine relatively easily. It has skid plates, which might help a bit?

The Tahoe has the 5.3L Vortec, with the 4-speed auto, 4WD and a 3.73 rear end with tow package.

We rarely use 4WD, and frankly would ditch it, except occasionally we need it to back into a tricky camping spot. Like tonight, uphill on gravel, we wouldn't have made it without 4WD. Otherwise we run in 2WD.

I have stock sized Michelin tires, 265/70R17 on the stock Z71 wheels. When towing I run at 41 PSI, around town usually at 35 PSI.

I hope that's enough for now. I have done some driving modifications, when I was commuting to work in a Focus I was trying to get over 24 MPG on a tank, I don't remember if I ever did.

I'd love to get 20 MPG on the freeway, say on trips to San Francisco. At the moment we struggle to hit 18 MPG. Part of the trouble is keeping speeds down, on a 10 hour trip a few MPH adds up in real time saved, especially with a 2 year old in the car.

Looking forward to learning more and sharing our results!

Ian


Last edited by autoarcheologist; 04-06-2014 at 12:29 AM..
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Old 04-06-2014, 12:30 AM   #2 (permalink)
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Just remembered another mod, delete the bug guard on the hood. I've always hated it, but it does keep the hood chip free. I assume it must add some drag. Anybody know?
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Old 04-06-2014, 03:16 PM   #3 (permalink)
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You are off to a good start. I suggest adding a Scangauge or Ultragauge. I think one or both of them can monitor transmission temperature.

The large difference in mileage when towing implies that your largest gains will come from improving the Tahoe/trailer aero interface or aero tweaks to the trailer. Post a photo with the trailer attached, maybe we can make a suggestion.

Mileage increases with tire pressure, but some vehicles need to run less than sidewall maximum because of traction. I'm running 50 PSI in my tires, where the sidewall maximum is 47 PSI.

Useful driving techniques include DWB and DWL. DWB is Driving Without Brakes. Time traffic lights and speed limit changes so that you don't use your brakes. DWL is driving so smooth that your passengers fall asleep.
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Old 04-06-2014, 05:33 PM   #4 (permalink)
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Quote:
Originally Posted by autoarcheologist View Post
Lowering it - partly for handling, also for FE. I already have the kit, just need to find the time!
Quote:
We rarely use 4WD, and frankly would ditch it, except occasionally we need it to back into a tricky camping spot. Like tonight, uphill on gravel, we wouldn't have made it without 4WD. Otherwise we run in 2WD.
Which kit are you going to use for lowering it? Have you never considered an adjustable suspension setup?

Regarding 4WD vs. 2WD: are you sure a selective differential locker wouldn't provide the traction control required under those conditions?
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Old 04-06-2014, 10:12 PM   #5 (permalink)
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You don't need a Tahoe to tow that trailer. My grandparents used a full-sized sedan, a Dodge Monaco, to tow their '66 Empress throughout the U.S., Mexico and Canada.

If you wish to reduce the yearly average fuel consumption of this second or third vehicle, then a current Chrysler 300/Dodge Charger would fit the bill. And, more importantly, provide a more stable towing experience, not just better braking or fuel burn than any truck or truck-based tow vehicle [TV].

I think you are already on Airstream Trailer & Motorhome Owners Community, and, if so may be familiar with threads of this nature. I bring it up for those who are also considering a TT of this type (all-aluminum, lightweight, aerodynamic; and of higher build quality than Airstream).

I'd say that attending to TV tire/wheel choice along with the quality of the lash-up of TV & travel trailer [TT] is part of the mechanical baseline that will, ideally, reduce the number of steering corrections due to road irregualrities and winds, and reduce the amount of braking force as well as in duration. This will add considerably to FE.

Were I to use a Tahoe, then the police-spec version would be my ideal (as we used the police-spec Dodge Police Pursuit for imspriation in the 1960s and '70's as TV). Can't carry much and is likely somewhat compromised for towing, but your TT is not an average TT as you aware. Its' suspension, tires/wheeels and brakes deserve as much, if not more attention for best articulated-rig performance. As should your hitch, where a "virtual pivot point" type (ProPride or Hensley) is much preferred.

" . . I have stock sized Michelin tires, 265/70R17 on the stock Z71 wheels. When towing I run at 41 PSI, around town usually at 35 PSI."
I'd ask Andrew Thomson of CAN AM RV what they have used (London, ON) and look, again, to police spec. You'll want to know tire loads

http://www.trucktires.com/bridgeston.../WeighForm.pdf

As to other specifics, the grille block won't really be good for towing outside of freezing weather, and perhaps not then. More than one power steering pump has been lost in horsing a TT into a space in cold weather by the lack of fan airflow and overheated that component. Etc.

Records are what matter most, and a complete set of scaled weights is part of your laudable pursuite of best FE. Seen from only a slight distance, safety will also be improved.

WOODALLS: Ron Gratz Weight Scale Chart

Look forward to more.

.

Last edited by slowmover; 04-06-2014 at 10:29 PM..
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Old 04-07-2014, 12:47 AM   #6 (permalink)
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Argh, stupid KOA wifi lost my post. Will repost tomorrow.

Thanks guys, and yes slowmover I'm also on Airforums and again appreciate your advice and wisdom!
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Old 04-07-2014, 03:28 PM   #7 (permalink)
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We also have a Tahoe for towing duties and it's also my wife's daily driver (her commute is quite short).



I used to have a Silverado as my daily driver and I used it for towing. Then I started looking for a Tahoe so I could get a Civic for my longer commute. Since I was already familiar with these rigs, I knew what to look for and I specifically picked this year and drivetrain for efficiency.

Her Tahoe is a 2005 4.8L, 2WD, 3.42 gears and was already lowered by the previous owner. As a 2005, it has the low air dam and the electric fans. Stock, other than the wheels and lowering, my wife regularly gets 21-25mpg highway. She simply sets the cruise control and goes, A/C always on...

You could get an '05-'06 Tahoe air dam and hook it up to your bumper for the OEM look. Look into the e-fans as well. It's easy to mount the '05-'06 fans on the older Tahoes, but you need someone to reprogram the ECU to trigger the fans.

You get better mileage than we do while towing (11-12mpg), but our trailer is a brick and it's way too tall. The high gears in ours don't help either, since with the trailer O/D is basically unusable (I'll kick it into O/D down hills, but that's about it)

On my Silverado, I left a hole in one side of the grill block since the tranny cooler is on that side. I pulled the upper block off while towing.


Good luck!
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Old 04-07-2014, 03:49 PM   #8 (permalink)
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I'm kind of going through this in my head now as well. I'm going to build an aerodynamic lightweight camper and am debating changing one of our daily drivers to better suit it. Now we have a 2007 Sedona good for 3500# and gets 17-23. I have thought of another minivan especially a modern Honda with the 8 speed but according to fuelly pretty much every minivan on average seems to get about 20 even though the honda looks more aero and is rated much higher by the EPA. The other choices would be a previous gen Rav4 v6, a current 2.0 ecoboost Escape, or a 3.5 ecoboost flex or mkt. The flex and mkt by far have the most power and get around 20 but also have AWD. A Tahoe like yours would be great towing but too much a penalty for daily non-tow use.
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Old 04-08-2014, 01:11 AM   #9 (permalink)
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Darcane, thanks for the info. Great to know that 20+ is possible, although you have the smaller V8 and lower gears, so it won't be that easy for me. But finding the efans and a chin spoiler shouldn't be hard. I do have the z71 package, so I might just have to fabricate a spoiler if the stock one doesn't fit the Z71 bumper.

Hersbird, if you're just getting into towing do yourself a favor and read up on this site: www dot canamrv.ca/hitch-hints/

slowmover referenced Andy's work above, and as a mechanical engineer I have to agree with his conclusions. You're right that you don't need something as big as a Tahoe, honestly your Sedona will tow almost anything you could build just fine. But if you're going to trade it, a minivan would be a great, stable tow vehicle bceause they have a low CG, have a long wheelbase to rear overhang ratio for stability, and independent rear suspension for better handling. They're also typically a bit smaller than an SUV for less drag. I say start with what you've got to keep the project simpler.

As for me, we drove 380 miles from Zion to LA today, towing the whole way into a nasty headwind. We averaged 12.4 MPG and 53 MPH, which included a slog through Las Vegas for lunch at Baja Fresh. We thought about driving down the strip, but settled for driving across it as traffic was brutal and we had another 4 hour drive ahead of us.

The first leg of the trip I averaged 12.7 MPG before hitting Vegas, this was with me trying to keep it under 60. At some point my wife took over and drove faster, but since it seemed to be more downhill after lunch we picked up a few tenths.

I will get a photo of the trailer and Tahoe and post it.
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Old 04-08-2014, 01:18 AM   #10 (permalink)
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Here's a pic of the trailer, need to get a smaller one with the Tahoe in it as well...

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