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Old 04-28-2014, 01:00 PM   #1
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Tow vehicle fuel question

We haul our latest 30' Flying Cloud with a 2008 2500 Suburban with the 6.0 liter engine. While we definitely notice the additional GW of 1,500 lbs over our earlier International our desire to move to a 2500 diesel pickup is not in the cards at the moment.

So, in traveling out west last summer/fall we experienced a very slow go in a couple of steep elevation climbs. We are talking about 1st gear at 3,000 rpms. We also found that in many remote locations only 85 octane fuel was available. The recommended octane for Suburban is 87. But, sometimes 89 octane would be available as well.

At an oil change stop I asked the Chevy dealer about that and they said that in that particular vehicle they recommend the 85 as long as we did not experience pre-ignition which we did not.

Upon getting home, a knowledgeable friend thought that we ought to burn high test in those situations since given the altitude and fuel we probably were losing 20% of our horsepower.

I don't mind spending the money on higher octane fuel if it will make a difference and does not create some other problem. Thoughts? Thanks, Jack
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Old 04-28-2014, 02:24 PM   #2
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Jack the rule of thumb is that you loose about 3% of horsepower per thousand feet of elevation. Higher octane will help some. But I think the 6.0L is 335hp so at 6,000 feet it only has 274hp left to help you up the hills.
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Old 04-28-2014, 02:34 PM   #3
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If the fuel you are using has a sufficient anti knock index/octane rating so that the engine controller doesn't retard the timing, you will gain nothing by using a higher octane rating. You should at least try and use 87 IMO.


At higher elevations the octane rating doesn't matter as much (about 2 points is the spread) so some areas use 85 as regular instead of 87. This won't cause engine damage or reduced power in and of itself.


The next point is that what it says on the pump isn't necessarily what you are getting. If you are in a rural area where they pump less volume, the fuel may not be as fresh, so the octane rating may be down from what it should be. Look for higher volume stations.


Then there is ethanol percent. You may notice a difference with increased ethanol, and it is also not always what it always says on the pump (often 5% or 10%), since it is mixed in at the bulk plant. If it is higher, that can make a difference.


You mention steep grades. At higher elevations your engine will produce less power, and you may notice this as well, separate from any fuel issues.


You can try a tank of 89 and see if it makes any difference to apparent power and fuel efficiency. If so, use it. It won't hurt anything. But if it doesn't help either power or mileage, indicating that it is preventing the engine controller from retarding the timing, it isn't providing any benefit, just draining your wallet faster.


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Old 04-28-2014, 02:52 PM   #4
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Gary and Jeff. I very much appreciate your feedback. Thanks! Jsck
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Old 04-28-2014, 03:01 PM   #5
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If you know someone who can try a test tow using a diesel of comparable size to your gasoline motor and side by side compare that would be an appropriate "test".

I would have my truck "dynoed".. So you "know" what power you have available.

Our Duramax Diesel 2500 just chugs along.... It is, for me, the right tool for the task. I get 23mpg running light on the interstate unless I get on the Troll Road where the limit is 85mph... That speed gets 16-18mpg.

When under tow, I get 11-13 mpg. Mileage, is speed and terrain dependent. I was getting better mileage but something changed with the diesel supply around here.

I am paying the same fuel costs as when I had a 6.0 Chevy gas motor when driving in town...and hands down better road mpg in the diesel.
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Old 04-29-2014, 09:08 AM   #6
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Ditto cwf Channing.
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Old 04-29-2014, 09:51 AM   #7
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Originally Posted by cwf View Post
If you know someone who can try a test tow using a diesel of comparable size to your gasoline motor and side by side compare that would be an appropriate "test".

I would have my truck "dynoed".. So you "know" what power you have available.

Not quite a fair comparison....most all modern diesel engines are turbo charged, maintaining base line power to very high relative altitudes. As pointed out earlier in this thread gassers start with a rating at sea level and lose power with altitude as you get farther from the coastline.
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Old 04-29-2014, 10:24 AM   #8
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It costs more to buy and run the big diesel pickups no matter how you slice it. Ram offers a small diesel now that matches many Airstream towing needs well. So many SUV's. But you will pay for that low rpm torque as well.

But what's the difference, the op has a gas engine, plans to keep it. He's asking about gasoline, which is what we are traveling through Idaho and Montana with today. Our rig is lighter and moving along nicely, but gasoline questions are more interesting without diesel answers.
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Old 04-29-2014, 10:34 AM   #9
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Hello Jack

In mountainous regions, somewhat lower-octane gas has been sold for years if not decades.

Gasoline engines at wide open throttle, unless turbocharged, will have a smaller fuel-air charge (less mass) in the cylinder at higher altitudes. This leads to the loss of power compared to sea level. It also reduces detonation. Therefore, at altitude, a lower octane fuel works just as well.

Fuels sold with 1-2 points lower octane than is customary at sea level reflect this reality.

The only time there's a problem is when you purchase low octane fuel at altitude and then drive out of the mountains.
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Old 04-29-2014, 11:22 AM   #10
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I just purchased a 2500 Silverado Diesel and I've towed with it once. All I can say is that it does have the wow factor and my wife loves driving it because it never says I think I can, it says I know I can. And the mileage is about 1/3 better than the 5.3 gasser I had.

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Old 04-29-2014, 11:48 AM   #11
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Having had four gassers and two diesels during my camping/towing years I can comment from both camps. My twelve years with my '96 Dodge Cummins was stellar. However, with today's truck prices and diesel fuel prices I doubt I'd go that way if I was starting anew. You do not mention your gear ratio in your Suburban. This is of major importance. If it is less than 3.73 you'll be in for some slow climbing. Having said this, unless you tow a lot in mountains with steep climbs at high elevations, don't sweat it if you like what you have. If you're on a four lane just climb slowly & enjoy the journey. If you're on a two-lane, pull over when you can to let faster traffic go around, and enjoy the journey anyhow!
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Old 04-29-2014, 01:09 PM   #12
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This may or may not apply to your towing a trailer but , I will share it for what its worth. I have a supercharged Bonneville. GM specifies 91 octane. Around here the choices are 93, 89, 87. I generally run the 93 and it runs great! Occasionally I get in a situation where the lower octane is all that's available. 89 seems to run fine, even the 87, until you mash the throttle on an entrance ramp and it seems to have less power. I never have pre-ignition ping, the computer senses pre-ignition and retards the spark thus limiting the power output. I have done several mileage checks and without exception i get better gas mileage with the higher octane. If I were you I would try running higher octane when towing and see if it makes an improvement. In the old days, I towed my Overlander with a 1988 ford 351, it had duel tanks and I would fill one with premium and one with regular. It would ping very noticeably when towing on regular fuel but, when running premium it would not ping and would pull much stronger.
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Old 04-29-2014, 01:17 PM   #13
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3000 RPM is not going to allow that engine to produce much horsepower. Ask the dealer what a safe redline is for that engine and what the RPM is for peak HP. If you are climbing hills you need to stay at the peak HP to get up that hill. A diesel will be able to pull a hill at 3000 RPM but a gasser won't develop enough HP. It is ok to run a gas engine at high RPM. That is what they are designed for.

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Old 04-29-2014, 02:16 PM   #14
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Originally Posted by Jack46 View Post
We haul our latest 30' Flying Cloud with a 2008 2500 Suburban with the 6.0 liter engine. While we definitely notice the additional GW of 1,500 lbs over our earlier International our desire to move to a 2500 diesel pickup is not in the cards at the moment.

So, in traveling out west last summer/fall we experienced a very slow go in a couple of steep elevation climbs. We are talking about 1st gear at 3,000 rpms. We also found that in many remote locations only 85 octane fuel was available. The recommended octane for Suburban is 87. But, sometimes 89 octane would be available as well.

At an oil change stop I asked the Chevy dealer about that and they said that in that particular vehicle they recommend the 85 as long as we did not experience pre-ignition which we did not.

Upon getting home, a knowledgeable friend thought that we ought to burn high test in those situations since given the altitude and fuel we probably were losing 20% of our horsepower.

I don't mind spending the money on higher octane fuel if it will make a difference and does not create some other problem. Thoughts? Thanks, Jack




I live in Colorado and go over lots of high passes towing my Airstream. The best fuel mileage is obtained by driving in the highest gear possible.
I go up the hill with no more than 3/4th throttle and let the transmission downshift when it is ready. Your vehicle will drop below the speed limit. If you hammer it in low gear trying to keep up with traffic, your mileage will suffer.
I agree with the other posters, unless your engine is detonating you will gain nothing with higher octane fuel at high altitude.
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