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Old 11-16-2008, 12:06 PM   #1 (permalink)
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Smile Efficiency from an inefficient vehicle?

Let me start off by saying that my vehicle is very hungry for fuel. The necessity of hauling and towing a trailer for work relegates my vehicle choice to a HD 3/4 ton. My truck was a '98 GMC 2500 HD running on propane until it was in a fire a few months ago. The cost of driving that truck on propane was quite low. One time I calculated the economy for a highway trip and found that driving easy I could get 27 MPG equivalent. What I mean by 'equivalent' is in order. Propane costs less than 87 octane here by quite a bit. You lose mileage due to the lower energy content of the propane vs. gas, but the overall net is a lower cost. So using the cost ratio between the two fuels multiplied by the actual MPG (Imp.) I came up with 27. Pretty good for a full size 4x4 truck with the largest canopy available. Note, I'm Canadian so I use Imp. gallons, but for the sake of clarity I'll only refer to US gallons in the future.

Now fast forward to the present. A 2006 GMC 2500HD 4x4 sits in the drive. Technically the motor should be more efficient through design (plus the old truck had 325k). Right now I'm in the very early stages of finding out what basic things can help the mileage. Tire pressure at 50 and then 75 psi seems to have made almost no difference unless towing a trailer... except that the ride is harsher. I've purchased all synthetic fluids and will monitor the result. Next was to change the tires to something slightly taller and narrower. After that, perhaps a computer programmer or burning the chip. I've heard that the programmers mostly play with the transmission shift points (auto) to feel more powerful but generally don't change the MPG in the downward direction much. The same person told me that the a company they used burned a chip in three similar vehicles for fuel economy and results were between 3 and 8 mpg. When you are starting off between 13 and 15 MPG that is a HUGE change! If I got 2 from such a change I would be very happy. A serious loss in power is what I expect from an improvement in mileage this large, and I am fine with that. My old truck had a 5.7l vortec which had covered 325k. This one has a 6.0l with much lower mileage and a higher starting point for power and torque. I'm willing to bet that I could happily get by with 1/3 less power than it has 99% of the time because I don't use that power the way I drive. BTW, I don't have a scanguage but the driver message center gives me a fuel economy read-out... which may not be totally accurate, but it shows trends and changes. The very best average I've gotten while not hauling anything was about 15.8 MPG on a tank (confirmed by my calculations with a log). So I am open to ANY ideas people have to improve this truck so long as the truck can still work as a truck... otherwise I would by a Honda Civic.

Oh, I almost forgot to add that I'm looking into cargo covers right now. Tonneau covers and canopies to protect tools that I often carry from the elements. They either have to be easily removable or collapse so large items can be carried. What about having a retracting soft canopy like the Softopper for covering most loads with a partial tonneau cover most of the time to reduce tailgate drag?

Thanks to all on the forum. Already I've learned so much from reading many threads (I found the site because of the Aerocap discussion).

So that's my story...

Cheers,
Kirk

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Old 11-16-2008, 03:29 PM   #2 (permalink)
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If you can see verifiable decreases in the energy needed to move the vehicle, say via a coast down test or similar, but no significant change in mileage, you need to alter your driving habits/gearing to allow you to take advantage of the efficiency improvements. You have essentially two things to look at, the energy needed to move the vehicle, and the efficiency of the engine. Ironically enough, the more energy it takes to move a vehicle, the more load is put on an engine and the more efficient it operates. Similarly, if you start making the vehicle more efficient, for instance through a sloped bed cover, lowering, tires at 50psi, etc... (see Big Dave's truck for examples), the engine will tend to operate less efficiently. In order to maximize efficiency you need to increase the vehicle efficiency by making it roll/move through the air easier, and insure that the engine is also operating as efficiently as possible through gearing or maybe driver habits.
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Old 11-16-2008, 04:09 PM   #3 (permalink)
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Three issues; driving habits, motor & drivetrain efficiency and aerodynamics.

Driving habits: These have already changed drastically. Being able to see exactly how driving style influences fuel consumption on the dash makes things very clear. It's a personal challenge now to drive somewhere and keep getting another tenth or half MPG. I drive more proactively, looking further in the distance to anticipate traffic and road conditions which allow better driving techniques. Also, due to the lower gearing of this truck, 4.10 vs. 3.73 on the old one, road speeds tend to be lower all the time. On the highway I drive 95-100 kph rather than 105-110. The peak torque RPM is something I've not found yet, but it seems to like 2000 to 2300. Of course when a road allows I'll drive at 80 kph closer to 1500 RPM. It's not lugging at all and the mileage certainly gets better.

If I understand you correctly, the motor works more efficiently the harder it's working, right? The easiest (and expensive) way to make the motor work harder would be taller differential gears. Going to 3.73 like I had before would make a difference and may be acceptable for power required towing/hauling. Much further and I fear utility would be lost. You mentioned Big Dave's truck. He has 3.08's but also much more power with the powerstroke 7.3 diesel. Many people I've met with diesels easily get mid 20 MPG figures around here. Quite often it's the people with lifts, mud tires and 5" exhausts. In spite of all the things they have done to ruin the vehicles' rolling resistance and drag, it is gained back with interest due to effective power tuning of the diesel. There is simply more to be gained from a modern truck diesel than a similar gas engine without having to make internal mechanical modifications.

As far as aerodynamics go, this truck will not be getting anything terribly out of the ordinary. No boat-tail, no wheel covers, etc. I like the idea of lowering and valences to improve Cd. As mentioned in the original post, I'm actively looking for canopy and tonneau cover solutions which should help. As an aside, the last truck got a very large canopy. It was the largest one I could find. That seems to fly in the face of reason when you are talking about MPG, but in this case not so. Hauling things around is part of my job. Rarely do I go a full day without hauling something and lots would not fit inside a car or SUV. By adding the topper I not only made life easier by eliminating the need for tarps etc, but also get away from pulling an enclosed trailer. Let me tell you that an enclosed trailer has massive drag. A bonus was that when I DID have to use the trailer, there was less drag with the large topper on than without! Now I had more space to haul which translated into fewer trips which nets a total savings in time and fuel.

Cheers,
Kirk
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Old 11-16-2008, 06:30 PM   #4 (permalink)
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A harder working motor tends to be more efficient, yes, up until you're hitting ~75% load/throttle, after which it can get a bit more inefficient since manufacturers seem to run a bit richer to safely get more power/torque.

In terms of gearing, IMO it's better to go w/ taller if you aren't looking to break the bank in terms of power output since you can always just drop a gear to get more at some speed. The 7.3L diesel makes more power at a lower rpm, ~175hp@1800rpm becauseit's turbocharged, and more or less sits at 200hp the rest of the way through. Your smaller engine makes more power but also has a much greater range of operating speeds. Another advantage diesels have is more energy per gallon, ~10-15%, which means a gasser getting 10mpg is equivalent to a diesel getting 11-11.5mpg all things being equal. They can also be more efficient at peak, but this is only a few percent difference. The biggest difference comes from offload engine efficiency, for instance VW's TDI engine from the early 90s is more efficient off-load, for instance at 25-50% torque/power at some engine speed, than the Prius' engine. This is another advantage for diesels, since they can be geared shorter w/ less of a drop in efficiency. Since people looking at efficiency tend to buy diesels, manufacturers tend to give gassers gearing that maximizes power w/o looking as much at efficiency anyway. For the most part, gassers gain way more from gearing changes at lower cruising speeds, say 75mph and below, than diesels do, although both can see gains.

To get some specific insight, look at what your max and min payload/grade/speed are in order to figure out what kind of gearing range you would need to maximize efficiency, lets say you need to be at ~50-70%+ torque at that speed/load, and look at the rear end/transmission options you would need to do that. Going with taller gearing may not be a problem since you could just use 4th instead of 5th, etc... when towing and you'll have the same power output at the same speed. The only disadvantage is that you wouldn't have as much pulling power in 1st, so if ya need to go up large grades towing a few tons at ~10mph it could be a problem, and you may not be able to tow as much up a tall grade going as fast, for instance you probably couldn't tow two tons going 100mph up a 3-5% grade or whatever the figures are, as well as go as fast unloaded in fifth. Otoh, you should be able to see 20+ average mpg easily since you'll have a much taller fifth that'll allow you to have the engine make more torque/operate more efficiently when unloaded, going downhill, etc...

In terms of aero, the best boat tails make great toppers for pickups if designed well. If you're driving w/ little or no load, it's in it's normal spot and you see better aero, and if you need to put more in it or are towing then you can prop it up in the normal topper shape, and if you need to haul something really tall just take the top off. It'd probably be fairly easy to do something w/ square tubing and sheet metal if concerned about cost.

Edit- I should add that at 5000lbs total weight even my little pickup can make enough power, ~50hp, to tow an unloaded four horse trailer up a ~1.5% grade in 3rd w/o getting near peak power (sticky needle), since it has a close ratio 4MT. Tossing in a taller rear end along w/ a 5MT would allow me to do the same, except I'd end up in second and I should be able to go a bit faster doing it, while getting ~40-45+mpg unloaded in fifth. It's all about gearing compared to power output over the range of speed/weight/grades you'll see.


Last edited by roflwaffle; 11-16-2008 at 08:43 PM..
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