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Old 09-05-2008, 10:43 AM   #1 (permalink)
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Why are tractor-trailer rig so much more fuel efficient

80,000 pound tractor-trailers rigs get 5-7 miles per gallon. Compare the weigh-fuel mileage ratio and a 5,000 pound auto should get about 80 miles per gallon.

What are the reasons for this and shouldn't autos be able to be just as efficient?

Just thinking,

Danny Harris
Arkansas

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Old 09-05-2008, 11:05 AM   #2 (permalink)
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if i were to guess i'd say it has to do with the gearing. along with that the motors never really work too hard, maybe 3,000rpm max? that is just a guess i know its not very high though.
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Old 09-05-2008, 11:12 AM   #3 (permalink)
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consider the weight to frontal area ratio of a transport truck to a 3000 pound car. 25x the weight with maybe 8x the frontal area. Their total air drag-to-weight ratio is much smaller. That same truck gets approx the same milage when it is dead empty, too (may save 1mpg if LUCKY). This reduces the weight/FE ratio greatly.... lots of reasons, really. Most prominant probably being that weight has a much smaller affect on FE than you'd think.
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Old 09-05-2008, 11:26 AM   #4 (permalink)
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Quote:
Originally Posted by MazdaMatt View Post
consider the weight to frontal area ratio of a transport truck to a 3000 pound car. 25x the weight with maybe 8x the frontal area. Their total air drag-to-weight ratio is much smaller. That same truck gets approx the same milage when it is dead empty, too (may save 1mpg if LUCKY). This reduces the weight/FE ratio greatly.... lots of reasons, really. Most prominant probably being that weight has a much smaller affect on FE than you'd think.
i could see this being true to the fact that once you have the truck moving it wouldn't take as much to keep it moving, just like any vehicle. my mileage shows really low accelerating but then levels once im maintaining speed. maybe the same applies to trucks?
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Old 09-05-2008, 12:39 PM   #5 (permalink)
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Quote:
Originally Posted by Brian03cav View Post
i could see this being true to the fact that once you have the truck moving it wouldn't take as much to keep it moving, just like any vehicle. my mileage shows really low accelerating but then levels once im maintaining speed. maybe the same applies to trucks?
And trains. I would guess that, given a flat straight surface, a 200 car train takes about the same amount of energy to keep moving as a 150 car train. The only difference is in the curves and grades.
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Old 09-05-2008, 01:52 PM   #6 (permalink)
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Yep...An object in motion tends to stay in motion until outside energy changes the velocity.

That could be friction, a curve, gravity, wind etc....

Once you get 50k+ lbs moving, it wants to keep going in the line set forth. so once it is at speed, it does not take that much energy to do relatively small changes. A higher gear can be used then to improve efficiency.

Jim
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Old 09-05-2008, 02:02 PM   #7 (permalink)
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Quote:
Originally Posted by Clev View Post
And trains. I would guess that, given a flat straight surface, a 200 car train takes about the same amount of energy to keep moving as a 150 car train. The only difference is in the curves and grades.
As far as grades are concerned, you get an advantage with more weight going down hill to counter the disadvantage going up hill.

Also, if gearing had anything to do with the reason that cars get proportionately poor mileage compared to large trucks then it would simply be a matter of changing the gearing of the cars to increase their mileage. The fact is, cars are already geared as they need to be to most efficiently do their job, as are trucks.
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Old 09-05-2008, 02:04 PM   #8 (permalink)
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cars can do this, but markets don't let it. the greatest power to weight ratio for 3000 pounds or so expected to do the 80s mph (typical car) is the 3 main boxer four.60mpg without einstein and close to nature has never been a surprise from that engine, even on old trannies and flaws thorughout, as all of them today are old even when they were new. Deisels in such small strokes is still a nuisance. The 8 cylinder diesel with the crazy crank thay have used for 50 years belongs in diesels and returns alot for power to weight as well. I see those coming around slowly, and liking it. The durmax, even ford has a little 4 something liter (rather backwards mentality, but hey, ford and long strokes where they don't belong seems appropriate to see such a tiny diesel)
Believe it or not, a deisel truck could do better with the 8 cyl. Tortured by a six cyl diesel my whole life..it remains the typical mainstream for big american rigs and it is terrible.
This question has been asked many times
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Old 09-05-2008, 02:14 PM   #9 (permalink)
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Another advantage big trucks have over our American cars is that the diesel engine is more efficient in producing power than the gasoline engine. this is due to the higher compression ratio that a diesel can and does run at.

If larger engines that didn't have to work so hard were advantageous FE wise then our large engine cars would get better mileage than our small engine cars, but alas the opposite is true. An engine that is working harder is more efficient. For instance, my GMC with its 4 cylinder engine has to work harder than the same pickup with the 6 cyl engine but my smaller, harder working engine, gets better mileage.
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Old 09-05-2008, 02:48 PM   #10 (permalink)
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Quote:
Originally Posted by bgd73 View Post
Believe it or not, a deisel truck could do better with the 8 cyl. Tortured by a six cyl diesel my whole life..it remains the typical mainstream for big american rigs and it is terrible.
This question has been asked many times
Why would an 8 be better over a six? I always thought that an I6 oil burner was the best of all worlds when it comes to packaging and power.

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