Quote:
Originally Posted by MadisonMPG
*did not read whole thread
Diesel is already more fuel efficient. Trucks have a higher load/Cda ratio. Stay at 70mph for nearly all of their trip. Made for hauling, not for city driving or spirited driving. So all of the gear ratios (18 or so of them) are made for highway. They rarely change speeds and they do all of the Hypermiling things because they have to.
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1] It's actually difficult to stay at 70 without a little too much power (or a lot), but a trip average above 60 isn't bad. (If you get trapped behind a slowmover with overtaking traffic, it may be miles and miles before you can pass and slowly get back to travel speed).
The industry trip plans all miles at 50 mph to account for terrain, traffic, weather, stops, etc. (A number that also works well for cars by the way). A truck near gross weight of 80k is hard to keep up to speed except on the plains and ONLY on the Interstate (or divided, limited-access roadways with bypasses around all towns). A truck down below 60k is fairly easy to keep up to speed (not in the Rockies or West Virginia though).
2] Most big trucks are part of a fleet, thus most transmissions are variations on nine or ten speed. As first gear is a granny, this means -- in use -- 8 or 9 gears for most trucks you see. An 18-speed is pretty well the province of owner-operators who REALLY have a thing for shifting . . you sort of never stop. I think most of us -- if we could spec a truck -- would go for 3.55 rear gears and a 13-speed. Depending on the type of loads carried I'll admit I still want in excess of 500 HP. But never again less than 400 (it's the torque that counts, and less than 1800 LBS/FT makes a long day ahead of 79k. Now, 2000 . . that's a line worth crossing) as a heavy load in hilly/mountainous terrain may mean more than 20-minutes to get up past 60 mph entering the highway. In a speed-governed truck, even longer. The horsepower and the extra few gears would make a great difference. It's all marginal, but it's how one uses that margin.
(TQ tells you if you can make it up the hill . . HP just tells you how fast you'll make it . . and that last 20,000-lbs is the camels back for ease or difficulty in braking and accelerating).
A 10" rest area stop in a car -- from ramp into and ramp out of, and down from 60 to back up to 60 mph -- in a big truck that's laden heavy can be 30" or more. Same exact time out of vehicle (less vehicle inspection).
Power really counts for a driver. Thus the interest in not losing mpg due to terrible aero. Would be great to really get it on at times when needed, believe me. The extra work for the driver is no small matter . . not enough power or gears makes it tough to
time the rig to what's happening on the big road.
3] Hypermile? I can't imagine pulse-and-glide in a big truck. Takes years just to learn how to choose the right gear every time. Etc. The one thing that matters is safety and that is full-time. The second is being on time. The rest is hardly considered worthy of attention as a fleet driver. So, if one equates safety with efficiency, then, yes.
Now, if we're limiting our truck driver discussion to cattle-haulers you can forget everything I wrote above.
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