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Old 08-06-2013, 07:15 PM   #51
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Originally Posted by cpaharley2008 View Post
I read someplace that it takes twice the amount of fuel to get a vehicle to 55 mph than at 35 mph. Just think we all could get 60 mpg if we drove @ 35 mph!!
Aerodynamic drag force - and thus energy consumption per unit of distance driven - varies as the square of velocity. In common terms, this means that fuel economy depends on the square of speed... or speeding up from 35 mph to 55 mph means the square of 55/35, or 2.5 times as much fuel per distance. This change with speed is equally true whether the vehicle has the aerodynamics of an airplane, or of a barn door.

Of course, aerodynamic drag is not everything: rolling drag (from tires, bearings, etc) is essentially constant regardless of speed, so the difference with speed is not as strong as that squared factor suggests. Decades ago, a rule of thumb was that aero drag and other drag were about equal for a typical car at 50 mph. Aerodynamics have probably improved more than the other factors, so the balance point is probably now faster; trailers are much like parachutes, and our rigs probably hit that equality point at an even lower speed, but the non-aero component is still significant at any speed.

Another factor is that engine efficiency varies with power output. It takes less fuel to put out half as much power... but more than half as much fuel.
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Old 08-06-2013, 07:20 PM   #52
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The Consumer Reports upper chart illustrates that drag isn't everything. Although aerodynamic drag at 55 mph is not much more than half of the drag at 75 mph, the fuel economy penalty is nowhere near 50%. It is, of course, still substantial.

Also, I note that the federally mandated fuel economy tests (in both the U.S. and Canada) include highway cruising speeds that are closer to 55 mph than 75 mph, so cars will be optimally configured (such as in transmission ratios) for efficiency at the lower speed.
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Old 08-06-2013, 07:30 PM   #53
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An interesting article in Consumer Reports August 2013. Speed does make a difference as does putting stuff on top of your car. Obviously towing makes a difference but drag and speed still have a big impact even when towing. See the following charts:
Now doubt this is a good general guideline, but I notice the all the speed tests were done with smaller cars, and might not correlate to all vehicles the same. Same for the wind drag. I know that while my Honda Pilot is affected by this a fair bit, my Ford F250 with the diesel engine is not affected much at all.
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Old 08-06-2013, 08:27 PM   #54
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I know that while my Honda Pilot is affected by this a fair bit, my Ford F250 with the diesel engine is not affected much at all.
I think two factors are at work with the F-250 diesel:
  • the rolling drag is huge (due to large tires and lots of mechnical equipment, especially if it is a 4x4), so it matters more
  • the engine is huge, and so it is closer to its ideal operating point at higher power levels - it takes more energy to push the truck the same distance at a higher speed, but it doesn't take the engine much more fuel to produce that energy
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Old 08-06-2013, 09:05 PM   #55
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Originally Posted by Brian B-P View Post
I think two factors are at work with the F-250 diesel:
  • the rolling drag is huge (due to large tires and lots of mechnical equipment, especially if it is a 4x4), so it matters more
  • the engine is huge, and so it is closer to its ideal operating point at higher power levels - it takes more energy to push the truck the same distance at a higher speed, but it doesn't take the engine much more fuel to produce that energy
Along the same line, is the reason that the Ford does not lose much in fuel economy when towing my 19, and gets what my Honda does towing. Not too much stops the beast short of braking once it gets going.

It really does make my 19 look small too.

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Old 08-06-2013, 09:08 PM   #56
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I do not think that beast would fit in my garage, what are those rails on the roof for?
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Old 08-06-2013, 09:12 PM   #57
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The truck is something like 22.5' long. You can't get much longer with a quad cab, and an 8' box.

That is a load of cedar planks a friend milled up. I am taking them home to make a cedar strip canoe out of.
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Old 08-07-2013, 09:27 AM   #58
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Cedar strip canoe! Drool. Was just remembering days of my youth in c.s. canoes last night watching The National, the Only in Canada item was on a man who makes birch bark canoes in NS. Did you happen to see it?

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Old 08-07-2013, 09:30 AM   #59
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Now to get this thread back on topic, sorry guys.

Gas mileage is one reason we are now leaning toward a 17b rather than 19 or 5.0 as want a good fuel efficient vehicle we can drive even at 110km near our new home (or 75mph) when we head south of us (not towing of course), and get good mileage - I can only see the price of gas continuing to head higher, and retirement funds won't be anywhere close to what we have now.

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Old 08-07-2013, 01:50 PM   #60
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Just got back from a 3,221k trip in my Ford Ranger 4L 4X4 pulling my 17b.
Loaded with beer and steaks I would guess the trailer weighed in at around 3000lbs although that can't be verified.

Using:
16.47 mpg .....$.32 per mile or
14.29L per 100k.....$.20 per kilometer
Based on todays Garibaldi Highlands, BC gas price of $1.41.9 per Ltr.
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