Date: Sun, 19 May 2019 10:29:18 -0700
Reply-To: David McNeely <davmcneely40@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David McNeely <davmcneely40@GMAIL.COM>
Subject: =?UTF-8?Q?Re=3A_So_what=E2=80=99s_the_most_efficient_speed_to_cruise_a?=
=?UTF-8?Q?t=3F?=
In-Reply-To: <252CA6CE-59D6-4600-86FE-07C3ADAEE20D@icloud.com>
Content-Type: text/plain; charset="UTF-8"
Hmmm.... . Of course, there are many variables involved. I'm sure that
Dennis's analysis (but he can speak to that better than I can) makes
certain assumptions, including that the only variable not held constant is
velocity. One that would be held constant I imagine would be that the
travel surface is perfectly horizontal. Any grade would alter the result.
The automatic transmission will downshift if need be as the grade
increases. With a standard transmission, on a steep climb one has to
downshift. With either, then not only does fuel use increase simply
because of the climb, but because of the gear ratio and increased rpms.
But without the downshift, the vehicle will eventually stall on the climb.
So, the question becomes not only at what speed is the vehicle most
efficient, which is answered experimentally (or theoretically either) by
holding all variables constant except speed, but under real driving
conditions, at what speed does the vehicle perform best in terms of fuel
use? I have satisfied myself that for me, with my 1991 Volkswagen Vanagon
GL Campmobile with original 2.1-l engine, with 185K miles, that speed, on
the highway, is between 55-60 mph. At that speed, I am comfortable driving
the van and typically get between 20-24 mpg. At higher speeds, it is
unpleasantly noisy, and I really don't feel comfortable with the vehicle's
tolerance of the demands I am putting on it. In very hilly terrain I may
even drop to 50 mph, downshifting as needed. I try to keep rpms around 3K
except when downshifted, then not over 4K. Oh, my tires are slightly
oversized (195R14) all weather (DOT mountain snowflake symbol) Vredestein
tires, and inflated at 42 psi front, 52 psi rear as recommended by the
manufacturer, though not adjusted for vehicle weight load. When I drive
it, I usually have water, camping gear, myself and my wife (both small
persons) loaded.
mcneely
On Sun, May 19, 2019 at 9:47 AM Jeff Palmer <w.jeff.palmer@icloud.com>
wrote:
> I am thinking of efficiency only in terms I fuel economy. I figured
> someone smarter than me has put together a graph showing fuel economy at
> different speeds. I sure hope Dennis it’s higher than 35!!
>
> Jeff
> Sent from my iPhone
>
> > On May 19, 2019, at 10:12 AM, Dennis Haynes <d23haynes57@hotmail.com>
> wrot:
> >
> > For most vehicle max economy occurs at the lowest speed you can travel
> in the highest gear you have without lugging the engine. Air resistance is
> the load. As the Vanagon auto does not have a locking torque converter
> there is the variable of slip which is massive at lower speeds, especially
> under 1,800 rpm, the stall speed. So probably somewhere around 30-35 mph is
> best. Over 55 air resistance goes up exponentially and so will horsepower
> required. There was a reason that was once set as a national speed limit.
> >
> > Dennis
> >
> >
> > -----Original Message-----
> > From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf
> Of Jeff Palmer
> > Sent: Sunday, May 19, 2019 10:36 AM
> > To: vanagon@GERRY.VANAGON.COM
> > Subject: So what’s the most efficient speed to cruise at?
> >
> > 2.1 auto. Curious!
> > Jeff
> >
> > Sent from my iPhone
>
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