Gas mileage depends on a whole lot of factors and people often confuse it with
efficiency. A gas motor is most efficient, as in you're getting the most out
of it for what you put in, at wide open throttle. This seems counterintuitive
unless you also take into acount that pretty much every car on the road has
plenty of excess power to maintain freeway speeds.
If you've ever followed the Super Mileage competitions, where Universities from
around the world go for the highest mileage, you'll realize that they are
achieving something like 3500mpg (not a typo). Of course, that is with bicycle
sized vehicles with aerodynamic bodies running on 1 cylinder motors. They
achieve the high mileage numbers by employing a "burn and coast" method. This
is done by running at WOT (most efficient) up to a certain speed, then coasting
down, then WOT throttle again, etc. This averages out to the best mileage, but
is not necessarily an approach you want to take on the streets.:-)
If you wanted to design a vehicle to get the best mpg possible on the freeway,
you would ideally design a motor with power output about equal to what is just
necessary to maintain that speed. Thus the motor runs a WOT all the time where
it is most efficient. Unfortunately, this leaves no power left over passing,
etc. Again, not very practical.
Your every day car's mileage depends on where in the motor BSFC (brake specific
fuel consumption) map it is operating, which takes into acount throttle
position, RPM, etc. How much throttle is necessary depends on what load you are
asking the motor to overcome. Contributors to that load are rolling resistance
(can't remember for sure, but I think this goes up linearly with speed) and air
resistance. The force of air resistance goes up with the square of speed, and
the power required to overcome that force goes up with the cube of speed.
I don't think there is any kind of ideal speed for all cars, with regards to
fuel economy. It really depends on each cars motor specs, weight, Cd, and
other factors. In normal operation, the common sense of keeping RPM's down,
easy throttle useage, etc. will probably yield the best results.
-Andy
--- Giles Douglas <giles@vy.com> wrote:
> Thats a good excuse - rates up there with the "but the dial only says
> 30mph" (looking at the tach.)
>
> My to-be-disposed-of-tomorrow Lexus RX300 seems to purr along nicely at
> 55mph, but any faster (or slower), and its gas mileage drops off hugely.
> Very noticable driving I5 to LA. You can either drive 55 and make it to
> the mountains without stopping, or 85 and fill up at Harris Ranch. I
> usually take the latter approach.
>
> And anyway, I can't autocross a RX300, so what use is it? Now, when my new
> audi allroad is broken in.....
>
> Which leads to another point. What class would the allroad be in? I'm
> guessing AS, same as the S4? (It has the 2.7turbo) Hopelessly uncompetitive
> of course, due to its weight, but at least it can get across the sand at
> Marina!
>
> Giles
>
> On Fri, 9 Feb 2001, Pat Kelly wrote:
>
> > I sort of feel like the 55mph deal was an excuse for the govt to rake
> > in more bucks from tickets. Yes, it was in response to the fuel problem
> > (crisis), and the other argument was it made the freeways safer because
> > of the lower speeds. Since then, part 2 has been proven incorrect, since
> > the deaths per mile have dropped significantly since the speed limits
> > were raised. As for part 1, since then, the automobiles themselves have
> > become far more efficient, and probably burn less gas at 70 mph than the
> > 70s cars did at 55.
> > Remember the cars that were built in the late 70s that had speedometers
> > that pegged at 85 mph? So you could be toodling down the freeway at an
> > indicated 85mph and really be going much faster. :) "Officer, the speedo
> > read 85."
> > --Pat Kelly
> >
> > Giles Douglas wrote:
> > >
> > > Doesn't it depend on the car (and more importantly, its weight and drag
> > > coeffecient?) But in general, I thought it was like 55mph (or used to
> be),
> > > which is why the US speed limits were set at that during the 70s oil
> > > crisis.
> > >
> > > Giles
> > >
> > > On Fri, 9 Feb 2001, Phil Esra wrote:
> > >
> > > > miscellaneous musing:
> > > >
> > > > I was just wondering, and thought maybe someone here would know. What
> is the
> > > > most fuel efficient speed to drive at? My guess is that it would be at
> > > > whatever speed you're in your highest gear with the engine turning as
> slowly
> > > > as possible. But is wind resistance great enough that that's not true?
> Any
> > > > thoughts?
> > > >
> > > > phil
> > > >
> > > > (ps--got the bearing replaced, thanks for all the advice--made it home
> over
> > > > the bridge with no drama--had to replace the hub too, $492, ouch...)
> > > > _________________________________________________________________
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