"Stormin Mormon" wrote in message
...
At 70 MPH, you'd have no reason to engage 4WD. You may be
grinding gears, there.

On my Chevrolet, 4WD is only used on sand, snow, and
surfaces which have some slip. My 1998 model is "engage on
the fly". My earlier 1989 model would only engage standing
still, with transmission in neutral.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
.

Audi Quattros are full-time four-wheel drive. A 1990 model would have a
torque-sensing center differential. No gear grinding.

--
Ed Huntress



"oldjag" wrote in message
...
I have an '90 Audi 200 Quattro wagon with about 260K miles
on the
clock. In the last several years I've logged four 6000 mile
trips to
the National parks. It's chipped for 15psi boost, with a
larger
intercooler to give about 250hp, so it goes okay for a 21
year old
car. I run the GPS during the trips, and the moving average
speed for
the trips has been fairly consistent at around 72 to 74 mph.
To
average 72-74 overall, I needed to run ~ 85 mph on the
interstate
highways, to average out the much lower speeds on secondary
roads
etc. On these trips my overall mpg has varied from
27-29mpg, with
fuel stops about every 450 miles. This is total trip miles
divided by
total gallons of consumed with two people and a lot of
luggage as well
as a lot of miles in the mountains and on dirt roads. The
GPS miles
run 5% less than the odometer miles, which is due to the
tire size
being slightly larger than OEM.
I've mentioned the numbers to a few people, but most say
impossible.
I remember reading literature that Audi put out when the
Quattro's
were first introduced in the '80's that said in testing at
speeds over
~70mph the overall rolling resistance of the tires is less
when the
driving torque is evenly split among 4 tires vs 2 tires even
when
losses of an extra axle etc are factored in. I think the
other factor
is that the car is turbocharged which probably makes it a
little more
thermally efficient, at least at higher speeds, and it's
fairly
aerodynamic. In daily driving around town the mileage is
lower,
around 18-20mpg which is not great compared to newer cars
weighing in
the 3700 lbs range. Be really interesting to see if there
are lower
overall losses at medium speeds with an efficient 4 wheel
drive
system. The older Quattros had 3 differentials, an open
diff with a
vacuam lockup in the rear, a Torsen in the center, and an
open front
diff.