The odometers are gear driven so the only variation there is tire size.
205/70-14's are actually smaller than 185/82-14's so the alloy wheels
with factory 205's add to both the speedometer and odometer error. On
the stock tires, driving at an indicated speed of 55 mph, you get the
added advantage that you are not dong 55. On some Vanagons your actual
speed can be as low as 48 due to speedo error designed to make you drive
slower. For a few years, if the speedo could read over 85 mph, the error
was 6% + 3 mph. 84's in particular had this effect. My 88 fox speedo is
so far off, I use the tach as my guide. On my Syncro's I went to
215/65-16 tires, (27.2" vs. 25.5 stock), and the speedo and odometer now
read ~1% under. Last Florida to New York trip did 16.5 mpg. I did 70
when I could.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf
Of Geza Polony
Sent: Saturday, April 01, 2006 12:25 AM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: MPG Question

One thing to take into consideration: the wide variations in mpg we're
seeing here could very well be related to odometer variations in these
20+
year old vehicles.  Bentley Section 90.30 tells us that, brand new,
odometer
tolerance is from -1% to + 3.75%, in other words, a spread of almost 5%.
Couple that with over- or under-sized tires, and you could get
variations of
20% easy.

I've noticed that going taking the exact same route from our home to our
vacation place yields different mileage readings on the odo. And I mean
exact same route! Maybe five miles difference over the course of 400
miles.

Claims of driving X MPH for such and such a MPG may be erroneous, too.
It
turns out the speedometers have a total tolerance, new, of almost 18%!
(Bentley 90.30) Incredibly, VW indicates that for a real 50 MPH speed,
speedometer readings can be up to 58.7 MPH! In other words, when you
think
you're going 75, you may only be doing 65 or so--and that's with
absolutely
stock tires.

What are we really measuring, then? This is all ballpark figuring, when
you
look at it that way. EPA must calculate MPG using measured distances,
rather
than odometer readings, along with carefully measured gas.

I was originally asking a mechanical question. Is there something that
typically cuts gas mileage down even though the car runs fine? What
would
that be, if so?