The BMW 528e has that vacuum-based mpg gauge.
Porsche 914 tach also reads 'backwards' ....
Tho that one isn't' really backwards, it's still 'clockwise is more', it
just has zero at the top.  A porsche 914 tach is like that too, zero at the
top, or something goofy like that.  
  Before the days of electronics, a vacuum gauge was used to drive by for
economy, just like you say, it's the highest vacuum you can get, which
occurs at low throttle openings and rpm..
  And yes, cars built to meet fuel mileage laws trade off something for fuel
mileage.
My 88 740 turbo Volvo is programmed to always shift up quite early.
You can turn off that feature so it won't go into the very high top gear,
and keep from lugging it.  But every time you shut off the car and restart
it, it defaults to high mileage mode, and if you drive normally, or half
gently, it'll never even get over 1,800 rpm.  Most late 80's and later cars
are like this- they'll shift up way early ( auto trans of course ) if you
let them.  Lots of lugging going on ! 

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
BenT Syncro
Sent: Wednesday, November 21, 2007 5:25 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Strange & Neat instrument on Samba

On 11/21/07, Chris Lisica <wavanagon@gmail.com> wrote:
>
> It appears to go up to 20, that can't be it! Hahahaaha, I crack myself up!
> Just kidding, I had to.
>
> I looked at it again, and it does look like it could be MPG. That would be
> cool to have.

It's probably a vacuum gauge which some cars came with as an MPG meter. I
can't remember which one of my cars had this feature. I think it was an old
BMW. Anyway, the goal was to keep the needle to as high as possible. Running
high-RPM's dropped the needle. The bad thing was it encourages me to lug the
engine.

AC Cobras had reverse reading tachs so maybe this is from a kit.

BenT