Hello Bill
     I have a full set of 5 guages in my van.  Volt, Amp, Water Temp, Oil
Pressure and Vacumn.  On I use the most is the vacumn guage.  With it you
can tell alot about your engine.  At Idle you can tell if any intake valves
are leaking or if you have a vacumn leak.  At speed you can see just how
hard you are working your engine.  It will also let you know when a tune up
is due, by a steady drop in vacumn over time.  It will also help you save
gas by showing you the best throttle position to run in for economy.  Some
vacumn guages are even marked for this purpose.

     Just my 2 cents worth, cheers
Colin Moran
'86 Syncro GL
"The Bronze Brick"
188,000 and still going