Here's a great source for gear ratios of commonly found Vanagon transaxles...
http://volksweb.relitech.com/helmut/GEARBOX.HTM
By multiplying your 5th gear ratio by the final drive, you get an overall ratio.  As an example, .77 times 4.83 will
give you 3.7191 overall.  You can calculate the "magic number" for your tires by noting the rpm off your tach at a given
speed.  For instance, if you have a 4-speed in your 86 and standard gearing (0.85 4th and 4.83 final equals 4.1055) and
you note that at 3800 rpm you're going 70 mph.  Then your mph/1000 rpm is 18.5.  If you take 18.5 times 4.1055 you get
76.  This is your magic number.  If you divide any OVERALL ratio into it, it will give you mph/1000 rpm for that gear.
For instance in your van with a 1.23 third gear, you get an overall of 5.9409.  If you divide this into 76 it gives 12.8
mph/1000 rpm.  So in 3rd gear at 5000 you'd be going 64 mph.  And so on.  This magic number changes with your tires and
gearing.  Go to the included website and start calculating.  My 5-speed has a 0.82 5th with a 4.57 final.  This gives
just a touch over 20 mph per thousand (the magic number on my Wildcats is 75).  Of course these numbers are only as good
as the accuracy of your tach and speedo, but give an "apples to apples" comparison number for your Vanagon.

-----Original Message-----
From James Ballen
 
A question for Jeff, and for those of you that have used
these 5-speeds with a WBX...
Are the 1-2-3-4 gears geared the same as a standard 4-speed?
What is your typical 5th gear cruising speed/RPM at 60-70 mph
on a freeway?
How much is mpg increased?