You may not actually be going as fast as your speedometer indicates
since the Vanagon speedometer is at least 15 miles per hour optimistic
at the higher speeds.  In the Bentley it states that at 1000 rpm in
high gear you are going 18 miles per hour (29 kilometers per hour) with
the 4-speed transmission.  I don't think the 5 speed has a higher ratio
in 5th, just different spacing of the gears.
Divide your indicated rpm by 1000 and then multiply that by 18 and you
will get your true miles per hour.  For example: your engine rpm is
4,500.  4500 divided by 1000 = 4.5.  Multiply 4.5x18= 81 miles per
hour.  My 1.9TD conversion (with gas transmission) shows 80 miles per
hour at about 3,700 rpm.  It sounds and feels like 80 but is really 67
miles per hour.

Wade Yorke
Nova Scotia
'84 Westy 1.9TD