Just a reminder that there is a number at the bottom of most every VDO speedometer which shows the revs per mile that the speedo is calabrated for.
Depending on which wheels/tires the Van came with, you will find different numbers on the speedo.

Also something to take into account if you are looking for a replacement.

Places like Palo Alto Speedometer offer a mail-in calibration service if you need to have your speedometer dead accurate.

Todd
'88 Westy