I've not heard of folks using a gear adaptor on a VW speedometer. It would be cool if it were easy.
So you know, Sean, the Speedo in your car takes its signal from the left front wheel, not the transaxle. It's very common for them to be off for two important reasons: 1., the reproduction Speedster gauges used until recently came from China and are of marginal quality (they are not VDO/Porsche 914 gauges). And, 2, because almost nobody (except me) runs the VW Bug-spec 165/80-15 tire, preferring instead these days the beefier 185/60. That takes the sidewall from 5.2 inches to about 4.3, and the rolling height from about 25.4 to 23.6, and thus the distance covered per revolution from almost 80 inches to 74—a bit more than an 8 percent reduction.
You've probably got a combination of both issues—inaccurate gauge and "wrong" tire size.
The fix for this I see most often in people mounting a GPS or their phone on the dash and reading speed from that.
Second most common is to ship the gauges to North Hollywood Speedometer or Palo Alto Speedometer, both specialists in the marque. They will rebuild the gauges to VDO or better quality and calibrate them to your preferred tire size. This process can take a month and run into money, but I never hear people complain about the service those two venders provide.
I did have North Hollywood rebuild/reface the 914 gauges I sourced for my Spyder. They're like jewelry.
Before (these are Chinese repops):
After (914 gauges, surgically altered):
