you need a gear reducer. they are normally a little box that threads onto the tailshaft where the speedo should go, and then the speedo cable threads into the gear reducer. i have one by VDO on my car, and its reading accurate.
I had the same problem with my Challenger when I switched tire sizes. I simply ordered a new speedo gear ($9) with the correct number and size of teeth and swapped it where the spedo cable goes into the tranny. Couldn't you do the same on an Opel?