Are Speedometer's adjustible?

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.

For the Opels do you really need a gear reducer?

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?

Any good speedo shop should be able to help you out. Check the Yellow Pages and give them a call. HTH.

Ron