Henrik, the good news is that the original speedo at 200mph means that real driving (where you care about the speed to avoid a ticket) is all bound in a tiny angle anyways...but the only way to actually get a reliable reading (as with the tach) is to have a speedo shop (with a speed controlled motor) turn (at 1620 rotations per mile) and set the needle. Setting a tach requires a function generator...
...and this reminds me of the saying for traffic signals in Asia, that they are not for rule, or recommendation, but ultimately there for only for decoration...could be that you just satisfy yourself that the gauges are turning and you are done (!)