Hi Jan,
...that you will not be completely satisfied with the specific numbers you provide for your transmission - there might be a mistake, a typo, or something else.
The only solution is to drive the Ibiza in 5th gear from one odometer reading to the next, and then measure the distance traveled.
(A measurement range that is as wide as possible reduces the influence of measurement errors, for example, due to backlash in the gear flank

).
Then you will have absolutely authentic data that you can convert to km/h at 1000 rpm, etc.
"By the way, while you're at it, measure the actual rolling circumference of the tires. If you calculate that value, you can even determine the overall gear ratio and then experiment with different gear and axle ratios until you (hopefully) recognize the data for your gearbox somewhere."