Hello Guest, you do not yet have the necessary rights to see the contents of our FAQ articles. Register in our forum, briefly introduce yourself and your Zetor in words and pictures and actively participate in discussions in the forum, then you will automatically receive access to the specialist articles in the FAQ.
Make sure you have the 40kmh overdrive transmission. Then install the ring and pinion of an 8145 and the 20.8r 38 rear tires of a 16245. (requires cutting the mudguards on the older cab) this will get you to 53kmh.
you need the ring and pinion of the smallest rear wheeled 30kmh 8145 and put the biggest tires on them. Perhaps the front wheel drive gears also have to be swapped.
Not sure about the UR2 but the difference between 25 and 30kmh UR3 is the ring and pinion. So you need the fastest ring and pinion of a 30 or 40kmh transmission. You might as well stuff the six pot in the 8145 and save the hassle of clocking out the ring and pinion play after reassembly.
The Ursus importer did it here on an Ursus which ran near 50kmh on 18.4R38. A 16211 (yes, 2wd special: it was used by a company that used it to haul truck semitrailers from field to market) with 8145 ring and pinion and big tires ran 53kmh.