Convoy speed would often be 80, but that was so vehicles at the back could catch up, although there would be one clapped out old unit along that would struggle to crack 90.
I downloaded the user manuals and repair and overhaul manuals (all from Frontline Machinery), and yes, I have no life 
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On the injection pump there is a governor. "The governor limits the idle and maximum engine speeds to 700 rpm and 2 800 rpm respectively."
So, if you had the gear ratios and tyre diameter, then you could work it out. But with only 124kW at 2800rpm, then I don't think it would give you much over a ton.
That must be a relatively late mod, as the ones I was using in 90's weren't limited 
"Interesting" set-up with the forward / reverse arrangement for the gears as well. Saw one of our drivers showing off one day with a close as possible racing start & had it up to 80k in top gear in reverse before he ran out of road & had to stop! 
8 forward and 8 reverse
Fairly sure easy fix as either 'Unidan Engineering' or Unimog Central' advised of one of these sitting at the states max speed limit all the way from Melbourne to Queensland after limiter removed hills included.
Also spoke to a ADF personal who followed a rally car quite a few years back who the mechanics had 'supped up' to keep up and went well over the ton!
Mogs can be electronically limited and/or limited by gearing. You can adjust the gearing to get the old 404's from the 1950's up to over 110km/h.
The Australian army ones are 435's (aka U1700's) which can be anywhere between 80 and 110 km/h depending on the governor, gear box, portal axle ratios and tire sizes. The RAAF ones are governed to 2600rpm, the Army to 2800rpm.
If you've want more than 90kph, you'll need an axle swap to the 'fast axles' but at a loss of lower ratios.
Top Speed 12.5x20 tires= 85 kph
Top Speed 14.5x20 tires= 90 kph
Top Speed 14.5x20 +fast axles= 110 kph
The standard axle ratio for the U1700 is 6.4:1. With the optional high speed ring and pinion the ratio is 5.4:1 for the U1700.
The newer civilian 5000 series are electronically limited to 96km/h and the 5023's to 90km/h.
They are primarily an off road implement carrier/tractor so top speed isn't as important. If you want a rock crawler you can get auxillary under gearboxes that bolt on and give you the following in forward and reverse...
-3.19:1 for a crawl ratio of approx. 271:1
-5.76:1 for a crawl ratio of approx 490:1
-55.87:1 for a crawl ratio of approx. 4750:1
This is an older mog in super crawl = 1 foot a minute at 1500rpm in first gear
https://www.youtube.com/watch?v=eAf8hLRS3CI
