Unimog Club UK

With an ex-snow blower, 35mph down hill with the wind behind it but we have had 40mph out of ours once :O never again
Being a snow blower yours will have increased hp from the factory, normally it would have 34bhp being a 67 411c, but a snow blower will have had 36 or 38 from the factory (njc I think this is the 36 hp version you mentioned), I can't remember which off the top of my head.
I wouldn't try fueling it up, without pump Mods that's generally a waste of time on the om636.