I wanted a wheel/tyre combination that was around the same diameter as the those on the Unimog etc, but with a more realistic width and sidewall profile.
I've found that the Rock Crusher XT 1.55 tyres have the desired proportions, and fit very nicely onto Wheel 43.2mm D. x 26mm Technic Racing Small, 6 Pinholes (56908).
Can't show them on the Unimog, as mine isn't currently assembled, but here are side by side comparison pictures: