Made small correction to the math behind Rolling Resistance/Friction force. Now you can use realistic coefficients from wikipedia: Rolling resistance - Wikipedia
Light Tank is now configured to have 0.04 coefficient on sand, 0.02 on rocks and 0.01 on concrete, which is a realistic estimate from an old manual on tank design. Certainly it all can be tweaked and has a large dependency on tank mass. So for example if you have a 60 ton heavy Tiger and 5.4 ton heavy Panzer I tank they speed can be drastically different when driven off road or on the cobblestones.