Some vehicles seem to have some gears misconfigured. For example the Gunter and Sarka will stall on second gear when driving on dirt roads, even if you use SHIFT to apply more throttle. On 1st gear you're fine and will reach max revs (redline), but when you shift to 2nd gear the car will lose speed and eventually stall unless shift back down to gear 1. This is not realistic, it feels like you driving up a fairly steep hill even when the road is horizontal.

Uphill all cars but the M3S and Ada really struggle on dirt roads, I suspect this is intentional although maybe a bit too exaggerated. It feels as if gravity is somehow too strong. By contrast the cars will rush downhill...