Friction for wheels is complex but you can make a few simplifications. First think about the difference between driving on concrete or on soft grass. The wheels sink into the soft grass, making little depressions or holes. So any time you drive anywhere, you have to drive up the slope of the depressions that your wheels are in.
So driving on soft grass is like driving up a concrete slope at 20-30 degrees angle. If you calculate for that slope without friction then you have a good chance of being able to drive at the desired speed on grass.