What is the coefficient of friction of a turf tire on grass?

[Carlos Esteves]

I need to know what is the coefficiente of friction of a turf tires rolling on grass. I'd also like to know how to calculate it.

thank you

 

[John]

not easy: turf tyre on hard ground and dry grass - good.
on soft ground the treads bite in.
on hard wet or soft muddy - very low (vehicle gets stuck)