Thrust Calcs give out absolutely meaningless data. They do not take into account all the dynamics necessary to even give an approximation. I used a fish scale like Dick H. suggested and got totally different figures. While not scientiffic, they proved to be alot more realistic than what I was getting off those thrust calcs. If you do this test, try to do it on a smooth flat surface such as concrete or asphalt as the resistence to rolling that grass gives, will affect the results.