All sites, whether those that you pay for a result, such as
Towsafe or the free ones, such as
Whattowcar,]http://www.whattowcar.com/]Whattowcar[/url],[/url]are only as good as the data which has been fed into them. I don't have any experience with Towsafe but Whattowcar definitely has serious shortcomings in this respect, using some pretty inaccurate figures in some cases. Whattowcar should therefore be used with caution.
If you already have the relevant data, such as the weights and engine performance figures out of the respective brochures or owner's handbooks, it is quite easy to calculate the match yourself and obtain a result which is no less meaningful than the professional websites. They do no more than what you can do yourself and the only advantage that they have is to make quick comparisons between various different combinations possible.