mmm... you might have a point. That said- Weapon:5 still seems a bit light for a 2+ ton truck at 60mph, even if it is comparable to a frag grenade (yes, yes, a normal person is pudding either way- but we have to worry about a lot more than normal people in this system, so degrees of pudding matter). Maybe solvable with something more along the lines of aspects... the frag grenade affects a whole zone, the mack truck gets a "Squish" aspect or summat.
Now, a regular car hitting someone at 60mph... that I could agree with for Weapon:5, using the frag as a benchmark for comparison.