I think everyone who has done their research agrees that fine thread is stronger than coarse thread in general. For anything that is mechanical on the truck I make sure to use fine thread and grade 5. Don't forget that your local hardware store may sell grade 5 fine thread bolts, but generally do NOT sell grade 5 nuts. This makes your strength grade 2 in combination. Not good. I purchase nuts from McMaster-Carr (fine thread in just a few sizes) so I have what I need for the local purchase bolts.
These days I use stainless on my projects so have to be really careful to get hardened stainless equiv to grade 5 or grade 8 in certain places on the truck.
