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A couple weeks ago while sitting in a boring job, I was comparing Ork Deff Koptas and War Buggies. One thing that ended up coming out of it is a chart I drew up that compared the To Wound or Armor Pen roll needed versus the vehicle armor and toughness of the shot-at unit. I ended up concluding that Toughness is roughly equal to an AV of 4 points higher. (It takes a Str 6 weapon a 4+ to Glance on 10 AV, and a 4+ to wound on Toughness 6, and so on.)
There are several important notes to this including creatures are slightly more liable to hurt by smaller arms fire (to wound pattern is 6+, 6+, 5+, 4+ vs to pen is -, 6+, 5+, 4+) and creatures are "safer" from Str 10 weapons (which will auto-pen/glance AV 10/11 but will still fail to wound on a 2+).
This makes comparisons between Monstrous Creatures and Vehicles more easily made when considering the number of wounds and hull points as well as the "effective toughness" of the model. However, the fact that vehicles can be destroyed in one hit via the Explosion result, and the difficulty of giving a vehicle any save at all renders them significantly more fragile that that of monstrous creatures.
How accurate would you say these conclusions are? Is there much of a saving grace to vehicles considering how fragile they are in comparison to creatures?

Only to a point. Two important differences:

1) Vehicles can be crippled or exploded before the HP out

2) Models with Toughness can be affected by poison, instant death, and lower ranges of ST than vehicles.

Effectively, vehicles are MC's that can be crippled or Insta-Death'd on any to-wound roll that exceeds the minimum required to wound them, and almost overwhelmingly the only save they can ever claim is a Cover save, very rarely Invul for a couple exceptions and never an armor save. They also largely are hilariously easy to kill in CC. Their only upside is that they can't be hurt by weapons who's S number is 7 lower than their AV, or generally 2 S categories that could hurt an equivalent MC on...6's, and stuff like Force weapons/poison/etc doesn't affect them.