Last night I finally got to update my maths program to accommodate all tricks of the 6th edition's new vehicle rules. Then I did some massive numbers crunching for the durability of different types of vehicles against different types of (ranged) weapons, with the idea of both figuring out how vehicles would fare in this edition and comparing it to what we had in 5th.
The results that I got can be roughly summed up in but a few statements, as the differences between 5th and 6th mechanics actually appeared to be quite consistent across the board.
1. For practically all vehicles that became
HP3, we can roughly consider their durability in 6th being
1.5 times lower than it used to be in 5th. It is almost precisely so for AV12 vehicles, slightly worse for AV11 and AV10 vehicles and somewhat better for AV13 vehicles. This effectively means that right now it's about 1.5 times easier to kill a 3HP vehicle in the open than it would've been in 5th. This also means that right now killing said vehicle protected by 5+ cover would be roughly as easy as it was to kill it in the open in 5th (still slightly easier, but not by much).
Example:In 5th edition 12 krak missiles fired at BS4 had ~60% chance to kill a Wave Serpent in the open.
In 6th the same 12 BS4 kraks have ~89% chance to kill a a Wave Serpent in the open, and ~65% to kill it when it is protected by 5+ cover save of whatever origin.
2. For practically all vehicles that became
HP2, we can roughly consider their durability in 6th being
2 times lower than it used to be in 5th. In particular, AV10 open-topped HP2 vehicles, compared to 5th, are more than twice as easy to bring down by massed S4 fire and almost twice as easy to bring down by massed S5 fire. For higher S weapons the difference is smaller but that's because even in 5th the chances of such weapons killing that vehicle were too high to double them - probability doesn't go over 100%
And while 5+ save slightly improves the situation, it still doesn't raise the durability anywhere close to 5th ed. level, especially against weapons of S6+.
3. Specifically in regards to 5+ saves of all kinds, those saves only make some really noticeable difference
for vehicles with 3HP, and then only against weapons that glance no better than on 4+. And of course they don't make even nearly as much difference as 5th edition's 4+ cover used to make, for any vehicle whatsoever.
And that's about it, really. Of course there are endless minor details, and of course for each vehicle the situation is slightly different - but those three statements pretty much sum up the general picture of the change in vehicles' durability that the 6th edition has brought us.
Oh, and about LRs - those became softer as well, but in a very selective manner. They would die considerably easier and quicker to multiple lascannons and especially lances. The effectiveness of close-range meltas has remained virtually the same. When it comes to LR-killing, lances and meltas now correspond to each other roughly as 2:1 - i.e. 10 BS4 lances have about the same chance of killing an LR as 5 BS4 meltas at close range (~71%). The effectiveness of lascannos is about two times lower.
S8 is still kind of useless against LRs - you'll need 30+ S8 shots at BS4 to strip 4HP with any degree of reliability.
Feel free to PM me if you're interested in any specific numbers.