Okay, here's another post I wrote on another forum. It's pretty basic and requires you to do some link searching, but it explains how speakers behave in relation to off-axis, and on-axis alignment, and what to look for when choosing speakers.
Here's the original link.
I'm going to use a couple of examples, and some response graphs to help you understand this.
I'll start with the Peerless Exclusive 7". Scroll to the bottom of the PDF where the response chart is.
You'll notice the "blue", "red" and "green" response graphs. Labeled at the bottom you'll see "On Axis", "30 Degrees", "60 Degrees" respectively.
Looking at the graph you can see that the upper end response of the driver lowers dramatically the further off axis you play them at. Now if your driver side door sits 60 degrees off-axis of your listening position (which most doors fall in that area) then you can get a good idea of what the upper end response will be. In this case the graph shows about 1750 hz before it starts to collapse, and is probably useable up to about 2200 hz.
Your passenger side driver will yield a considerably higher response due to it be much closer to on-axis than the driver side, so you might start to hear some bias from the passenger side should you try to run the set up to 3500 hz.
Now let's look at the Vifa MG 4" midrange
Here you'll notice that on-axis response is great. Near 15k flat, with extension up to 20k. Npdang tested this driver and mentioned that it can almost be used without a tweeter. By the response graph we can see that.
However, now let's say we're building some kick pods, and due to some reason, we can't get them completely on-axis, but rather 30 degrees off-axis is the best we can do. You'll see that the 30 degree off-axis response graph basically tells us we can use these midranges up to about 5k before any real degradation of response. Pretty nice.
Now, for low end response. For tweeters, the general rule of thumb is twice the Fs (Resonant Frequency) at 12 db. A higher slope (i.e. 18 or 24 db) can get you closer to the Fs, but we'll use 12 db for now.
Let's first look at the most common tweeter on this board... The LPG
The Fs of this tweeter is 1850 hz. Doubled that is 3700 hz. Now, try coupling that with a 7" driver mounted in a door 60 degrees off-axis, and you have quite a gap between 2000-3700 hz. Almost a full octave. Now to be fair, let's look at the upper end extension. The on-axis, 30 and 60 deg graphs look almost flat up to 20k. Very nice. These can be mounted in some sail panels firing horizontally across your front stage and you can get great results from them.
Next is the Seas Neo tweeter.
Here is a tweeter with a much lower Fs. 1170 hz, using our rule, can be crossed at 2340 hz. The specs say 2500, so we're pretty close. With a 24 db slope you could get 2200 hz out of them. These would be much better to mate with a set of Exclusives mounted 60 degrees off-axis. However, their top end is nowhere near that of the LPGs. You can definitely see that these would lack the top end "sparkle" that so many people refer to when talking about the LPG's. These will be much more neutral and laid back on the top end. Not a bad thing, as a lot of music doesn't go any higher than 15k. You will also notice a huge difference between on and off-axis. A major consideration when considering how to mount them.
These are just some suggestions on how to "guess" at appropriate crossover points. The graphs give you a good idea of how to tell how a driver will perform in a given installation. These graphs in no way indicate how a driver will sound, nor how they will perform at the upper and lower limits of their capabilities.
I hope this helps you, and hopefully, many others out there. I know it's rather lengthy