Cabin gain. I mean there's not much you can do about cabin gain. You can tune for cabin gain some, but you still want to tune your box for that range you want vs where the peaks are. Your cab is an enclosure itself. You know if you like cut a hole in your roof and stuck a sub in it, your cabin would basically be a giant sealed box that had a certain resonance with that specific sub and that specific airspace. So the shape of your cabin plus the actual air volume/mass is what gives your cab it's audio peaks and dips characteristics. Pickup trucks have the weirdest acoustics out of any vehicle group I've seen. I strongly believe it has to do with a pickup's cab usually being wider than it is long/deep. I really pay attention to where I put ports and subs in a pickup.
If your truck has deadener, like dynamat or something, it'll make you peak lower more than likely, because it adds mass to the metal. Adding mass = lower Fs.
