I've been lurking on this forum for quite a while now, but finally joined to post some interesting findings I made today. I own a bluetooth-based OBDII adapter, which I have paired to an old Android phone. When I have this adapter plugged in to the car and I run the right app on the phone (it's called Torque, for those who are interested), not only am I able to read the DTCs stored on the ECU, but I'm also able to access a bunch of realtime data going over the bus. I have this old phone set up as an additional gauge cluster, giving me such information as boost/vacuum pressure, throttle %, intake air temp, etc. Here's a photo of the gauges I set up:
Having this customizable gauge cluster is quite useful and informative, but it obviously isn't directly related to this thread. The interesting part of this, though, and what's related to this thread, is the fuel gauge that I can pull from the realtime data. As you can see in the photo, it was reading around 87%. That number wouldn't normally be interesting, but in this case it was, as this was right after a fill-up and the short drive back home (not even a mile). Before the fill up, that gauge read around 12% or 13%, and that was when the dash read 5 MTE (12-13% comes out to be right on 2gal, which seems like a logical amount for the 'reserve' tank). After the fill (which was 12.35gal; the pump shut off slightly before and I rounded off to the nearest dollar), the guage on the dash didn't even touch full, while, as mentioned, the OBDII/CAN data for fuel level read less than 90%. I think this can confirm that the fuel tank isn't getting completely full at most pumps.
I have noticed that how far the nozzle is inserted seems to play a big part in having the pump click off early for me. If I put it in as far as possible it will pretty consistently shut off way early at most any pump. I have the best luck putting the nozzle in all the way, then backing it out a bit. I think the fuel filler neck design on our cars is very sensitive to this, and to get it to act correctly with the pump shut-off we need to determine the proper insertion depth.
On my next visit to the pumps, I'm going to try and keep the car on in accessory mode (engine off, of course) in hopes that I can still read the fuel level realtime data. This will help me to figure out just how much is actually in the tank when the pump goes off, and how much more could possibly go in.