Fuel Evaporating?

This is probably a really stupid question, but I couldn't find anything from searching.

Is it possible for fuel from the gas tank to be evaporating? Like if the gas cap is bad or something?

It seems like when I start up my '98 4.0L 4x4 Auto after it's been sitting overnight, the fuel gauge will read about half a "tick" lower than when I parked it. My MPG is fairly low (15-16mpg), so I'm trying to figure out if maybe that has something to do with it. Of course, it's entirely possible it's just a symptom of fuel gauges being inaccurate. Or maybe it's just because it's cooler in the morning, so the volume of the gas is less?

Do you have kids with drivers lisences? Girlfriend, Wife? Everybody accounted for.

...but seriously, my fuel gauge will move when I'm going around a corner(left turns it goes down, right turns it goes up or uphill, or downhill) and then when it's all settled down it'll read differently than when it's moving.

I suppose it could evap., but not just from an iffy cap. If all your rollover vents were missing, then maybe.

I just know to ALWAYS keep the top on my Tequila bottle.

Ha, I'll have to watch my fiancèe a little more carefully to make sure she's not going on any joy rides, lol.

Yeah, I'm sure it's just that fuel gauges are simply inaccurate and fluctuate based on a lot of factors.

when you park for the night turn the heep off and wait 2-3 then turn the key on so your fuel gauge comes on and see if you get a different reading. this happens in my peterbuilt and i don't no why.

The XJ like so many older vehicles do not have electrical stabilizer on the guages. That means they bounce around quite a bit based on the motion of the vehicle and to a point by voltage fluctuations. This is very noticable on the fuel guage. Driving down a bouncy road, taking a corner fast, going up hill or down hill or side ways on an incline causes to guage to swing wide.

As said in an earlier post, the guage is as accurate as it will be if the XJ is on a level surface and not moving.