Okay guys, it's working perfect now.
I think Ducimus is right, unlimited fuel.
But it wasn't by my choice. It appears that quick missions start by default at lower realism settings? At least, mine were resetting to 42 realism for some reason. And I think I didn't catch that on my first test mission.
I just retested it on several missions with my usual realism settings, and it works perfectly now.
Thanks everyone for the help.
