A buggered stat would explain why she returns decent enough fuel economy on a run, but round town, straight after startup, she's derisory
Accept its a heavy tank with a big engine and no direct drive from engine to wheels under 50mph.
Your readings sound perfectly normal.
Not to me, if mine was that bad there would be no reason not to have either the Interceptor or V12 Jag I've always fancied. And some modern tech on each of those would make worthwhile improvements for a daily driver.
See this was my issue - if I'm really doing that kind of mpg, then why don't I get the XK8 / R I've wanted for years? Also Interceptor or a lovely old XJ12 sounds like fine choices. Christ, a Hummer wouldn't be far off that figure (not that I'd ever want one) Ten minute drive today, gauge read about 87, just off the bottom, really. Generally she sits at 90ish. Which I've always put down to a poor gauge, knowing how 'approximate' they are. However, a new stat isn't going to hurt anything.