Naturally depends on the setup. So many swaps out there. My basically stock Tintop with 235 75 15 (28.5"ish)s gets a gps calculated 16 around town. Haven't had an opportunity to get a highway reading yet.
I've read about people getting in the low 20's on a truly stock one, low 30's on a completely loaded TDI westy. mid 20's with a 1.8T.
I think 24 is doable in a perfect condition bone stock Westy. If you "hypermile" it. High pressure in the tires, under 55, lots of drafting, no ac, etc. Wouldn't count on it though.