My driving is mixed but always at least some highway, and I'll usually see 18 or so; not as high as 20 but rarely less than 17 or so.
The "economy" gauge is actually a vacuum gauge, and therefore responds immediately to changes in engine loading or throttle position, but says nothing whatsoever about anything over time.

__________________
Craig Bethune

'97 SL500, 40th anniversary edition

'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)


SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)