I get about 27-29 mpg mixture of city and highway driving 30%/70% around 55-60 mph (on the highway part) and I get about 31-32 mpg on 5%/95% (65-70 mph) highway driving on my 1997 3.5 24v SOHC. My secret is the K&N drop in filter, 89 octane-10% ethanol at stations on the "Top Tier Gas" List
http://www.toptiergas.com/retailers.html only, and lightly accelerating in my car so then the engine runs under 2k rpms at all times.