I would run a full tank and calculate it off the odometer and the actual gas usage. Those trip computers are notoriously incorrect. Not to mention the reason that it drops sitting in your driveway (I think) is that the gas mileage for 0 mph is 0 mpg. So it's taking that time at 0 mph into whatever moving average it uses to calculate mpg.
I think you'll find that if you look at your mileage when you re-fuel (by calculating it yourself), you'll do much better -- I get 21-25 in all city driving and closer to 30 (29.75 is the best I've done!) on the highway in my MY97 with 93 octane. Even with spirited driving, I don't think I've had an entire tank average below 20 mpg.
Hope this helps.
-Rich
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