I have an 03 convertible and get poor mileage. 80% of my driving is highway. I just pased the 7,000 mile mark. The car spends 99% of the time at or below 2000rpm yet I only get 20mpg. Is this typical? My code reader comes up empty. The dealer told me that was about right. The window sticker says 24 city and 29 highway. I can't believe I can't get at least 24. Any ideas?