automatic transmission takes more power, and thus will decrease fuel economy. i have the XRS with 6 speed manual, and i was getting worse mileage when i was driving 60 miles each way for work that was almost exclusively freeway about 70-75 mph (~27mpg) as compared to my current 13 miles each way which is more in town (~30). not much of a difference, but gear ratios do make that difference.
jdhog, if you drive on the freeway and keep your RPMs up around 3500, your mileage will likely suffer.
libmanj, if it isnt the XRS, make sure you only put in 87 octane gas. anything more than that is just wasting money, because the motor in the base and XR models is designed for the lower octane gas. this could also crap on your mileage a little as well. only the XRS is designed specifically for 91+ octane. that is part of being high compression.