You really need to record the fuel usage and mileage over several fill ups to get a reliable idea of the average consumption. Start and finish with a brim full tank. Also bear in mind that your speedo and therefore odometer will overread slightly, so will show slightly more miles than you have actually covered.
I used to have a 216Si manual, which did an average of about 42 odometer miles per gallon when relatively new. After about 9 years and 108k miles, it was averaging about 37mpg.
An autobox will always cause a higher level of fuel usage, but even so, I would expect more than 30mpg; probably around 34.