I use CarCare software to enter my fill ups. Cool software.
www.carcaresoftware.com. The reports are awesome! You can run reports comparing gas station mpg (Shell vs Chevron vs Exxon, etc), MPG vs octane, mpg vs month, etc.
I have a company car so have only had 3 fill-ups since 1/11/11 delivery. 890 miles on car. Weather has been cool, better mpg, but expect when it gets 100+ deg in central Texas it'll drop 2-4 mpg.
So I have 19.4mpg 90% city with 89 octane. Not had a hwy drive yet. This is with a Roto-Fab CAI & Solo Mach XLT catback exhaust. Expected the CAI & exhaust to give me a little more mpg but apparently not yet.
Might try 5 fill-ups with 89 octane and 5 fill-ups with 93 to compare.
Here's one nicely developing trend since purchased, all 89:
Jan'11 - 18.86 mpg
Feb'11 - 19.80 mpg
Mar'11 - 20.43 mpg
I rented a 2010 Camaro 1LT from Hertz in Chicago Oct'10. Drove it to Minneapolis, to Milwaukee & back to Chicago. Broke 30mpg on DIC!