Yeah, be meticulous with how much gas you're putting in the tank and how many miles were carried on that amount of gas. After a few tanks you'll get a good idea of where you stand in FE.
To facilitate driving style improvement, a way to monitor your MPG usage is highly recommended. Waiting for a tank to be refilled doesn't help one fine tune driving at any time before that. A scangauge or something similar will help in this (assuming it's for your 2005 car).
At less than highway speeds, weight and tires are the biggest factors to consider as they will have the biggest FE impact moreso than aero.
TIRES
> pump tires up (some go as high as sidewall max, but I just went to 40psi because its TOO bumpy any higher... it's a Scion... you feel everything on that stock suspension!!!)
> consider Low Rolling Resistance tires
DRIVING
> choose your routes carefully to minimize stop-go, idling and congestion
> read over the 100+ tips if not done so already. (
100+ hypermiling / ecodriving tips to increase gas mileage - EcoModder.com)
WEIGHT
> lose weight (the less your engine has to lug around, the easier it will be on the guzzling)
With all of that, I bet you'll eek out at least a 5mpg jump, especially with changing your driving habits. The nut behind the wheel is the most significant and easiest thing to mod by far.
A grill block has another benefit besides aero and that is if properly designed can facilitate faster warming of the engine to its optimal thermal levels. But don't expect the world in MPG from just a grill block. From my own tests, a full block gave me a boost in MPG from 3 to 5 (!!!) but the overheating was not worth it so I opted for a mix (full upper, half lower) which is a gain of 1 to 1.6mpg. But I won't make any conclusions until I get my tank refilled and my SG recalibrated on another tank (just to be sure) and of course do more testing in more diverse conditions.