Just wanted to pass along this nice link if you're interested in plotting your mpg vs. your weather conditions over the course of your tank. Daily high/low temp, precip, etc. are available by zip code lookup with the link below...just change the zip to get yours.
AccuWeather.com Past Month Weather Data for Danvers, MA
To further the mpg increase discussion for my Prius as the weather has begun to heat up...
Over the 9 days I've been running this tank the avg. temp has been 63 degrees (estimated 58.5 mpg for current tank with grill block plus added material to block). My past tank avg. was 53 degrees (56.5 mpg for the tank with initial grill block). A couple mpg increase...10 degree increase...extra grill block material added (the 2 mpg could be the weather and/or the extra material in the grill block).
Whatever the case may be there many, many other factors that could be fueling the increased mpg (if I'm driving during the hotter hours of the 10 degree increase, traffic, etc.). Bottom line is the mpg is going up for now!