I feel your pain, haha, my mpg calculation from yesterday's fillup was 29.45mpg, 25% lower than the previous tank, which was 39.32 mpg. Morning temperature yesterday here in FL was in the 40s. Winter being one of the main reasons why the mpg dropped by so much.
I went the same routes as my last tank, and the tire pressure was lowered to 33 PSI from 35 PSI. Driving while not caring much for the mpg may also have contributed to the lower mpg.
At least our cars ain't getting below 20 mpg like those behemoth SUVs (H2, Escalade, etc).