Really bad MPG since winter hit??

[omodos]

hello, a few months back I was doing predominantly motorway driving after adding redex my mpg went up about 3mpg , not sure they sell it in the u.s, now that I have started town driving - schools runs etc it has dropped by 3mpg....maybe i'll add another dose of the stuff see what happens

http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_16 0212_langId_-1_categoryId_255221