My fuel economy instantly got worse, i was consistently getting 11.96km/L which was about 395 km from 33L. now i only get 300kms from the the same 33L, almost 25% less kms

according to http://www.miata.net/garage/tirecalc.html the old tyres should have had a circum of 2177mm and the new of 2210, which is only a 1% increase, but i realise the different brands will measure different. my gps showed 4% different speeds when doing 100km on my dash, between the tyres. this leaves 21% of unexplained kms dissappearing.
do you guys think the little zuk is just working harder to push the bigger tyres and therefore giving less kms? would the extra 60kgs make that much difference? is there anyway a new muffler could reduce fuel economy (didnt touch rest of the exhaust)? a combination of all 3 (which would be the biggest contributor?)
any ideas? getting almost 100kms per tank less makes a considerable difference to my fuel bill, so i would like to get it back to the 12km / L i was getting.
i hope all of the above made sense, it does to me, but im special
