Fuel economy gone down ?

[JudgeM]

Hi All,

My Vitz been rather thirsty lately. Just wanted to check if we can pinpoint a reason. I use 91 petrol and injector cleaner every 2500kms

I used to get about 6 liter / 100km now I am getting about 7 liter /100km

First winter with this car for me.

It's winter now, not really that cold like for some of you North guys. In the morning it goes as low as 5c but within a short while its in the 10s. I don't think this would be enough to warrant the increased consumption ?

I have been using the heater alot, does it affect the fuel consumption significantly ?

Also, I think spark plugs haven't been changed for long time. Because they are the iridium dual sharp point type (most expensive type) apparently lasts 100k km. My car has done just under 110k km now. Is it worth changing ? Car starts fine though.