I did an spi to zetec engine swap. The engine runs fine there's no engine codes I had an emission test done, the readings were what i'd expect from a brand new engine which it basically is. (4500 km on it) Had an O2 sensor go so i changed it. Anyone have any ideas what it could be. My grandfather has the same car/ engine and gets 600 km to a tank. My car get's 400 km to a tank. I'm easier on my car than my grandfather. Anyone have any ideas.

what plugs are in there? air filter clean?

Air Filter was brand new, and plugs/ wires were also new.

How are you determining your mileage? I have two different foci, and the low fuel light comes on when different amounts are left in the tank. Both cars get nearly same mileage, but one will go amost a hundred miles longer before the low fuel light comes on.

they might be new plugs but are they motorcraft plugs? i heard they work the best. i dont use them personally... lol

the wires r motorcraft so in all likelihood the plugs r as well. I'm thinking it's the coil as it's from the spi motor (had 320k km on it).

i didnt even know they were interchangeable..

They are indeed the exact same had an spi and zetec coil side by side. Unfortunately the zetec one i did have had 450K km on it so it wasn't of much use.

I would definitely replace coil and clean the MAF

I wouldn't replace anything until you calculate the actual fuel mileage (or fuel kilometers, which just sounds weird). This nonsense of 300km to a 'tank' give no basis to how many liters of fuel have actually been used, simply that you've traveled 300km's.

make sure the plugs aren't bosch platinums either.