Outer Limits 4x4 Board

I did a 3rz conversion into my hilux about a year ago now, It was getting arouond 6.8-7.0km per litre (thats reading straight off the dash, i understand that the tyres make it a difference reading) but in the last 1000km or so its been dropping down to around 6.3km per litre. My driving style hasnt changed at all.. I have cleaned the air flow meter and the air cleaner has been replaced, tyres are at the same pressure (40psi). any ideas on what else could be causing this?
John