I'm using NGK?s...and I would only dream of reaching 100 km/h...usually I'm doing 70-80 km/h <img src="/forums/images/graemlins/sleeping.gif" alt="" /> and always driving economical.
Around 12 km/100 km sounds reasonable to me, but I'm not even near that.
Came to think of the sender units and sensors mentioned in earlier posts...when the engine is cold the automatic choke keeps it at 2000 rpm and dropping nicely as the engine gets warmer, can I rule out any of them knowing that?