I've found out that Lexus (well, basically all manufacturers) misquote their fuel use figures by several percent.
Since they generally calibrate the speedometer to read slightly higher than actual speed (so you don't get pinged by a camera for doing 2-3km over) this affects the onboard L/100km calculations.
I recently changed my wheels and tyres to what several GPS units have shown me to now have dead accurate speedo readings... ie. when it says 100km/h I'm actually doing 100, not 95.
My average fuel numbers have increased since I'm not travelling the extra 5% worth of kilometres per litre :(
For example, Lexus quote 9.1L/100km as average for the IS250... using tyres which achieve correct speedo readings changes this figure to ~9.6L/100km.
Not happy to see my numbers up, as slight as it might be.