I own a 2005 Jazz with 15" alloy wheels which I have owned for 10 years. All this time, I regularly got readouts of between 56 -61 mpg on the cars computer on a regular 30 mile, mainly motorway, journey. (The highest was 66 mpg). Just before Christmas I had the two front tyres replaced with the same size and make (Yokohama), and a new battery to replace the original one! Since then the computer readout has dropped to 47-51 mpg for the same journey. Odd??