Larger tyres don't correct the speedo. They change where the needle points. The ECU actually knows quite well (on standard tyres) how fast you're going - it's the crappy needle that is about 10% out. If you change your tyres and your needle looks right, then your ECU is now 10% out AND so is the odometer. It's a sad problem. Nissan don't have any legal obligation under ADR to fix it and so they won't. The best you can do to get an accurate speed measurement is to use a GPS.
Ok, the fuel - bigger tyres require more force to turn them over, so with the same engine and gearing, that means more fuel. Our tyres are just 2% larger than standard and we're using more fuel than we ought to be.
Then there's the speed. In the 1800-1900rpm range, the D40 YD25 engine is nearing the top of its torque range and at 2,000rpm it's right at the point where the engine develops the most amount of torque for the fuel going in. If you put more fuel in you get more rpm but not a lot of extra torque - so anything over 2,000rpm at cruise will result in an increase in consumption. On standard tyres that's 95km/h, and that should be just over 100km/h (actual) in your car. Anything beyond that and you're effectively throwing fuel away.
You can change the torque curve with a chip - especially one that adjusts the turbo vanes. Chip It and Unichip come to mind.
