With wider tyres the fuel consumtion will increase because there is more drag, the large alloys will probably be heavier than the small steels, which will increase fuel consumption, the engine will have to work harder to turn the wheels to get up to speed, the larger rolling diameter makes the car higher geared and more fuel will be required to generate the same rise in revs, which will increase consumption.
However because of this higher gearing once rolling fewer revs will be required to maintain the same speed, so should reduce consumption, but this is likely to be offset by the sincreased drag and weight.
All of the different variables, gear ratios, rolling diameter, fuelling/ignition timing etc will have been calculated by nissan to give the best results in a variety of driving situations, changing any of these is likely to have an adverse effect on performance and economy. Having said that its likely to be noticeable, but not enormous.
You really want to keep the rolling radius as close as possible to the standard wheel tyre combo, which means that 185/35/17's are probably your best bet tyre wise, if this width can be kept under the arch, these would fit nicely on a 6.5 or 7 inch rim.
The first columb shows the stock 155/70/13's, the second columb shows the 185/35/13's (Available from mytyres for just over £50 each).
Diameter 547.2mm 561.3mm
Circumference 1719.1mm 1763.4mm
Speedo error 0% -2.51%
Reading at 30mph 30mph 29.25mph
Reading at 60mph 60mph 58.49mph
Ride height gain 0mm 7.05mm
Arch gap loss 0mm 7.05mm
I used
www.willtheyfit.com to generate that data, just enter the stock micra size (155/70/13 on a 5x13 rim, of offset 45mm), then insert the specs for the 17's and try some tyre sizes.
Here it is with the variable i used already in.
The offset is the difference between the mounting face of the wheel, and the center line.
To calculate the offset of your wheels, measure how wide they are, this can be total width or tyre mounting face to tyre mounting face, just make you use the same datum in a mo. when you have the width, divide it by 2. Now you need to measure the back spacing (the distance between the mounting surface, and the inner rim (or inner tyre mounting surface). Work out the difference between the back spacing and half the width, thats your offset, if the back spacing is greater than half the width you have a negative offset, if the backspacing is smaller than half the width you have a positive offset.
, It's mainly for the aesthetics. Although I still want to know how bigger seizes affect economy. 
