The manual will not say that if u change the consumption will increase.
I verified this with engineers at Nissan, common science, and common sense, and google results.... summarised below
ALL other factors are equal - threads, softness of tyres, driving habits etc.....
If the tyre is now 195 instead of the recommended 185, than there will be more contact with the road (hence wat people say more grip). More contact with the road means that there will be more friction. Science 101 will tell you that this will means that the car will need to do more work to overcome the friction, more work done by the engine, more power more fuel. (of course too small a tyre will loose grip - so optimum is given by the maker of the car - who knows best)
If the tyre is now 60 instead of 65, then the car will be lower, cos there is less "air" in the tyre between the car and the road. This will mean that the bump in the road, you can feel it "better".
and hence more bumpy ride.
If the tyre size is different from the specifications, than one turn on your axle in the engine will means more than 1 turn or less than 1 turn in the wheel and hence the distance traveled. What this means is that the odometer readings will not be accurate. Over the long run - a few 10K of km, the differences will become large enough.
I also physically checked a few Latio and found that if they put in a different profile, their tyres stick out of the rims - like going to burst like that... not sure if this is a good sign or not.
Also, as I understand it, the gear ratio etc for the car is designed for the tyre profile specified by Nissan. A different profile would surely cause some strains... what exactly I am not sure at this point.
If we want better grip, we can go for the same tyre profile, but choose the type of tyre/ rubber for wet conditions, which is usually the softer tyres if I am not wrong.
One thing to note - in racing like F1 etc, they wanted to have wider tyres for more tractions, better stability etc but they also change tyres very frequently, and the tyres uses a different type of rubber compositions than the normal ones that we use and they use Nitrogen too. The pattern in the tyres are also different for different race conditions and the softness, hardness of tyres are also factors.
One conclusion I get out of this exercise, there are a lot of science to tyres. I do not want to follow the tyre shop guy blindly (they did not explain to me the drawbacks of a different profile, only say better grip and a lot of people change and no problem! ).
I would rather play safe and follow the instruction from the people that makes the car, who would have optimized and created a balance between tyre profile and other factors FOR our normal diriving - and not race driving....
Just my 2 cents worth
- not meant to shoot anyone down or any tyre shops or any tyres down
- just sharing my experience and research....
CHEERS! 
some References
http://www.answers.com/topic/tire-code
Wider tire NOT really mean more grip . .
http://newscionxb.com/printpage.html;topic=1976.0
Wider tires effect gas mileage:
http://www.saturnfans.com/forums/showthread.php?t=68454
Hope I'm not too late with my two cents. I watched a program on "SPIKE" where they were replacing the wheels and tires. The wheels were bigger and wider (15x6 to 17x8). The tires wider and lower profile. They also put bigger brakes on this car, after testing. The tire and wheel weight was close to stock. The final analysis was; slower acceleration (by fractions of a second) because the "centrifugal weight" of the wheel tire combo was moved farther from the center of the wheel. This was also true for the stopping distance (increased) until the bigger brakes were installed. I might not have all the specs. correct but the general idea I'm trying to get across is, IMO, it will look better but perform worse, except maybe cornering will be better.
Thanks for the sharing.
learnt something. I'm a noob in tyres things.
It does makes sense in having more air will take more impact thus less "feel".
One thing I always been thinking.., In F1, cold tyres runs faster right? or is it the opposite?
and one more thing, Bigger profile tyres doesn't also means more distant to complete 1 revolution? if so, it would have appeared in the odometer that it cover lesser distant? thus the illusion for higher FC?
One other thing bothers me too. sorry ah, very the layman illustration, If a Brand A car is designed with a specificed tyre size, wouldn't that the odometer is adjusted to suit 1 complete revolution cycle to complete example 1 meter? if profile changed, than 1 complete revolution cycle will means a slight deviation? But in terms of great distances travelled, it will means great differences?
