There is an mathematical equation, but in simple words more friction you the more you consume.

I tried with a friend of mine (who ownes a porsche boxter) to drive with 10 liters fuel with stock tire and the way back with wider tire (and other 10 liters fuel). Obviously condition were not exactly the same but really near. Well a the end we cover 120 km and 105 km.

Could you please tell about the front rear ratios. i'm interested in it

 

There have been a lot of discussions about tires and performance. The search function of this site works very well and here are a few threads to get you started-

General tire replacement:
https://www.s2ki.com/forums/showthread.php?...ight=Tire+width
https://www.s2ki.com/forums/showthread.php?...light=Tire+Size

Tires recommended for autocross and other competition:
https://www.s2ki.com/forums/showthread.php?...?threadid=19189
https://www.s2ki.com/forums/showthread.php?...ight=Kumho+Tire

Basically, the front OEM SO2 tires measure correctly at about 205mm cross section, but the rear tires are closer to 245mm, not the 225 that they are labelled. All else being equal, this gives the rear more grip than would be suggested by the label. Thus, if you want to maintain the same front to rear grip in a non-OEM tire, many people in this forum recommend buying 205/55/16 front tires and 245/45/16 rear tires to maintain the same contact patch ratios front to rear.

Regarding the effect of tire width on fuel mileage, yes a wider tire is likely to require more energy to accelerate and maintain speed, but I doubt that going from a 225 to 245 width would affect mileage as much as 10-15%. Other factors such as carcass and tread design and tire weight have important effects on mileage too. Lastly, as previously mentioned, the stock rear tire actually measures closer to 245 than 225, so I doubt that there would be any appreciable effect from going to a non-OEM tire in the 245 size.