What mods do you have done to the car, such as intake and exhaust? those will affect your milage.

Also the tires will affect your milage. more tire to spin = more work for the engine to do = less efficiency. also, when you change your rim and tire size, unless overall it is EXACTLY the same overall height as stock, your odometer is going to rack up miles slower. had a car once that we went from a 70 series to a 75 series tire on and the speedo was off by 5mph, showed we were going 5mph slower than we actually were. our fuel milage numbers were always off after that too.

One more thing, when you go to a wider tire, you are going to lose milage. More contact area with the road means there's more tire to move, therefore more work the engie has to do. All of those things could be having an effect on your milage.

If you've changed any part of your exhaust, you are also going to notice a change in fuel milage. your engine is dependant on the backpressure of the stock system to run correctly. If you have a higher flow system on your car but still dumping the same amount of fuel in as your system was seeing with the stock exhaust, most of your fuel is actually flowing right out the exhaust valve when they are overlapping the intake valves during their duration of lift because there isn't enough backpressure to hold the fresh fuel/air flow in. That would also create a lean condition in your engine which will eventually cause damage.

Hope this helps.