Give the car's make and model, mileage and any maintenance history. Also, give information about the mileage you were previously getting versus now. Bad spark plugs usually have other symptoms than hard starting. Tell about drivability. Does the car misfire, backfire, etc. Is it noticeably sluggish when accelerating? Were the plug (and associated wiring) replaced per manufacturer's schedule? As well, fuel mileage tends to increase in Summer and decrease in Winter so comparisons cannot span the seasons. Fuel blends change as do many other factors that contribute to one's fuel mileage.
When I try to crank my car it sputters the first time I try to crank it then starts up after the second try. I have also noticed very very bad mileage recently. Is this because of my spark plugs?