You're going to get piss-poor gas mileage. 4.10 gears are going to keep the motor turning at a higher rpm in every gear. The lower the numeric figure on the gear-set, the better the gas mileage & more top-end gearing/speed (ie: 3.08, 3.27, etc); the higher the numeric figure, the better acceleration and worse gas mileage (ie: 4.10, 4.30, etc).

If you're modding a car and concerned with gas mileage, maybe you should be building a Honda?