I was just wondering..... does anyone have a formula to calculate the gas milage according to the load on the vehicle?

For example, i the gas tank is full vs half full, weight difference is 10gallons X 4L/gallon X appx 1kg/L X 2.2lbs/kg = appx 90 pounds. 1 passenger is apppx 150lbs, and luggages additional.

I recently drove from LA to SF 100% highway and got a dissapointing 24 mpg. We had 3 people in the car and trunk with about 60lbs of luggages. I am used to getting 26mpg on the highways and I wonder if the added weight could offset the gas milage by that much.

 

I thought it was somthing like for every 100lbs you lose 10% gas mileage. I think I heard this on Click and Click Car Talk or something.