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Actually I'm surprised that one of our resident math/science/engineer types dont have an equation for this problem.
But we know that the issue is drag. Time is much better spent (and later enjoyed) smoothing out the shape.
That's why we always focus on:
1. increase psi (cheapest fix) worth 2mpg
2. Grill block - warmer engine & some aero gain
3. Belly pan - can improve are dramaticly AND is invisible.
All of my record setting cross country drives were done with more that 500 lbs of luggage in the car and trunk.
And back in Sacramento It was a struggle to get egual (much less better) mileage results.
so here's some math.
2002 Infiniti Q45 3800 lbs. add 500 lbs which is more than 15% of the original weight.
Get 34 mpg.
Remove the 500 lbs when i get home. Never got any better than 33mpg.
Note: went over the Great Divide......lots of down hill coasting!
your potential 30 lbs + new material 7lbs = 23 net loss. I bet it wont make a difference.
but try it your self. Get a spare tire and add it to the car. test run. Then take the tire out. test run. My gut says less that 1/2 a mpg improvement.
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