No the 28% does not add for each one its a curve when adding them.
If you get 20mpg and add one you would get 25.6mpg and from there:
2x 32.77mpg
3x 41.94mpg
4x 53.67mpg
5x 68.71mpg
I think I will get myself 10 of them to go from an avg 27mpg to 318mpg
, that's right suck it hybrids.