there are two main things ....
1) human effort is equally involved wrt after market mods...
2) how much willing u r to get averages.
your foot work counts...coasting....drafting....passenger load.....engine power to body weight ratio counts.... so a lot things.....lighter rims....bigger intake....bigger down pipe....wheels thickness ....carbon fiber hood/trunk/body panels.......lighter seat.....
so a lot of things are there if u want to get ideal averages but your intention to get the best is most important.....you cant think about hypermilling with a v8 sitting under the hood or a turbo turbine not spooling enough
so its all about what you want.....and u can go to any level u want or u can spare....