Start with the stuff that affect the drive drain. crap tyres, low tyre pressures, worn suspension parts, tracking, etc etc will all contribute to poor mpg.
Also is your engine healthy?? If its working harder than it needs too then mpg will be affected. A remap is not always the answer.
Remapping a unhealthy car will just give you headaches and lead to more expense.
Start with a fit and healthy car and improve your driving technique and go from there