I owned my previous car from 34k to well over 200k miles. Always kept an eye on fuel economy, and obviously it gradually got worse over the years. (Average of 70 mpg gradually reducing to low 60's)
I then had a fuel injector issue, and as a last resort to fix it, I dropped the fuel filter housing and filled it with redex to flush the system through. Not only did it fix the issue, but fuel economy sharply rose back up and to 67/68 mpg. It's now part of my regular servicing schedule to put redex in the tank.