I've changed the Exhaust & Filter and I'm using Shell Optimax (Higher RON) so It's not really a controlled experiment but i got an extra 50-75 miles out of my last half tank.

Depends on how you drive I guess, put your foot down & use the extra power & you'll not get any better millage, drive
gently & the better breathing may give you better mpg (?)

Marv

-------------------------- '99 Mondeo ST24 Nice quiet Milltek cat-back exhaust Noisy K&N 57i Wanted: 3L block and a ST200 / SVT to pull bits from