Anyone notice how when their cars were new parang very fuel efficient tapos as it gets older parang sumasama?

I (just) noticed this on my 2002 Civic Vti. Dati umaabot ako ng 13+ kms/liter pag mostly hiway at naka-blaze na gas. Ngayon parang hirap na ako umabot sa 11. And I think my previous cars also behaved similarly -- another civic and a lancer. Ngayon ko lang na-realize kasi medyo bigla dahil last year yakang yaka ko pa mag 12+.

Dati naman nakamana ako ng 8 year old car (colt) and while hindi siya ganun ka efficient to begin with, parang hindi ganun kalaki ang degradation after 3 years.

Ganun kaya talaga? That FC degrades on a decreasing slope as a car gets older? Sa inyo gano kalaki ang degradation as the vehicle gets older?