I use standard plugs (not Iridium) and the eight plugs have been in for two years. I intend leaving them another year before I change them. Air filter is just bog standard from a local motor factor. It has been in two years and about 18K miles, so I may change it next service. Oil is just cheap 5w-30 synthetic.
Cat was replaced two years ago but I actually noticed a 10%
drop in mpg immediately after replacing it.
I do set my cold tyre pressures to 35 psig and regularly feel my wheel centres after a trip (in case of any brake rubbing).
I tend to drive for economy, with little brisk acceleration, unless road conditions demand it. On a downhill stretch, if there is nothing behind me, I will let the car pick up speed by its self, rather than by accelerating (I am retired, and when I am in the car by myself I am seldom in a hurry). I seldom drive fast. I find that on the open road a GPS speed of around 53 mph (just under 60 on speedo) suits me and returns best numbers.
I do have my ScanGauge E fitted to binnacle and it displays my trip average and instantaneous mpg, and I constantly try to keep the latter higher than the former.