Well there are a number of items going on here

1) Old oil with contaminents will not 'slip' as easily as new oil
2) Oil make differences (all within spec but different tolerances)
3) New air filter will affect air/fuel ratio espcially if it was fairly well
contaminated
4) Tyres pressures - despite the manufacturer's owner's handbook
recommendations to check tyre pressure (and fluids) weekly we all just about
never do this.  So with tyres the pressures can lower slowly over time.  We
don't see/feel the degraded pressures but this sure does affect the fuel
consumption/rolling resistance.

Any proper service will enforce the routine itmes we should be checking
weekly.

All the above could explain you new lease of fuel life :-)

Big bonus points for self service by competant! owners ( NOTE! - outside of
manufacturer's warranty period)

1) You save money
2) You know the work has been done
3) You learn, learn learn
4) You can keep far more detailed notes of the service items, adjustments,
problems, rectifications, paint touch-ups, etc. etc. than ANY garage IF you
are prepared to put the "Tender Loving Care" into your service routines.
Making detailed notes at one service makes the next service so much better
in terms of coverage, possible problems, ordering parts like brake pads etc.
ahead of the servicing day.  Makes lifes so much easier.

Nick /////