I think that cars today 'go out of tune' far more slowly than cars of yesteryear.
Carbs have gone.......as have points which would wear and change the timing. I remember seeing the air filter from a Cortina 2000 GXL MK3.
The car was returning about 14 MPG and the filter which was as black as Obama's arse had never been changed. The points were black and pitted, and the plugs which were supposed to be changed each year/12000 miles were wet and black.
Couldn't get it to rev above 3500 RPM.
I remember going with a mate to buy a Chevette.
It was in reasonable condition but the engine was as rough as a bear's @rse and the owner clearly clueless. We took a punt on the fact that it just needed some TLC.
I drove it home and it was barely drivable.
We thought we'd go over everything to give it a good service. On removing the rocker cover to adjust the tappets, we couldn't find any. Just a mass of black jelly that wobbled a bit when you started the engine.
The air filter had totally clogged with oil from the breathers, the points were barely opening at all and chunks kept plopping out of the drain plug when we drained the oil.
A new set of filters, plugs, points, oil and coolant change, check the timing and adjust the mixture and it drove like it had just come out of the factory.