In older times the compression ratio almost dictated the the grade of fuel needed. In the 60/70/80s I used to run a TR engine in my race +4 at 12.5:1 and it demanded 5 Star (101 RON). However engine management systems have been transformed over the last decades to computor control of ignition and fuelling so the fuel demand can be very different
I think that all European vehicles from the last 20 years must be capable of running on 95 RON unleaded, not sure of the regulation but no doubt could find it given a few days of searching!
Most newer vehicles will have knock sensors (as pointed out above) and these were specified to ensure that "high-speed knock" (which is different to the "pinking" or "pinging" which occurs at lower engine speeds) did not not destroy the engine. This was a particular problem in Germany when cars were held at high speeds for long periods on the autobahn (especially prevalent in southern Germany on the A8)
However some performance engine manufactures have used the knock sensors to optimise engine performance based on the higher octane fuels (97/98/99RON) and thus extract more performance from the unit. So if you have such an engine in your vehicle, and are after the last drop of performance, then the higher octane fuels can make sense. For the rest of us, we are potentially wasting money by purchasing the extra octane.
