Most modern cars have engine control computers and sensors that can tell not only whether your current tank of fuel is contaminated but also whether you would benefit from higher-octane gasoline. Yet few (if any) cars readily communicate those data to the driver. Why not?
Many car engines are designed with higher compression ratios that require “premium” gasoline for optimal performance. These cars can still run on lower-grade fuel: They rely on knock sensors to detect the failure of low-octane fuel to resist detonation and can retard ignition timing to counteract it. However this timing adjustment reduces engine efficiency and power, so typically drivers want to avoid it. (Conversely, higher-octane gasolines are sometimes sold at such a premium to regular that their higher cost might outweigh the efficiency benefit to engines tuned for them.)
But gasoline octane rating is not the only factor that determines safe engine timing. Air density, which decreases with altitude and temperature, also affects detonation. Fuel that works great in summer or mountains may bog your car down in cold or sea-level conditions. Only your engine knows for sure whether it’s running optimally, or whether it would benefit from a bump in your fuel tank’s octane.
Apparently some aftermarket engine computer interface devices (e.g., the ScanGauge or the DashHawk) can allow a driver to monitor engine timing retardation in realtime. Ideally manufacturers should convert these data into useful dashboard information. Perhaps something like, “Your current fuel is handicapping the engine. Increase tank octane by 2 for optimal performance in current conditions.”