2005 Corolla, 299,000 miles. Bought new, have maintained it well, but never used fuel additives. It does NOT get top-tier gas 95% of the time. Around 270,000 miles the check engine light came on and it began idling really rough. Short on time at that point so I poured a full bottle of Si1 in from O'reilley's. After 3/4 of a tank, the light went out and it was running smooth. Now, 30k miles and almost two years later, same exact thing...in a half-tank, the light is out and is running smooth. These are the only two times I've used fuel cleaner in this car.