Well if you can stomach the price difference and the results are consistent, it doesn't really matter what's causing them.... just keep doing it.Prior to the V-Pover/Optimax I had run a tank of Chevron 91 and Circle K 91 and saw little to no difference. So I can't say that the V-Power is that much better than the other premiums since others in other threads have said it takes multiple tanks for the ECU to adapt without a reset. But if this trend in MPG maintains, I will continue to use premium as it is a 5% premium at the pump, that appears to yield near a 10% increase in MPG. Others have said their cost per mile is less with premium. Plus, I think the car runs better on premium.