I have to run premium in my car and my old V10 Dodge runs like crap on anything below 90 (I dont drive it much so generally run ethanol free 90 in it and pay the extra .50)
Car mileage has dropped about 15-20% the last several weeks and I keep it very well maintained and been suspicious about what is up lately
There is a way to get around having to buy premium fuel. Water/methanol injection can raise the octane considerably. If you are kind of handy, you can come up with your own system. Racing systems are a bit too expensive to be practical for street use. But it might be worth looking into. Lots of mixtures will work but the optimum for most cases is about 30% methanol to 70% water. A side benefit is that it keeps your combustion chamber and valves clean as a whistle.