The particular Gas you put in your car does not effect mileage per-say.
Is "premium" a cleaner fuel? Possibly (tends to include more ethanol as an octane boost). But for the most part, gas is gas is gas. The Octane level is just a rating of how quickly it burns (Higher = Slower) and does not include the other additives and other crap in the mixture.
You want to run the lowest octane you can, while keeping the engine happy (not knocking). Since you have a regular 3, I fully assume it takes regular gas. A speed 3, however, has a much higher effective compression ratio, and it needs the higher octane fuel to stay happy. Its not that the higher octane gas has more "power" in it, its just a requirement that the fuel burns slower and more controlled to insure proper combustion in the speed 3.
In conclusion, buy regular gas and save yourself lots of money.
