So I'm seeing a ~5mpg bump.
Last Friday I went to fill up, tank basically on empty, as you can see from my picture I filled up on premo 91 for the price of regular. I was on my way to the casino and unfortunately I used up all my luck at the pumps. *sigh*
And I know, premium isn't going to give my car magic horse powers (I've decided my car runs on 'donkey power' any ways), or give me handies any damn time I please. But it is a pre-Christmas miracle that I coincidentally have seen a ~5mpg increase in efficiency.
The car? 2007 Mazda6S 3.0L Duratec/MZI V6 5-door hatch, 6spd auto
I always clear my trip odometer when I fill the tank, typically I get 375-400miles on the 18gal tank. So about 22-23mpg combined.
Right now my trip odo is at 450km (280miles) and I've used 40L of fuel, 10.5gal. So 27mpg.
Driving habits have not changed, cold starts are around -5*C to 2*C and I let idle a few minutes. Tires are the same and same pressure, still on the same oil change interval, and I did not install an intake cyclone wizzbitch fan!
I have the OBD-II Torque running, and it's telling me the same thing. Sure it may not be exactly accurate (pretty F'n close though), but it is at least showing the same basic increase in fuel sippy-ness.
Conclusion, unicorn jizzed a rainbow in my gas tank.