After adjusting for the 9 km that wasn't taken into account into the fuel mileage calculation, I got 30 mpg instead of 28 mpg for that tank of mostly premium gas. It was still a bad data recording event.
I've talked to my mom's boyfriend this weekend about it and he was shocked. He said he filled it to the top, 3 pump clicks and then took it for a 9 km spin around town. He claims premium gas gives him about 10% better fuel mileage versus regular (he drives a Nissan Versa hatch w/ 5 speed manual).
I'll have to experiment with different fuel grades myself, but the price difference is substantial. If premium is about 15% more expensive than regular, but only supposedly gets you 10% better FE...then I'll rather take the cheaper gas.
