Which is cheaper: Regular or Premium Unleaded fuel?

The table below represents all the fuel I’ve bought for my little car for almost the last 12 months:

I only use my car to drive between my home and the local train station, and occasionally I pick the kids up from sports after work. Other than four traffic lights and expected annual weather/temperature changes there isn’t a lot of variance in conditions. Sometimes I’ll get all four lights green, sometimes all four will be red. Of course stopping and starting light this effects mileage. I try to run my car to as close to empty as I dare and then fill it to the brim every time, although sometimes I break this pattern by necessity.

Given this data I want you to tell me if it’s cheaper for me to run my car on Regular Unleaded (highlighted in green) or Premium Unleaded (in yellow) which can be anywhere from 2c to 10c more per liter, or even if a solution exists.

I’m not interested in fuel use. I don’t care if I use more volume of one fuel than the other per kilometer. I want to use less dollars per kilometer.

I’ve discussed this with some workmates who have come up with various results, all of which are a bit wishy washy for my liking. For example, it was found that I seem to be buying my Premium Unleaded cheaper than the Regular, on average. I guess this is because I was buying more Premium before Christmas when it less expensive than Regular is now.

If you take simple averages, then it’s costing me 35c per 100km less when I run on Premium than when I run on Regular. But I don’t think it’s that simple, and I don’t quite know how to go about it.

Any mathematicians out there want to give me a hand with this? Never mind the whole “if you don’t run your car on the right fuel it harms your engine and costs longer in the long term” red herring. I just want to know which is the cheaper fuel per kilometer traveled.