I have been doing my own fuel tax for the past 10 years and I am still out one where it is best to fuel.
In Canada the cheapest place is Quebec because as usual the road tax is the highest in North America with @ $.1720 / KM while Ontario is @ $.1430.. The State of Missourri is $ .0462 or roughly the quarter of Quebec.
This is only one factor.
There are several others that need to be entertained, they are as follows. Most of these relate to US bound traffic.
1) The price in the region
2) The exchange rate.
3) The GST /PST tax that you get back, or version of in Canada which you do not get back from US sourced fuel.
4) Your fuel consumption
5) method of payment and transaction fees.
Therefore one needs a complicated little algorith that takes all these parameters into account.
We always get a sizable return check from the government because we fuel a lot in quebec and run mostly US miles.
That in itself does not mean that it is the most cost effective.