I will try to help with any question you have, the short answer is you pay for two things at the pump, taxes and fuel. You can not change the amount of tax paid once the IFTA report is done. So take the pump price minis the state tax and buy the cheapest fuel.

Here is my first QT IFTA, this only shows the tax due to each state I ran in, no matter where I bought fuel these numbers will not change.

Now if I take todays fuel prices from http://www.truckmiles.com/FuelPrices.asp and pretend that that was the price for the entire QT we can compare the differences.

I am going to use IL and IN in this example for 2 reasons, 1 this is a common situation with these two states that many people choice the wrong place to fuel. 2 in my lane that I run todays IL price is the lowest without tax, and IN is the lowest at the pump.

Todays pump price in IL is $4.048 and I bought 5,419 gals in 1st QT, that comes out to $21,936.11, the tax in IL is $.411, so I paid at the pump $2,227.21 in taxes. My taxes due were $1,580.04 so I have a credit of $647.17, now I subtract that form my total spent for my new cost after IFTA of $21,288.94 true cost.

Now lets do IN, pump price of $3.913 times 5,419 gals, for $21,204.55 paid at pump. IN tax of $.16 means I paid in $867.04 but owed $1,50.04, so now I have a balance due of $713.00, now I add that to what I paid at the pump for a total after IFTA of $21,917.55 true cost.

Now we take the true cost comparison IL $21,288.94 minus IN $21,917.55 for a loss of $628.61 from buying IN fuel.

Now go back to the easy way of using the pump price minus the state tax, IL $3.636 at 5,419 gals is $19,703.48, and IN $3.741 at 5,419 gals is $20,272.48, this shows you that you spent $569.00 more in IN. The reason this does not match the number above is the added surcharge tax in IN and KY when you file IFTA.

Sorry this is so long, hope it helps.

Please ask more questions if you have them.