S2k calculates fuel cost using WAC (Weighted Average Cost) also referred to as WAM (Weighted Average Margin). In the fuel inventory history you can see how this is calculated:
1. The old weighted cost per gallon
2. The new purchase cost per gallon
3. The new purchase gallons (based on purchased # on fuel invoice normally based on NET in most states)
4. The opening stick reading for that tank on day of delivery
5. The new total gallons after deliver
6. The new weighted cost per gallon
S2K takes the old cost (1) x gallons in ground at beginning of day of delivery (4) then gets the new cost (3) x new purchased gallons (3) and divides by the new TOTAL gallons after the delivery to get the new WAC (2)
($3.4194 x 12,057 = 41,227.7058) PLUS ($3.5336 x 8769 = 30986.1384) DIVIDED BY 20,826 = $3.4675