I have purchases with rates that change over time that I want to track- Take gasoline for example, I buy 10 gals today at $2.50/gal for $25, then next week buy 5 gals at $3.00/gal for $15, etc. Sometimes there are even more variables, so back in the example, I want to track miles driven between each fill up- so I could later calculate $/mile or $/gal average, etc. I am using a standard double-entry accounting program, with support for commodities (stocks, etc) but I am not sure how to best track this information. Any suggestions? (Note: I'm looking for general ideas, not suggestions on which software to switch to 