You will need to do it a query. Here is an example of how I do it for tracking gas mileage.
SELECT tblFuel.fID, tblCar.cName, tblFuel.fDate, tblFuel.fGallons, tblFuel.fMileage, tblFuel.Cost, DLookUp("[fMileage]","tblFuel","[fID]=" & [fID]-1) AS [Prior], [fMileage]-[Prior] AS Difference...