Compare 2 columns on different sheets & return current % difference as year progresses

Greetings,

I need some help with the following problem:

Workbook Layout
I created an Excel 2003 workbook that tracks various categories for my small business, including mileage, parts, labor, tax, invoice total, profit, etc. I set it up to have each month of the year be a separate sheet with an annual summary sheet after the 12th month. Each category has it's own column with a "totals" row at the bottom (see pic).

Annual Summary Sheet.jpg

Formula Needed
Everything is set-up and working great, but I would like to add a row under the "Totals" row of each years summary sheet that shows the percentage difference of each category between the current year and the previous year. The catch is that I would like the formula to calculate the percentage difference as each month progresses. Meaning I cannot simply divide the totals from one annual summary sheet with the other because it will give me a negative/inaccurate value until the current year is over. Rather, I would like to compare whatever amount of months that have passed in the current year with the corresponding months of the previous year.

My first post so I hope I have clearly described my issue.