[brucepick]

First off, great concept! Second, I think it's difficult to build the calculations to actually do this properly.

With your car's mpg going up and down regularly simply due to seasonal climate changes, and gas prices also going up and sometimes down regularly, a determination of what you WOULD have spent on a given fillup without these and those mods would be somewhat chancy.

If you had a whole year's worth of data for the unmodded car you could compare your current modded mpg vs. your unmodded mpg for a given month. But, having been at this for a few years, I know that even the weather conditions for entire months will vary from year to year.

Changing needs of what distances or routes to use and even what time of day to drive can also significantly affect your mpg. My point is, it's just about impossible to get a real apples-to-apples comparison.

OTOH, I very happily did the best I could on such a calculation, and figured that over the first three years owning the Civic, I saved enough on gas to cover the cost of the car itself, when compared to the car I used to drive. However, I did ignore the cost of the mods... Just too much data in the mods cost component, which I had not saved anywhere convenient. It was nice to see the estimated result though.