[elhigh]

Scroll up to the top of this page and while signed in, see where it says "Garage & Tools." Click "Garage."

Look for the "Add a new vehicle" option, click that and fill in your info. Then at every fillup, update the Garage page with how many miles and how many gallons. The Garage automatically generates a graph so you can easily see how this fill did compared to other fills.

You can't add a flag to the graph to call out when mods were added (Ultarc in particular is lobbying for this) but it's easy enough to note it in the comment slot for the fill.