I recently moved from a rural area to an urban one, and my economy tanked. However, I walk to half of the places I shop now, and rather than going through a tank (700-850 miles) every week, it's looking more like a tank every ~3 months. Given that, almost nothing I do to my car will have a positive ROI. However, it's one of my favorite hobbies and I justify my spending on it mostly by the skills I learn and the enjoyment I get.
I don't ever intend to sell my car, but one factor I consider is reversibility of mods, most of which can be pulled off and sold, and reduce the aftermarket value of the car if left in place. For example, when I splice the harness, I prefer spade connectors rather than soldering into it. When I pull a part off, it goes into a big box of spare parts. Even my headlight projector conversion is entirely reversible.
Bicycles are a bit hard to quantify, because it's more than the gas saved. A bike (vs no bike) reduces the number of cold starts and short trips in a car. It expands one's transportation options - in many cases it's faster and more convenient for me to bike, nevermind the economics. It has health benefits, and it removes another vehicle from the roads, which is of benefit to everyone from both environmental and traffic congestion perspectives. And, bicycles too have resale value.
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Only semi-related, but one of my favorite pictures of MrMoneyMustache:
