This question forked from another question about budget performance upgrades from another user. Weight reduction came up, and rather than hijack that discussion, I thought I'd start a new one here.
Since most people need their vehicle to remain functional and compliant, where's the dead weight one can remove?
EDIT: Functional means doors, seats, windows and all external sheetmetal need to stay.
Depends on your comfort level. AC system (complete, condensor, compressor, evap, piping, etc), stereo, and every bit of interior except the seats and critical controls. Glovebox, airbags, related control modules, wiring, door panels, carpet, insulation, sound deadening materials, even the entire dashboard can be removed and the critical instrumentation and controls remounted with brackets. Lawn tractor or racing batteries are enough to start most 4-cyl engines, and can be relocated to better weight distribution.
Chrome trim, body decorations, bezels, bumper covers (not required by law) hood blanket, engine covers and non-essential trim, cruise control components, the list is almost unending, but return on effort starts to diminsh (eg headliner)
The question is how willing you are to drive such a beast, and if your performance/efficiency goals couldn't be realized with a chip and a change of driving style.
This is not conjecture, I've done the above many times over, racing both Showroom Stock and Improved Touring cars with SCCA for decades, that still could be streeted and inspected in New York, albeit terribly uncomfortably.
The spare tire and jack are a good place to start. Lots of weight there, and most people would not change a tire on the side of the road and have their vehicle towed instead.
This has the added benefit of freeing up space in the trunk for things people are likely to use, like booster cables, windshield washer fluid container and a jerry can.