[Harpkatt]

Tubes in cars are the same as tubes in a bicycle, albeit typically thicker. There's nothing special here though. One thing you should know is that Co2,is actually soluble in butyl rubber. This means that even if a tire is good and defect free air can escape directly through the rubber as it actually becomes a solution with the rubber. Thinner inner tubes will for this reason leak faster. Still deflating in one day would lead me to believe its not this.

Usually placing the tube in a large container of water does help find the leak. I would suggest checking again. After you find a hole lining the inner tubes leak up with the tire helps show where something may be poking through the tire into the tube. You've done everything the right way, so I would check again. Additionally look for any signs of the inner tube chaffing against the rubber or rim. Sometimes the inner tube can become pinched between the rim and tire when first installing and this can cause a leak. As an attempt to prevent this fill the tube just until it starts to take shape before installing in the tire.

Check near the valve on both tubes, a sharp rim can easily slice a valve stem, unless of course its a metal stem.

Good luck, you'll find it