I haven't seen this posted anywhere so I assumed not many people know about this.
If you have a custom car say with Franklin for example and want to give it to Micheal, get the car, call Micheal and ask to hangout. Go to the bar or whatever then drop him off. Immediately get out of your car and run further away then desired character . Switch to Micheal the quickly get in the car
TA DA you now have Franklins car with micheal. This is useful if you put lots of mods on a car at Franklins LSC and want to give it to Desired Character.