While Fehu and Sowilo broadly relate to empowerment or an increasing in personal power and wealth, Gebo rules gifting, as this makes you an "attractive recipient" in a sense. Further, Berkano relates to the family and nurturing, and can be used if your family doesn't normally support you, or views you as not in need of their support for a car.
You will have to think about which approach you wish to take here and what obstacles currently exist. Fehu would create the opportunity for this to occur. Sowilo would more broadly relate to personal empowerment. Gebo seems clearly related.
Raidho could be used both to create or project yourself as in need of a car, like if you are viewed as irresponsible or if the gift would be unwise, or some other obstacle which this rune could help you to overcome.
As Henu stated, be careful you are not trying to just "juice" your parents for their wealth. Positive affirmations for both yourself and them should be used, or perhaps you would just affirm the energy broadly to help you get a car (in the best way for you), rather than only targeting your parents. Maybe this manifests a situation where you can more quickly get a car, but you pay half and they pay half, or something like that.