Hi Latha,

I would say that,

"J'ai envie d'une nouvelle voiture" would mean,

"I feel like changing my car"

indicating a feeling,

whereas, 

"I need a new car" ( your car has broken down/been involved in an accident) would be,

"J'ai besoin d'/Il me faut une nouvelle voiture"

indicating a strong need,

"I want a new car" is, "je veux/voudrais une nouvelle voiture" closer in meaning to avoir envie de  but indicating a wish.

Hope this helps!