Question

why lemons are tied to motor vehicle

Answer 1

Well, the indians  tied lemon as well as the chilies  togather  to not in vehicles but also outside their houses and shops. The people who do this have believed that the this would prevent the Alakshmi( Goddess of misfortunate)  from entering in to homes or shops etc thus prevent them from the bad luck.This is the exact reason they tied the lemons to motor vehicles. 
But some believes it only as the superstitious thoughts.

On the other hand some people believes in the a scientific reasons like prevention of insects etc.

Answer 2

There are two explanations behind this:- One is Mythological and another is Scientifical. Fancifully, It is trusted that goddess of the incident, Alakshmi conveys misfortune to the shop proprietors or the agents. Keeping in mind the end goal to not enabling her to enter the shops or business places either, they hang these seven chilies alongside the lemon at their doorways.

As per Indian Mythology, Alakshmi likes Sour, Pungent and Hot things. That is the reason, Alakshmi will just come up at the entryway, gobbling up her most loved nourishment and fulfilling her taste and leave without entering the spots. However, Scientifically, the cotton string which is utilized to tie the chilies and lemon assimilates the acid from the natural product while it is new.

This keeps the pests and insects far from the shops, houses and business places.This is an easy bug spray which came into custom from antiquated circumstances, which deceives now as clarified above as a mythological reason.