The period of oscillation of a simple pendulum of length L suspended from the roof of a vehicle which moves without friction down an inclined plane of inclination θ, is given by

See the following force diagram.Vehicle is moving down the frictionless inclined surface so, it's acceleration is gsinθ . Since vehicle is accelerating, a pseudo force  m(gsinθ) will act on bob of pendulum which cancel the sinθ component of weight of the bob.Hence net force on the bob is Fnet =mgcosθ or net acceleration of the bob is  geff=gcosθ∴ Time period  T=2πLgeff=2πLgcosθ