[joe_bruin]

the error message means the following:
"cannot convert a function pointer for a function that takes in an int and returns a void to a function pointer that takes in a void* and returns a void*".

this code compiles fine.

#include <pthread.h>

void *Car(void *arg)
{
  return NULL;
}

int main()
{
  pthread_t cars[5];
  int i = 0;

  // your call
  pthread_create(&cars[i],NULL,Car, (void *) i);

  return 0;
}

this code generates your error:

#include <pthread.h>

void Car(int arg)
{
}

int main()
{
  pthread_t cars[5];
  int i = 0;

  // your call
  pthread_create(&cars[i],NULL,Car, (void *) i);

  return 0;
}

p.c: In function `int main()':
p.c:13: invalid conversion from `void (*)(int)' to `void*(*)(void*)'

have a look at Car again, something is strange about it (maybe you have a prototype in a header file that doesn't match the body).

btw, there is nothing wrong with "(void *)i" if you intend to have a pointer who's value is i (i'm guessing your Car function will take this value and convert it back into an int).