We like indentation here, but this is not Python, and indentation is no substitute for getting the braces right:
Code:
for(i=0; i<min_in_trip/5; i++) {
    for(j=0; j<300; j++) {
      fscanf(ifp, "%f%f", &temp, &temp2);
      numrevs += temp;
      gas += temp2;
    }
    if(j == 300)
      array[i] = findfueleff(radius, numrevs, gas);
  }