Larger circumference rims will decrease your standard speedo reading. Stock A15 in a 1200 with 4 speed depends on your diff ratio and how high you rev. Genuine 160 kph with out downhill or wind assist is achievable. There will be no speed difference between the 4 and 5 speed if the top gear final ratio is the same. If it is an overdriven 5 speed then you may find that 4th is your fastest gear - depends on whether engine has enough torque to rev out an overdriven 5th.