what do you top out at in 4th? What size tire do you have on it? Bacily it is because you have long gearing your tranny
If your car had x amount of HP, the engine could pull all the way to the red line in 5th but because it only has 106 hp, it can not overcome wind resistance and force of friction to pull all the way to 186.861mph@7000 Rpm with a 195x55x15 tire. If you had 300 hp and built the engine to withstand the high revs, you could get to the 186mph. The reason why you are toping it out in 4th is because your engine can not pull it self in 5th gear.
One good way to demonstrate this is in Grand Trissmo 3, buy a skyline and then make the gears as long as possible with no other mods on it. Getting up to 200 is extremly hard if not impossible. Now put all the mods on and now 200 is not that hard. Next make the gearing smaller to the point that by the time you finish your top speed run, you just almost make the car redline in 6th. That there is the fastest you can go with that HP.
Fool around with this for a little bit. Speed-gearing calculator
Change the red info to what your car is and it will do the rest.