NC 1.8 5 speed to 6 speed conversion

Technical Area Engine, Transmission, Exhaust etc
1

To make the motorway miles I have to do more bearable, I’m thinking of swapping my standard 06 NC 1.8 5 speed box for a 6 speeder from another NC.
How straightforward a job is this to do?
There is talk of different ratios used on the 6 speed boxes - how does one tell which is which?
Are the final drive ratios the same on 5 & 6 speed cars? I’d hate to change boxes only to find the taller 6th gear being offset by a lower final drive diff on my car compared to a 6 speed car!
Any pearls of wisdom appreciated!

  1. My model of MX-5 is: 2006 UK 1.8 manual_
  2. I’m based near: Gloucester__
  3. I’m looking for technical help or recommendations on: Gearbox upgrade from 5 speed to 6 speed
2

You’ll need a 6spd prop and clutch too.
Oh and some decent oil, the cold changes are terrible unless you get that right.

3

5th gear on the 6 speed is the same as 4th gear on the 5 speed. 6th gear on the 6speed is slightly shorter than the 5th gear on the 5speed. However, the end diff is slightly taller on the 6speed, making the 5th gear resulting ratio on the 6 speed a bit taller than 4th on the 5speed and resulting 6th gear on the 6speed is a bit taller than 5th gear on the 5speed. This is all with their corresponding final drives. If you combine the 5speed final drive with the 6speed gearbox then you have got the shortest of ratios. If you want lower rpm’s on the high way you’re better off swapping bot 6speed box and final drive. If you do, make sure you get a final drive with lsd if you’re on it anyway :wink:

Source: Roadster.Blog: Roadster NC Transmission & Drivetrain

4

I had two NCs this year: an 06 5-speed Option Pack (now sold) and an 05 6-speed Launch Edition. The difference in rpms, mpg, and noise on the motorway at 70 mph is miniscule. Certainly not worth the cost of a transmission swap. I actually miss the 5-speed. It’s a much smoother shifting gearbox without the cold shift problems of the 6 speed.

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5

Maybe 215 tyres would be a simpler solution??

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6

It doesn’t surprise me; at 70mph the wind noise component is of big influence because of the soft top, perhaps even bigger than engine noise.