- My model of MX-5 is: __2007 NC
- I’m based near: __Warrington
- I’m looking for technical help or recommendations on: Speedo drives
Put the old tyre size and the new tyre size in to this calculator and it will tell you how much the speedo reading will change.
I am thinking of changing my NC 16inch wheels to 17inch. What I would like to know is whether I have to change the gearbox speedo drive mechanism? Seen this having to be done on Wheeler Dealers a few times.
Cheers,
Les.
You won’t have to change or adjust anything. Just use a 205/45/17 tyre size on a 17x6.5 new wheel if you want to keep things standard.
It’s about the rolling radius. If you go up in wheel size but downsize in tyre profile and the radius remains the same the speedo will show no change. 
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I believe the fitment from factory was either:
- 205/50R16
Or - 205/45R17
I’ve been looking into this for my ND too…
My current tyre size is 195/50/16, but if I go to a 205/40/17 the rolling radius is more or less the same according to willtheyfit 
Hope this helps in some way…
Even on a 45 profile, at 120mph your speedo will read 117. As long as it doesn’t interfere with the drive train it’s not too much to fuss about.
Thanks for the heads up, very helpful.