I have tuned my ND 2016 up to 200hp but this number is high up the revs. I wanted to know the differential ratio for my car(I can work this out by putting the car in the air and rotating the prop shaft to see how many rotations it takes to complete a full rotation of the wheels, but I dont have a ramp to lift the car). I’ve also had so many different ratios suggested 2.866.1, 3.54.1, 4.1.1, I wanted to know if there is a differential from a different car or if the 1.5 ND has a higher ratio and if it was interchangeable. Same question applies to ABARTH 124
The manual1.5 and the 2.0 have the same gear and diff ratios as below. The difference in the MPH per 1000RPM is down to the tyre circumferences.
1st = 5.087 2nd = 2.991 3rd = 2.035 4th = 1.594 5th = 1.286 6th = 1.000 Rev = 4.696 Dif = 2.866
The Automatic has the following.
1st = 3.538 2nd = 2.060 3rd = 1.404 4th = 1.000 5th = 0.713 6th = 0.582 Rev = 3.168 Dif = 3.583
The latest brochure does not show any details for ND3. The above is from ND1 and 2 brochures.
I have a spread sheet that you can input any combinations of gears and tyre circumferences to give MPH/1000RPM if it helps.
Nice one, that’s some good info 
Do you know of a vehicle that has I higher ratio diff that is interchangeable with my car. Even if it means swapping the innards only. I’m looking for acceleration
If I may link this, I posted this over at Miata.net which you may find useful.
What’s not covered in the link is that you will need a remap to let the car know it has a different final drive. Otherwise the throttle body will not fully open, feels jerky, as well as both gear indicator and cruise control not working properly.
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Great info above, just one small thing, cruise still works as it comes from the speedo which is GPS based. The car hugely benefits from an appropriate remap though once final drive has been changed - and this is not just updating the diff ratio in the map as at least two of the “major MX5 specialist tuners” believe.
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Adam thanks a bunch lad. I’m one step closer to getting this job done now I have the correct info.
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Brilliant mate
Appreciate it, what a help this will be
Very welcome. Been part of a few of these swaps now all the way from sourcing parts to building diffs to fitting them and finally writing appropriate maps and flashing them on - always happy to share
‘‘The car hugely benefits from an appropriate remap though once final drive has been changed’’
this sounds like a very interesting nugget which needs a paragraph to itself… 
If you change the diff ratio, why does a remap need to occur?
in my head thats a mechanical change, i cant see how this affect the ECUs choices?
on the same ilk - i have a long sight for the NC gearbox swap onto my ND, and will need the fiat final diff. i hadnt forseen a remap for this being needed?
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The “workflow” through the ECU is entirely different to previous versions of the car. Its not the standard TPS vs RPM tables.
It uses a separate acceleration request table (TPS vs road speed) for each gear, and then the ECU goes away and figures out what spark timing, cam timing, fuelling, throttle body opening, etc is required to produce the requested acceleration for a vehicle with the NDs coefficient of drag, weight, gearing, tyre size etc.
So when you drop in a shorter final drive, the car will only ever maximally accelerate at the values set in the OEM map for the 2.86 diff - when in reality you can now accelerate ~20% faster in the case of the Fiat diff. I’ve seen it on logs from other cars mapped by other people, and been able to replicate it with my own experience too - the throttle body doesn’t open fully in order to limit acceleration.
Its not just as simple as multiplying them all by 1.2, since some values are negative and all the speed sites in the table are shifted “down” by that 20% too, if you want to maintain driveability. It requires careful editing of around 2600 individual cells to 3 decimal places do a proper job.
Can stick some screenshots up when on a computer if you wish
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golden information here!
thank you
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Adam can you give some advise on where I can source parts, which way was this achieved when you done the swap? Was it an Abarth rear diff used, or did you use an automatic and swap the innards. Or when I want to do the swap( get funds ) can I contact you to help or advise?
Thanks again
DK
I’ve done 3, all with different routes…
One was JDM parts from Yahoo! auctions Japan (1.5L car, 4.1 ratio complete diff from an automatic car, LSD unit from a JDM model, plus custom machined spacer and 2 sets of Ford Mondeo flywheel bolts!).
One was a 2L car, straight swap for Abarth diff from a breaker.
Another was 2L car using OEM 2.86 Mazda LSD, plus 5mm spacer / 5mm longer bolts between diff unit and crownwheel, all into a Fiat 124 open diff from eBay. Bolts were NB flywheel bolts and I have the CAD files if you wanted to get a spacer made (needs a little lathe work).
Not taking on work at the mo but feel free to call me (please call rather than text / WhatsApp) for advice, 07834 534 612
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