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As per the subject.. I understand stock NA offset 45mm for 14x6" wheels was changed to 40mm for 15x6" wheels on NB8A.

Does anybody know or remember specifically why? Just for a slightly wider track? Or did suspension changes in NB necessitate it?

Just curious...

The NA was actually 14x5.5
So the wheels were 12.5mm narrower, which is likely to be the reason.

Ah sorry, I meant NA8.. which was 14x6 as opposed to NA6 (14x5.5 as you mention)

http://www.miata.net/faq/wheel_weights.html

So the question still stands

Check redbook or manuals and the car is different in track and width. Different cars NA and NB.

As to why an engineer would decide that, unless anyone has inside knowledge to mazda design, it is all educated guess work.

Sometimes it is, what it is.

Redondo wrote:Check redbook or manuals and the car is different in track and width. Different cars NA and NB.

As to why an engineer would decide that, unless anyone has inside knowledge to mazda design, it is all educated guess work.

Sometimes it is, what it is.

But track width changes with wheel offset because it's measured from teh centre of the tyre/wheel

So it may not be the track changed and that caused and offset change, it's likely to be the offset change causing a track change.

According to redbook.com.au,
NB track: F 1415 / R 1440.
NA8 track: F 1410 / R 1430.

So it can't be purely a result of the changed wheel offset because it isn't the same change front and rear. We know that suspension geometry was tweaked for the NB, which I think would be the main cause for the difference - and perhaps also the main cause for the change in wheel offset.

Unless the A arms are different between NA and NB, then I cannot see why there would be difference in the wheel track.

NB just has pump out wheel arches and wider track. Cheapest way for improving handling for little effort.