The overall diameter of every space saver, temporary spare (call it what you like) we have ever had has been less than the wheel/tyre fitted to the car. This applies to Ford, VW, Kia, Nissan, posibly more.

They are only intended for use in an emergency and should be fitted to the rear and used for a max distance and speed. Its a bit of a faff having to change 2 wheels at the roadside I guess but if put on the front they will probably cause issues with the ESP etc and put a light on the dash.

A call to Mazda should provide you with a clue of the correct size.

Smaller diameter, but greater profile, so rolling radius is equivalent.

Some may be the same but definitely not with the brands mentioned above. All were smaller on rolling radius, when fitted the car was much lower on that corner.

Had time and carried out a bit of research.

The 18" tyres on a the Mazda 3 Sport should be 225/40 x 18 which has an overall diameter of 637.2mm giving an approx rolling circumference (unloaded) of 2002mm

This appears to be correct since the Mazda 3 when fitted with 17" and 16" wheels has a rolling circumference of 2001mm and 1990mm all of wich are within 0.5% which is negligable and well under the difference you would expect between a new and used tyre which is approx probably about 2%.

If the size quoted above for a 16" sopace saver is correct it has an overall diameter of 581mm which is approx 56mm less than the standard tyre and will lower the car by about an inch on the corner where it is fitted. This is exactly what I have experienced myself when fitting a spacesaver. The unloaded rolling circumference would be approx 1826mm, approx 9% less than the standard tyres.

Out of interest how did you secure your space saver in the boot and what did you do with the dirty 18" wheel after fitting the space saver?

I assumed the holdall would have been large enough to accommodate the punctured wheel, but looking at the site you mention, they are for the space saver only.

I would have thought correct space savers would be same rolling radius as original equipment normal tyres, so the size differences noted above is surprising.

On 2WD cars different sizes on undriven wheels won't be an issue transmission wise, even if fitted to a normal driven wheel the standard diff will cope fine for the mileage expected to be covered, but thinking about AWD systems some of which appear to be made of cheese presumably space saver sizing has to be the same RR as standard tyres.

Does this apply to Mazda AWD models, or is the Mazda transmission made of tougher stuff than some of the european models.

Even on 2WD cars the space-saver needs to be the same overall diameter otherwise the ABS will be constantly activated

Even on 2WD cars the space-saver needs to be the same overall diameter otherwise the ABS will be constantly activated

In that case wouldn't the ABS be activated at every corner and roundabout with standard wheels fitted?

No - it has a sight tolerance - in the same way, slight wear differences don't trigger the ABS.