You'd need to check what rim size is possible in combination with the brakes fitted. Some upper trim 'Sport' models have bigger brakes and thus might, for example, be capable of accommodating a 17in wheel, a lower-mid spec model might have the next size down brakes and be able to accept a 16in rim.

Either way, most insurers will likely accept doing this IF the manufacturer allows it - most dealerships can tell you what wheel and tyre combo is permissable for the brakes' size, and often this information is given in the handbook or on the plate (also showing the tyre size and pressures) on the driver's side door/frame.

I would suspect for a lower to mid spec Mazda6 of that era, you'd be able to fit 16in or 17in wheels and tyres as per the plate/handbook. 17 to 18in for the Sport models. Technically larger wheels can be fitted to most cars, but they and the tyres (presumably wider) must adhere to the law about fitting within the wheel arch (not sticking out) and being able to turn full lock to lock without impinging on the brakes and suspension as well as the wheel arch.

I suspect its unlikely that you'll have the cash to spend on replacing all 4 wheels and tyres (with whatever size), but if you did, downsizing can actually be cheaper than, for example, just replacing the wheels at the same size if they had seriously degraded. That's why buying a Sport (especially a 2.5L version) would not necessarily be a wise move if you are on a really tight budget, including ongoing maintenance, as the car would far more likely be shod on bigger wheels and tyres and thus the problems I spoke of earlier would come up.

You just have to weigh up the consequences of owning car that gives lower mpg, higher VED, rides more firmly, goes through tyres, suspension and alloys quicker (admitedly partly dependent on how and where you drive) and will cost more to replace those items, against having a car that looks a bit nicer and gets to 60 in a second less.

If you can afford it and find the ride to be fine, then great. Just don't discount the 2.0 petrol. Note that the reason why the 2.5s are priced very similarly to the 2.0 is probably because they are costly to run, and perhaps are a bit too firm a ride.

Edited by SLO76 on 20/05/2019 at 15:52