17x7 on an NB Mx5?

[fastfreddygassit]

17x7 (correct offset) will fit with no issues (Simon's rant notwithstanding).
I run them, meanmx has run them (he now has 17x8.25 +25), and cookiemonster now has them.
Charlie Brown is correct about keeping the weight down.
Aim for a total rim/tyre weight of <16kg if possible. Many aftermarket wheels are quite heavy.
My set-up is approx 16.7 kg. Its not light, but I run tein flex with street spring rates and the ride is awesome (just ask kevgoat!)

Your thoughts on having a larger diameter rim so that you can lower the car a tad more, are correct.
What many people forget is that there is less sag/ballooning with the 205/40-17 tyre than there is with a 205/50-15 tyre.
I swapped rims with my brother (Droo) for a month (the above combo) and I found my car was scraping consistently.
I rechecked rolling diameters, and whilst the 15's were smaller, it was not enough to explain the lack of ground clearance that I had.
It was the aforementioned ballooning. The same sidewall flex that provides a comfortable ride for the cardigan wearers meant that
I had lost at least 15mm ground clearance when static, and significantly more when the car was laden with my not inconsiderable weight.
Putting my 17's back on IMMEDIATELY improved my ground clearance (driveways, speed humps, freeway undulations.
And the car looked better as the 17's do "fill out" the guards when compared to the 15's.

Welcome to the forum, many of us have strong opinions on what should be done to the MX5. Read/listen to what is proffered but make your own mind up.
Though, I would strongly suggest that you keep your wheel/tyre combo weight in mind when you start looking at purchasing rims.
Bests of luck and post some photos of your ride when its done.

[Danny]

It sounds like a spring issue, twisting knobs will not fix a spring frequency issue. Do you know the spring rates fitted?

The higher the frequency, the stiffer the ride.
0.5 - 1.5 Hz for passenger cars
1.5 - 2.0 Hz for sedan racecars and moderate downforce formula cars
3.0 - 5.0+ Hz for high downforce racecars

Lower frequencies produce a softer suspension with more mechanical grip, however the response will be slower in transient. Higher
frequencies create less suspension travel for a given track, allowing lower ride heights, and in turn, lowering the center of gravity and hence a siffer ride.

How do you check the frequency?

[MattR]

Not so much the frequency, but the rate which is expressed in kg/mm or for old school people like me lb/inch, basically a measure of the weight needed to compress the spring a certain distance.

If the PO went all out to make the car rock hard to get the "handling" right then he will probably have gone too heavy in the spring rate so that it takes a lot of force or weight to compress the springs and ruined not only the ride but the handling of the car. A big problem with MX5's is that people put too stiff a suspension in them, that is the spring rate is too high and the dampers valved so that they are also too stiff to allow the car to work as it was designed to work. The suspension needs a bit of give to let the car lean a little to get the best out of it.

Easiest way to check spring rate is if you have removed a spring then you can put a nice heavy weight on it and measure how much the spring compresses.

If you are happy with the way the car handles and rides now then I wouldn't be too worried until it becomes time to replace the dampers and then you can look at getting it right, especially if the dollars are tight.

[Brewster]

So did you get the 17 x 7 rims or what?

[project.r.racing]

it doesn't sound like a spring issue to me. it sounds like someone that hasd never dríven in a car with coilovers.

best to swap them with someone that wants coilover so you can get the stock suspension rachael.

go smaller wheels with larger aspect ratio tyres if you want more comfort. but to be honest, tyres are only 10% of the picture. the springs and dampers is where you'll find 90% of comfort issues.