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i currently have the 205/45 r17 from the NC mx5. Now i want to widen its track a bit to 215 but not sure whether to go 45 or 40 r17.

using the miata tyre calculator:

Specification Sidewall Radius Diameter Circumference Revs/km Difference
205/45-17 92mm 308mm 616mm 1936mm 516 0.0%
215/40-17 86mm 302mm 604mm 1897mm 527 -2.0%
215/45-17 97mm 313mm 625mm 1964mm 509 1.5%

So as you can see, at 100kph on speedo:
40 tire would actually be 98kph
45 tire would actually be 101.5kph.

which would be more suitable?
would ride compliance (e.g. comfort?) be affected if you go 40 vs 45?

also, any tires anyone could recommend around 200 mark? i got quoted toyo proxes 4 @ 180 each fitted or kumho ast @160. but they seem not so flash tyres.

go the 45's & gain the 1.5% in diameter.
on my NA my wheels are up 6% on original & according to my GPS i'm doing 101km/h or 102km/h when the speedo says 100km/h.

I have just fitted 215 45 x 17 tyres to my NC and can give you the following information.

Speedo error is negligible.

Road Ride Comfort is sensational

Track work disappointing at normal pressures.

I have been experimenting and have found that 36 all roand still gives a nice ride and good handling but for the track I have been running at 40 psi. I am still experimenting with pressures and will post more infor as it comes to hand.

I have just fitted 215 45 x 17 tyres to my NC and can give you the following information.

Speedo error is negligible.

Road Ride Comfort is sensational

Track work disappointing at normal pressures.

I have been experimenting and have found that 36 all roand still gives a nice ride and good handling but for the track I have been running at 40 psi. I am still experimenting with pressures and will post more infor as it comes to hand.

cool!

i just booked mine in for 2moro for 215/45/17.

yoko a.drive.r1's for $190 per corner/fitted/balanced.

what tyres did you get?

They are a premium Goodyear product, and perform brilliantly.