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My model of MX-5 is: __RF le
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I’m based near: __Blackpool
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I’m looking for technical help or recommendations on: __
Just upgraded from 205/45,s to 215,45,s
All seems good and the 215,s were cheaper
Went for ps 4,s again
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How do they compare for diameter, width and aesthetics ??
Slightly wider,seems to drive better but also look better
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Cheaper too, so win/win 
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If you go for a wider tyre and keep the ratio the same, the overall diameter of the wheel/tyre will increase. This is because the sidewall depth will go from 45% of 205mm to 45% of 215mm, so the overall diameter should increase by 8mm (215*.45-205*.45 x2), which means that the total circumference will increase by 25mm (8mm x pie, or 3.141), ie presuming you have a 17" wheel, from 715mm to 740mm, or by 3.5%. This would only be an issue if there is lack of clearance in the wheel arch or if the speedo uses the tyres/wheels to calculate the speed. I changed the tyres on an MG Metro many years ago and the speedo read incorrectly, so I had to change the pickup gear coming out of the gearbox. I have no idea how the MX5 calculates its speed, so hopefully should not be a problem.
Having done this I am sure that someone will say I am wrong, as that is how these threads work… hey ho!
You’re spot on pretty much yep. And 4% is nothing. I worked in a garage when studying to make a few quid. You could get two brands of tyre, both the same size and aspect ratio etc, and they could be out by more than that. Some had almost 20mm difference in diameter, but they should have been identical. It’s all a bit questionable to me.
A new tyre comes with 8mm tread unless it’s some track special. If you swap it at 2mm that’s 6mm less radius or 12mm diameter. That’s a 6% difference. Obviously that’s true of the larger sizes as well as smaller. The larger the diameter the smaller the resolution tho 
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215’s seem fine speedo wise and just remove some of the slop between the speed reading and sat- nav speed. The speedo still reads slightly lower so on the safe side.
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I think you have got confused somewhere down the line. Your percentage is way out, as are the diameters. Changes is less than 1.5% difference. The diameter theoretically goes up by 9mm on a new tyre, roughly 616 to 625. You will lose more than that in wear over the life of the tyre and far more depending on what pressures you run.
If you are talking about circumference then you are nearly at 2m not 715mm
It is a far simpler calculation to say what is the width difference, 10mm x 0.9 = 9mm
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Here be the maths, I’m bored
215 vs 205
(205*.452)+(1725.4) = 616mm
(215*.452)+(1725.4) = 625mm
625/616*100-100 = 1.46%
Worn vs new assuming new is as per aspect ratio
(205*.452)+(1725.4) = 616mm
takeaway 6mm of tread
(205*.452)+(1725.4) = 616mm - 6*2 = 604mm
616/604*100-100 = 1.98%
Worn 205 vs new 215!
625/604*100-100 = 3.48%
So a new 215 will be much larger than a used 205, A new 205 will be slightly larger than used 205 and ever so slightly larger than a used 215. I never known a speedo be less than 3% off, in my experience it’s more like 5-7% so a 215 will be fine and make your speedo more acurate. I have 215s on my MK3 and the speedo still under reads a tad.
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Don’t forget that if you have changed from the standard tyre you will need to tell your insurance company.
Just like everyone else does? and with changes of wheels, exhausts too? I know you shoudl be how many people do?
To add to this I went from 205 to 215 in the summer with Bridgestone 215/45R17 91W XL‘s. Better in every way, running on lowered Bilstein/Eibach mix. Look better, more sure footed and from the back a beefier look 
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Wow I have been educated I honestly never knew that, I thought if a profile was 45 then it was 45mm from wheel to road 
No kidding and I’m over 50 
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