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Correct Tyre Size?


ShinyAndy
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I have a 2008 Focus 1.8TDCI Titanium which has 17" wheels, according to the "ficha tecnica" which is an official document similar to the logbook in the UK it says that the car should have 205/50/17 tyres. It has a most distressing collection of cheapy TDFs fitted at the moment but they are all 225/45/17 and I am keen to fit some decent tyres (probably Goodyear Eagle F1 Asymmetric 2 - good enough for the ST then must be good enough for a Satan's fuel powered shopping car) but want to make sure I get the right size.

It appears that the 225/45/17 is a much popular tyre and so cheaper and probably why it has them on. I know it might knock the speedo out a bit but wondered what should really be fitted if anyone knows?

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A quick check on a tyre size comparison like http://www.etyres.co.uk/tyre-size-calculator.htm shows that either size is OK

Tyre diameter is a function of both width and profile, so of you choose a wider tyre you have to match it with a lower profile to offset - which has been done correctly here.

You'll notice that the 205 size has a 50 profile, while the wider 225 size has a lower 45 profile to compensate.

Although the 225 is wider, the actual tyre diameter is just 3mm different - which is a lot less than the difference between a brand new tyre and a part-worn tyre!

The 205 has a diameter of 637mm while the 225 has a diameter of 634mm which is just 0.47%. So the speedo will be almost exactly the same whichever tyre you choose.

So the choice is yours.

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I was more worried about what tyre was actually supposed to be fitted as OEM as you can fail the MOT test here if things don't match your paperwork. I assume the wheels are 6.5x7 so is fitting a 225 wide tyre across the rim pushing things a bit?

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  • 2 weeks later...

Fitted some Eagle F1 Asymmetric 2 (in 225/45/17 as couldn't get them in 205/50/17!) last week. My God, what a difference.. that's 600€ well spent indeed :D

No more horrible road noise, no more crashing over potholes, no more constant traction control light blinking, no more understeering off on a roundabout at 10mph (the roads here are something to behold, more marble than tarmac in an attempt to stop the roads melting). I'm actually looking forward to some rain so I can see how they perform in that!

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