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I don't understand tires very well, so I'm hoping you guys can double check this.

I talked to the tire shop today about getting new winter tires put on. I have a 2009 Ford Focus SES with 215/45R17 Bridgestone Blizzak tires that are worn out. The tire shop wants to replace them over to 205/50R17 GTRadial Champiro tires, saying that they're a cheaper and better option for winter.

Is there anything wrong with doing this? I tried looking at some tire calculators online (this and this) and it seems like they would fit and don't deviate too much from what I have, but I'd rather double check before I go ahead with it.

From what I understand, is it correct in saying that the new tyres would be thinner but taller? Or am I reading this wrong?

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Do you remember which GT Radial Champiro they're recommending? There are a bunch of them. That said, Blizzaks are a very well respected snow tyre (at least over here in the US) that performs very well, whereas GT Radial appears to be one of those brands nobody's ever heard of. –  Timo Geusch Oct 11 '12 at 14:28
    
He wasn't clear. From what their website tire recommendation engine through out, I'm assuming it's a WT+AX, but I'll double check on Friday. –  ECRD Oct 11 '12 at 20:07

2 Answers 2

up vote 6 down vote accepted

The new tyres would indeed be narrower and slightly taller - the first number is the width in mm, so the second tyre is 10mm (3/8") narrower than the first, and the second is the height as a percentage of the width, which works out at roughly 96mm and 103mm respectively. Just to mix things up even more, the third number is the wheel diameter in inches...

With the new tyres your speedometer would be slightly out as the new tyres would have a 2% larger circumference (so for each revolution of the wheel you've gone 40mm further than you would have done with the old tyre, but that is a pretty negligable amount, not much more than the difference between a totally worn out tyre and a brand new one...

Many manufacturers suggest a range of tyre sizes that will fit as well as the recommended size, but generally you can go a couple of sizes either side of the recommended one and still fit happily on the wheels. Certainly one size out won't cause any problems.

The one key thing to check is your insurance - many companies (at least in the UK) want to be notified about changes to tyre type, size or speed rating, although I doubt they'd load your premium.

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That's exactly what I needed to know. Thanks mate. –  ECRD Oct 11 '12 at 14:18
    
+1 for the insurance mention - if you forget and your tyres are not the correct spec then they can refuse cover in the event of an accident... –  Mauro Oct 15 '12 at 13:16

Following-up on Nick's answer, the recommendation from Tire Rack is actually 205/45-17. That size would have the taller, thinner profile but with a smaller outer diameter. That smaller number after the slash is the aspect ratio, indicating that this size would be slightly wider than it is tall.

So, no, I don't think there's anything necessarily wrong with their particular recommendation of tire brand. I don't agree with their sizing however, preferring to stick with either the original or the one recommended by Tire Rack.

Check your owners manual for additional information.

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