Tyre Size Variations

[Mr Pig]

Are all tyres of the same stated size actually the same size?

In bicycle tyres, the stated spec of the tyre is largely fiction. I'm beginning to wonder if car tyres vary too.

My Leon came with 205/55/R16 tyres. I've bought a set of FR alloys for it which have 225/45/R17 tyres. I had both sets of wheels out of the shed today and I noticed that the FR wheels/tyres are a good bit larger in diameter. Hard to measure exactly but it looks like about 20mm of a difference. The tread on the tyres is about the same depth.

According to the spec of tyres of those sizes there should only be a difference in diameter of 3mm, the 225/45/R17 being fractionally larger.

Anyone else ever noticed this? Tyres from different brands being wider or taller than they say they are? As I said, in mountain bike tyres such variation is the norm with Maxxis tyres typically being narrower than the claim to be for instance. If the difference is large enough it might be noticable.

 

[LouG]

Tyre rolling radius can vary between brands. Is it a problem? You could fit 215/45-17 tyres.

 

[Mr Pig]

Tyre rolling radius can vary between brands. Is it a problem?

I have no idea. I reckon the car will sit about 10mm higher and the wheels being larger will effect the gearing. Is it likely to be noticable? I don't know.

 

[CupraGeezer]

The difference in rolling radius between a new tyre and a worn tyre is about 5 mm. Whilst that's only half the figure you're talking about, I can't say I've ever noticed the difference in gearing. Even 10 mm on radius would only equate to about 3%; I think you'd have to be usually sensitive to notice that.

A 10 mm change in ride height may be more noticable though.

 

[Mr Pig]

A 10 mm change in ride height may be more noticeable though.

Like I said, I don't know. The FR is 15mm lower than the SE so clearly Seat think that yes, a small change in ride height matters.

It's kinda academic as I've bought the wheels anyway and they're going on the car. It was more that I wanted to find out if anyone else had noticed differences in tyre height like this.

The 16'' tyres are Goodyear and the 17'' ones are Maxxis. Good chance some of you have never heard of Maxxis but they are a brand of a very large Taiwanese manufacturer.

 

[EFF11]

This Youtube video is a comprehensive guide to fitting aftermarket wheels and tyres..You can watch the video and then go to the 1010tires.com website to use the calculators featured in the video.

Also,one of the documents in the link to Seat manuals that Paddy6789 posted is the Wheels,Tyres and Rims Manual. This has images of all Seat wheels with part numbers and dimensional data. It also has tables listing appropriate tyre sizes for each rim and for recommended tyre brands.

https://drive.google.com/drive/folders/1M4ofxxMgsFjE7hNWg6l4KuifelmdMUu0

 

[Mr Pig]

You can watch the video and then go to the 1010tires.com website to use the calculators featured in the video.

Thank you for the links. I've already looked at a similar tyre comparison chart which gave similar results to the 10-10 site. Both say there is a very small difference in the overall diameter of the tyres, 10-10 says 0.83%. My point is that what I'm seeing is a larger difference than that.

 

[LouG]

The difference in rolling radius between a new tyre and a worn tyre is about 5 mm. Whilst that's only half the figure you're talking about, I can't say I've ever noticed the difference in gearing. Even 10 mm on radius would only equate to about 3%; I think you'd have to be usually sensitive to notice that.

A 10 mm change in ride height may be more noticable though.

It's actually 5mm. Radius is half the circumference

 

[martin j.]

Tyre of the same size but made by different manufacturers can/do come in different rolling circumferences, possibly due to tolerances being used, there are charts to compare tyres sizes to ensure if you change wheel size all other operations are correct.
Radius is half the diameter btw.

 

[Mr Pig]

Pulled the two wheels into the shed and got a ruler out. Tread depth is almost identical on both and the difference in diameter is about ten of eleven millimeters, not the twenty I thought it looked like. Still more than the three it's supposed to be.

 

[jefalad]

Pulled the two wheels into the shed and got a ruler out. Tread depth is almost identical on both and the difference in diameter is about ten of eleven millimeters, not the twenty I thought it looked like. Still more than the three it's supposed to be.

Tyre diameters in data tables are typically the industry standard nominal diameters, however a tolerance is permitted. The diameter of a new 205/55R16 can be 625 to 639mm, and a new 245/45R17 can be 628 to 640mm. Clearly a difference of 10 to 11mm in diameter between these two tyre sizes when new is possible.

 

[Mr Pig]

Clearly a difference of 10 to 11mm in diameter between these two tyre sizes when new is possible.

That's great information. Thank you :0)