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Different tyres diameters: what's the limit?

2K views 6 replies 4 participants last post by  Rand 
#1 ·
Last week whilst away on holiday one of my 225/60/17 V rated tyres picked up large construction screw and suffered a non repairable puncture. As luck would have it it was on a Sunday! Fortunately I have fitted the car with a full size spare and carried out five wheel rotation. So the spare was fitted to the road wheel (rear off side). I then had to purchase a new tyre for use as a spare. Of course Subaru's OEM size isn't very common in UK, but I managed to source an in stock Hankook of the correct size.

Now if I should have a puncture in the future I will have road tyres with current tread depth of 4.5mm and a spare with a tread depth of 7.5mm. What are the specifications for mismatch in tread depth for the 2014 Forester? Given that I might have to use the spare in an emergency what would be the distance and speed limitations to prevent damage to the drive AWD system? I normally change road tyres when the tread has worn to 3mm, so the maximum mismatch would be 3mm road tyres to 7.5mm spare tyre.

I am glad that I took the precaution of fitting a full sized spare otherwise I may have been facing the replacement of four tyres, of which there would have been a 24 hour delivery wait.

Oh and now the damn Hankook doesn't allow the load area floor to sit flat. And I really don't want to deflate it just in case it shouldn't re-flate when required.
 
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#2 ·
Well, your 5 tire/tyre method has paid off; now you need a little luck with the remaining 4. Assuming that the Hankook spare is a different make from the other 4, tread depth alone may not be a good comparison for overall diameter. Ideally, all tire CIRCUMFERENCES would be within ¼", (sorry for not doing the conversion ). For me, that translates roughly to a max tread depth difference of about 3/32". Converting that, it looks like your 4.5mm max difference would exceed that guideline.

As a start, you could compare the actual circumference of your new spare with one of the other 4, then do the math for tread depth. For my similar situation, I used a tailors tape with the tire off the car and ignored the nit-picking "rolling circumference" comparison.

In any event, that spare should be OK for a short drive to get a replacement. Cheers, Jon.
 
#3 ·
Jon
Thanks that's useful info. I knew the awd was sensitive to tyre diameter, but it's a deal more so than my previous awd cars KIA, Discovery, Mitsubishi and Volvo. I've had two punctures in the last 2 years, more so than in the last 25 years. So hopefully the laws of probability will be on my side until I am down to 3 mm
 
#4 ·
The 1/4 inch difference hasn't been quoted by Subaru since they started using cost-saver spares, so I wouldn't take much notice of it. The circumference of the latter differs by several inches from the standard wheel.

In any case, on our twisty UK roads, the differentials are working all the time to accommodate wheels rotating at different speeds.

I bought my full-size spare tyre from a German supplier, as the same size/type Geolanders aren't available in the UK.
 
#5 ·
The 1/4 inch difference hasn't been quoted by Subaru since they started using cost-saver spares, so I wouldn't take much notice of it. The circumference of the latter differs by several inches from the standard wheel...
However, that "donut" spare is restricted to the rear axles, and, on some Foresters, a fuse must be installed to defeat the AWD.

Although the ¼" may no longer be hard and fast, I understand that the 4 on the ground need to be "matched", else damage to differentials is a higher risk.

Subaru surely does need to be a bit more explicit about just how much difference in tire size can be safely tolerated.
 
#6 ·
I have no problem buying the OEM Yokohama G91 tyres although they are not held in stock by any of my local tyre dealers, even the Yokohama specialist. So there's always a 24 order period. But my main gripe is that 225/60/17 V99 is not a common size so alternatives are less, even going for H rated and 103 Load Index doesn't open up the choice significantly. Still hopefully no more punctures and I can get back to five wheel rotation for Winter 2017.
 
#7 ·
.25" is circumference

3/32" tread depth converts to 2.38mm so they will probably round it off to 2.5mm?

Haven't seen metric spec owners manual.

This is of course a joke as they give us in north america... compact spares that are massively different.
 
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