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gazman

Member Since: 17 Aug 2015
Location: Liverpool
Posts: 652

using a smaller sized diameter and width wheel as a spare.

it's only for a few days until big dave can get me my new spare. would it shagging the duffs etc if I blew a tyre and had to put on a smaller spare.
The tyres on the landy are 33/12.5/15 and the spare I have is a 235/7016 2014 - current ..... 2003 defender td5 90 (my car)
2009 - current .... 2005 zx10r

22nd Apr 2016 2:12pm

Martin
Site Admin

Member Since: 02 Apr 2007
Location: Hook Norton
Posts: 6605

2015 Defender 90 Puma 2.2 XS CSW Montalcino Red

Doing the calcs, 33x12.5x15 is the equivalent of 318/72R15 in metric

Doing a comparison with 235/70R16 (Discovery size?) gives:

Specification Sidewall Radius Diameter Circumference Revs/km Difference
318/72-15 229mm 419mm 839mm 2636mm 379 0.0%
235/70-16 164mm 368mm 735mm 2310mm 433 -12.3%

12% is way, way, way too great a difference for the spare to be OK with your normal tyres

1988 90 Td5 NAS soft top

2015 D90 XS SW

22nd Apr 2016 4:39pm

gazman

Member Since: 17 Aug 2015
Location: Liverpool
Posts: 652

thanks.
I won't carry anything and just call a breakdown service if something goes wrong. 2014 - current ..... 2003 defender td5 90 (my car)
2009 - current .... 2005 zx10r

22nd Apr 2016 5:31pm

Christian1402

Member Since: 17 Feb 2016
Location: Cumbria
Posts: 214

2009 Defender 110 Puma 2.4 XS CSW Indus Silver

Martin,

Sorry for hijacking but what about a 285/75/16 using a 265/75/16 as a spare? It's basically what I've got and I was hoping I didn't have to buy another tyre. There can't be much difference, half an inch in diameter?

5th May 2016 10:01am

pjb

Member Since: 08 Apr 2009
Location: Sunny Oxford
Posts: 1244

265 & 285 are just different widths that is not a problem as a temporary spare its only different diameters that cause a problem

Yours are both 16" spares with same 75 profile ie tyre height

At least that is what I thought but I'm wrong - see below

2020 P300 HSE

Last edited by pjb on 5th May 2016 10:50am. Edited 1 time in total

5th May 2016 10:38am

Martin
Site Admin

Member Since: 02 Apr 2007
Location: Hook Norton
Posts: 6605

No, the tyre height (sidewall size) is different. The "75" means "75% of the width".

A 265 tyre has sidewalls of 199mm
A 285 tyre has sidewalls of 214mm

Specification Sidewall Radius Diameter Circumference Revs/km Difference
265/75-16 199mm 402mm 804mm 2526mm 396 0.0%
285/75-16 214mm 417mm 834mm 2620mm 382 3.7%

30mm difference in diameter, nearly 100mm difference in circumference (3.7%)

1988 90 Td5 NAS soft top

2015 D90 XS SW

5th May 2016 10:45am

pjb

Member Since: 08 Apr 2009
Location: Sunny Oxford
Posts: 1244

oops

I've been wrong for so many years

2020 P300 HSE

Last edited by pjb on 5th May 2016 10:48am. Edited 1 time in total

5th May 2016 10:47am

Birdy

Member Since: 07 Oct 2011
Location: Côte d'Azur
Posts: 865

2011 Defender 110 Puma 2.4 XS CSW Santorini Black

Not quite true, in the sense that the "75" is a percentage of the tyre's with, so in this case the overall diameter will be about an inch less. Whether or not that's of any significance I'll leave to the experts.

Peter

Edited to add: Martin beat me to it!

5th May 2016 10:48am

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