Wheels/Tyres: Spacesaver size

[Ron_XC90]

Hi all,

I'm looking to buy a spacesaver wheel, but am confused about the specifications of the spacesaver tyre in the user manual. according to the manual the size of the spacesaver is T125/85R16 for an XC90 with 235/60/18 wheels.

When I stick this into the tyre calculator, it gives a much lower circumference than the 18" original wheels (see attached).
For example an S80 has a spacesaver size T125/80/17 which is still not right, but closer to the circ as the 18" originals.

Few questions:
- Can anyone check the size of their spacesaver (with 18" original wheels)
- Will an S80 wheel fit the XC90

Many thanks

Ron

[Jim314]

I just slid under my wife's 2007 XC90 and the compact spare is a T 155/85-18. This is what is given in the Owner's manual.

The four tyres on this XC90 are 235/65-17 which has a nominal circumference at the tread extremely close to 235/60-18.

The unloaded diameter of the XC90 tyres is close to 29 inches--29.10 inches for 235/60-18 and 2322 mm nominal unloaded circumference. The 235/65-17 tyres have an unloaded diameter of 29.03 inches and 2316 mm nominal unloaded circumference.

T155/85-18 tyres have a nominal unloaded diameter of 29.59 inches and nominal unloaded circumference of 2362 mm.

I wonder about the tyre calculator you are using. Could this calculator be giving the effective loaded circumference rather than the nominal unloaded circumference? That is, does it take into account the deformation of the tyre under load?

[Ron_XC90]

Thanks Jim!

So it looks like the right size for me to get is the T155/85-18, as it is pretty much similar circ. as the standard 18" wheels

I used this calculator (further down on page): http://www.carbibles.com/tyre_bible_pg4.html
It looks to me that it calculates unloaded circ.

thanks again!

[Jim314]

Originally Posted by Ron_XC90

Thanks Jim!

So it looks like the right size for me to get is the T155/85-18, as it is pretty much similar circ. as the standard 18" wheels

I used this calculator (further down on page): http://www.carbibles.com/tyre_bible_pg4.html
It looks to me that it calculates unloaded circ.

thanks again!

The values shown in the picture of the tyre calculator output for 234/60R18 are not the same values given by the widely used Miata tyre calculator (http://www.miata.net/garage/tirecalc.html), which gives the same results as manual calculation.

For 235/60R18 tyres the manual calculations are

diameter = (235 mm)(0.60)(2) + (18 in)(25.4 mm/in) = 739.2 mm

circumference = Pi x Diameter = 3.14159 x 739.2 mm = 2322 mm

[owenfackrell]

Originally Posted by Jim314

I just slid under my wife's 2007 XC90 and the compact spare is a T 155/85-18. This is what is given in the Owner's manual.

The four tyres on this XC90 are 235/65-17 which has a nominal circumference at the tread extremely close to 235/60-18.

The unloaded diameter of the XC90 tyres is close to 29 inches--29.10 inches for 235/60-18 and 2322 mm nominal unloaded circumference. The 235/65-17 tyres have an unloaded diameter of 29.03 inches and 2316 mm nominal unloaded circumference.

T155/85-18 tyres have a nominal unloaded diameter of 29.59 inches and nominal unloaded circumference of 2362 mm.

I wonder about the tyre calculator you are using. Could this calculator be giving the effective loaded circumference rather than the nominal unloaded circumference? That is, does it take into account the deformation of the tyre under load?

Mine is the same size as this and I have the 18" alloys.

[Jim314]

Ron, from your picture of the output of the 'carbibles' tyre calculator it appears that one must click on the 'calculate' button to actually perform the calculation for the tyre size entered. Possibly the results for a previous size remain until one clicks on the calculate button. If this is done, the carbible calculator should give the same results as the Miata calculator.

[Ron_XC90]

Thanks Jim/Owen, at least I now know which size to get, although finding one looks a different story...

[Jim314]

Originally Posted by Jim314

I just slid under my wife's 2007 XC90 and the compact spare is a T 155/85-18. This is what is given in the Owner's manual.

[snip]

T155/85-18 tyres have a nominal unloaded diameter of 29.59 inches and nominal unloaded circumference of 2362 mm.

I think the nominal diameter of T155/85-18 is 28.4 inch, and circumference 2264 mm.