Wheels/Tyres: 20" rims, tyre size?

[Stu V8 2007]

I'm a bit confused. On my OZ 20" wheels, I have fitted 275/40 tyres, but others with XC90s and 20" rims I have seen have 255/45. If the rims are the same, shouldn't we have the same size tyres? Can someone please clarify this for me? Doesn't the differing size on the same rims mean a different rolling circumference?

(car is a 2008 V8)

[SwissXC90]

275 is bigger than 245 and diameter and circumference is bigger. Speedo accuracy will be wrong.

[Odysseus]

This website might be useful. https://www.tyresizecalculator.com/

[SwissXC90]

And radius is bigger too. The car will also be higher and have more ground clearance

Baasicallzy, they are the wrong tyres for the car.

[Stu V8 2007]

Thanks I appreciate everyone's help.

OK sorry but struggling here.

Mine are 275 (not 255 like yours), but isn't that simply dependent on the width of the rim, ie maybe my OZ wheels are possibly wider, and the width doesnt affect the circumference? What am I missing.

On the profile, mine are smaller, ie 40, as opposed to 45, but isn't that figure just a percentage of the width, so my wider tyre would hve a smaller (40) profile proportion yet give the same overal circumference?

As you can see I'm confused so please be patient. I did look at the tyre calculator but couldn't work out what I wanted to find out. It doesn't ask you the rim width.

This size tyre was on the car when I purchased it second hand, I when I replaced the tyres I would obviously ask for the same size. The tyre fitters didn't raise this, and the car went to Volvo for laser alighment and they didn't mention anything.

But now you have me worried!

[SwissXC90]

Comparision
https://www.tacomaworld.com/tirecalc...5r20-275-40r20

Your 275s are wider than the standard 255s.
You may, or may not, run into width issues inside the wheel wells, especially when at full steering lock, or full suspension travel, or both.

/40 is smaller than /45, but by very little.
See the comparison for the amount.

[XJSDriver]

As I understand it the diameter/rolling circumference needs to be the same as designed for the vehicle i.e. if you change this then gear ratios and speedo will be different to the design for the car and creates all sorts of issues.

The width of the tyre e.g. 275 vs 255 does not affect rolling circumference, but will of course affect suspension geometry and rolling resistance but is often done but must be within permitted tolerance for the wheel and any good tyre fitter will be able to tell you what width size you can fit for a particular wheel.

So as pointed out, when you change either rim size 18",19", 20" etc. or tyre depth you need to change both to keep same rolling circumference e.g. if you decrease wheel diameter you need to increase tyre wall depth or visa versa - This linked given by Odysseus https://www.tyresizecalculator.com/ gives a great explanation with diagram of this.

If you purchase a space saver for the XC90 it may well be different wheel diameter to your car but the tyre depth will ensure same rolling circumference so is Ok for all XC90s! Weird but true.

[Stu V8 2007]

Really appreciate the help you guys are offering, and I'll look at the links.

My alternative, as the rims are in a poor condition, is to replace the rims and tyres and strt again with the correct configuration.

I am therefore looking at a set of wheels which say:
20" 8.5J, does that sound ok?

And then fit 255/45/20 tyres to them?

It's a V8, so don't know if that matters.

Cheers.

[SwissXC90]

Tyoe of engine makes no difference

I have stock 20" rims from Volvo, mine are

CRATUS 20x8 on Pirelli Scorpion Zeros 255/45

[TruckbusUK]

Wheel with (J value) also plays a part of the decision so does the offset value (ET) of the wheel, as you don't want to move much from the original scrub radius value if at all (wheel width and offset can easily upset this.)

I'm sure the XC90 has negative scrub radius ... most McPherson strut and ABS equipped cars have, also the aspect ratio of your tyre is the ratio of the sidewall to the tyre width, so it important to understand the impact of what your proposing to the rolling circumference.

Find a good calculator and use it for all your wheel values so you see what the outcome is ... I know its another link, but worth a look ....

https://www.wheel-size.com/

Really hope it helps your dilemma.