Stupid wheel / tyre width question....

[Jakg]

Sorry in advance for the stupid question.

My car has 225/45/18 tyres & 7.5J wheels

This means the tyres are 22.5cm wide, and the wheels are 19.05cm wide (7.5")

Why are the tyres so much wider than the wheels?

I'm trying to work out what other wheels will fit my car (i.e. looking at how much space I have, and what I could fit with a different offset / wheel width / PCD adapter) without rubbing, but this nugget of info had me confused...

[Caulkhead]

So the rubber hits the kerb before the rim does.

[cuprabob]

The other thing you have to consider regarding space for wider wheels is the offset of the wheel.

It's normal for the tyre to be wider than the rim unless you go for the stretched look.

[Jakg]

Caulkhead said:

So the rubber hits the kerb before the rim does.

I've seen tyres with rim protectors a few mm thick, but my tyres don't seem over 3cm wider than my wheels!

cuprabob said:

The other thing you have to consider regarding space for wider wheels is the offset of the wheel.

I'm trying to work out (without having to buy the wheels first!) if it'd be possible to fit my "dream wheels" - but they are wider, with a different offset, and require adapters. I just need to make sure all my sums add up...

[cuprabob]

Play with a wheel calculator like this one

http://www.1010tires.com/WheelOffsetCalculator.asp

[jagracer]

Jakg said:

Why are the tyres so much wider than the wheels?

The measurement for the width of the tyres will often go from quite a way down the side walls where the tread finishes.

[Jakg]

jagracer said:

The measurement for the width of the tyres will often go from quite a way down the side walls where the tread finishes.

The other thing I was wondering is where the 225 measurement actually comes from - is that the tread, the contact patch, the widest point of the tyre?

[v8will]

Rim measurements aren't made edge to edge as such but at the bead seat, a 7.5" rim will be actually close to 8.5" in reality.

Have a read here, http://www.comp.co.uk/info/technicalinfo.asp

[cuprabob]

Jakg said:

The other thing I was wondering is where the 225 measurement actually comes from - is that the tread, the contact patch, the widest point of the tyre?

225mm is the width of the tread.

45 is the height of the sidewall expressed as a percentage of the tread width, in your case 101.25mm

[JR]

v8will said:

Rim measurements aren't made edge to edge as such but at the bead seat, a 7.5" rim will be actually close to 8.5" in reality.

Have a read here, http://www.comp.co.uk/info/technicalinfo.asp

This is the answer to the OP's question. If the manufacturer had increased the width of the rim the next size up would be 8" or 9" overall and that's 228/229mm which is wider than the tyre, nominally; manufacturers don't like to cut it so fine that the make and model of tyre determines whether the tyre or wheel is wider.

[Si_steve]

I have 235's on a 7J rim for the Saab...On my astra I have 205's on 7J. The Saab needs the wider profile

[Frances The Mute]

There's some misleading information in this thread.

The nominal section width is between the widest point of the sidewalls - not the tread area or from shoulder to shoulder etc. This measurement will also differ dependent on wheel rim width and inflation pressure.

A 225/45R18 tyre is operable on rims between 7J and 9J - giving an actual measurement of 240mm to 255mm. The recommended size is 7.5J so you're fine there.

Depending on what you're looking to achieve, you need to start measuring (or experimenting) to get the right fitment for what you want. Good luck with that...

[GC8]

Remember too that three different manufacturers '225' tyres will be three different sizes. Theyre a nominal size, rather than an exact measurement.

[Studio117]

A manufacturer will set a certain overall diameter for the tyre where the speedometer will be accurate.

eg the standard tyre might be 195/55/16 and the sporty model maybe 215/45/17

The overall diameter is very similar, so the speedo will still be accurate. So If you want to go wider or narrower than the standard size, adjust the aspect ratio to suit. The tyre size calculators posted are very helpful.

Say your offset is et45, a wheel with et42 will fill the arch a bit more.

Edited by Studio117 on Wednesday 26th September 06:49

[Captain Muppet]

Si_steve said:

I have 235's on a 7J rim for the Saab...On my astra I have 205's on 7J. The Saab needs the wider profile

The "profile" is the number after the width in the tyre designation.

[Marf]

Check the manufacturers specs for the tyre you're using. They will list what wheel widths are suitable for what tyre width, giving a max, ideal and min wheel width.