UKC

After a puncture, attempt to use one of the goo repair things and then a 5+ hour wait for breakdown recovery (who didn't recover me) I am going to buy a spare wheel (probably a space saver type).

It may seem an obvious question to car people but.. how do I know what size to buy. 15 or 16 inch seem to be the options. We will avoid the question of why we are still using inches and jump straight to  what on my current wheels am I measuring to check or will my tyre markings tell me?

Wheels are a bit more complicated than tyres. The tyre markings will give you the wheel size, but you'll need a specific wheel geometry to make sure it fits to your hub. Details should be in the owners manual or you could look up your vehicle online.

Some cars now have brakes which don't fit under space saver wheels (the narrow tyres steel things most cars use as spares). The downside of going for a full size one is that many current cars don't have space to put any wheel, let alone a space saver, which means that a spare in the boot will take up a lot of space. Space savers are also speed limited which may make the full size spare the more desirable option. 

Rims are always (well, almost always) measured in inches, for both diameter and width. Tyre widths are in mm, and the aspect ratio is a ratio so is what it is. However, if you're going full size then aim for a matching wheel for what you've got, i.e if you have a 195/50/16 then get another wheel and tyre that matches this (including the rim width which should be stamped behind one of the spokes). The advantage of this is you can carry on driving your car as normal until you get the original wheel and tyre sorted. Best bet is to get another wheel from a car like yours rather than any old wheel to ensure you don't have any issues with PCD or bore differences.

If you're planning to get a space saver then try to get one from another version of your car. If you can't get an OEM one then use https://tiresize.com/tyre-size-calculator/ to find a space saver tyre size which is within 1-2% of your existing wheel/tyre. You can go for a space saver in either 15" or 16", it really won't make a difference other than the brake clearance issue I mentioned (and assuming the PCD/bore fit). If your car is AWD then the safe advise is that it will need to be <1% diff to minimise the risk of any transfer box issues.

The best bet is to post what model/year your car is and I'm sure we can give some very specific advice as to what will and won't work!

Tyre markings will tell you the rim size. Its the number after the letter R on the tyre markings. It'll be something like  xxx/xxR16 which would mean a 16 inch rim. But you also need to know the number of studs and the bolt circle diameter as a minimum. Probably best to go to the manufacturer of your car for the correct one. Or maybe get a full size one from a scrap yard as another option. 

There will almost certainly be a specific spare wheel/tyre combination for your make and model of car which will have been type approved for use as a spare on that vehicle and which will fit in the well in the boot.

Finding out what it is might not be 100% straightforward, though. Best bet is often to find an active online forum specifically for your make and model of car (rather than a random climbing forum, ahem) and ask on there.

Just knowing the rim size of your existing wheels is not enough, because a space-saver tyre will likely have a much higher profile (and therefore a bigger sidewall) than the low profile tyres which are standard fit on most cars these days. As well as making sure you have the correct stud pattern and enough brake clearance, you'd need to calculate the total diameter of the wheel/tyre combo, if you see what I mean?

I did a bit of research into this when I got my current car, although I gave up and didn't actually get a spare.

The important bit is the bolt circle diameter and spacing. If this isn't correct the wheel won't fit the car. There are online tools to find out what you have on your car and what other wheels will fit.

You should aim to get the outer diameter as close to the regular wheel size as possible, but it's actually not that big a deal if it's a bit different. On a spacesaver you are only going to be doing under 50 for a short distance anyway.

Rim size, width and profile will all be very different to your regular wheels, so don't worry about this. It's only the bolt spacing and outer diameter that matter. You can calculate outer diameter: <Rim size (in mm)> + (2 * <width> * <profile> / 100 ). So my 205/55R16 wheels will have a diameter of (16 * 25.4) + (2 * 205 * 55 / 100 ) = 632mm. A 125/80/R17 is the same diameter, but there are many sizes that will work.

What car is it?

As has been covered, there are lots of considerations, some you can research like the hub/stud pattern, or calculate like the rolling radius, others you pretty much can't like the caliper clearance, that's by trial and error unless you can find the original fitment.

Where are you going to stow it?

jk

mostly covered by folk above, but sites like mytyres will spec the wheel for you if you put in the vehicle reg.  I've a full set of spare steel rims with winter tyres that I got from them years ago an no problems with fitment.

As an aside it's definitely a gripe of mine that modern cars don't seem to have space for a spare wheel and rely on the foam / gel sealant instead!

Cheers,

Stu

I have only got tyre seal kit too, despite there being space for a spare wheel. Thought about looking for a spare but would need a jack too. 

It's a  Vauxhall Corsa 13 plate. 1.4 Sxi  

I can't imagine it has any noteworthy modification

I can get a jack for free no problems but would need to get a wrench (or whatever they are called).

It's so annoying... I'm good at changing a wheel, could have been o the road in 5 mins but no... 5 hours later!

There is still the space and everything for a spare in the boot, I think it would fit a full spare but it is a good space for jump leads, spare dog towels, smuggled drugs etc

In addition to the bolt pattern and spacing, you also need to know the offset - the amount by which the rim sticks out from the centre. If the offset is wrong the wheel will either foul the arches, suspension, or brakes.

What tyre numbers mean:-

e.g. 205/60/R15

1st number (205) is the width of the tread in mm. 2nd number (60) is the height of the sidewall expressed as a percentage of the width - in this case 60% of 205 - 12.3mm. 60 is considered standard, anything smaller is 'low profile', proper offroaders will have 70 profile tyres. The 'R number' is the diameter of the wheel in inches (16).

Often, two widths of tyre will fit on the same width of rim, the narrower tyre will result in the modern boy racer look where the rims stick out further than the tyre but get kerbed to frock.

Put '2013 corsa spare wheel' into google, Ebay ad shows an OEM wheel tyre and changing kit advertised for ~£45.

jk

Or just as useless, cars that do have a space to carry a spacesaver (what I believe the Americans call a donut) but are mounted underneath the boot not inside the car, like my Zafira but many other cars do it too.  Alternatively you can see that design flaw as a blessing not a curse and "borrow" someone else's one night. Mine lasted 8 months, which is 7 months longer than I expected before it vanished. Vauxhall can sell you an anti-theft device, which consists of an easily cut nylon webbing strap.

Does offset actually matter for a space saver? I've never heard it mentioned as a requirement and they are so much narrower that they probably clear everything.

I think @jkarran found the OEM spare kit for you.

At £45 it sounds very likely to be a space saver rather than a full-size spare wheel+tyre.  If that's the case then the existing wheel well in the boot is very likely to be too small to take a full-size spare securely so if you wanted to go down that route then you'd need to think very hard about how it was going to be secured. You really don't want a full-size wheel and tyre coming loose and flying around inside the car in the event of an accident.

You could always try jacking up one corner, removing the existing wheel+tyre and trying it for fit/security in the boot well. That would also give you an opportunity to read the full specs off the wheel, including offset etc, as these are usually stamped somewhere on the inner side of the wheel.

Or just buy the OEM kit.

Thanks, pretty sure I will go with @jkarran's find and hope for the best. 

I've no idea, I just felt it important that people considered offset when thinking about wheels as no-one else had mentioned it