Wheels & Tyres

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When choosing wheels and tyres for your 306 there's a lot of technical data you may feel you need. Some of it can seem daunting the first time you look at it but it really is pretty straight forward. So to help you understand things better we thought we'd give you a simple guide. It is confusing because wheel and tyre manufacturers mix metric and imperial measurements in almost all cases.

Wheel Dimensions

Rim Diameter

The obvious measure meant you're probably already aware of is the rim diameter. These are fairly easy to understand, simply it's the inner diameter of the tyre that fits the wheel. Typically this will be either 14, 15, 16 or 17 inches for the 306. It's almost always given in imperial size.

Rim Width (J)

The rim width gives a good pointer to how wide a tyre you're going to need. It also tells you how deep the rim is going to be into your wheel arch. Too wide and it won't all fit inside the arch. This is usually specified as a J fitting and it's typical to see it in the for 5J, 6J, 6.5J or 7J where the numeric part is once again in inches. So a 6J rim width indicates a width of 6 inches, 6.5J is 6.5 inches and so on.

On the 306 once you start hitting 7J the width can become an issue and rub the inner arches and the outer arch too. You may find you need to use an arch roller to "flare" your arches outwards a little, or at least fold the lip in.

Bore Size

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If you've ever fitted a wheel you'll notice it isn't the 4 bolts that actually hold the wheel in place, but a small ridge in the centre of the hub. This hub needs to be matched by the bore size of the rims you are fitting. Without ensuring the bore size matches the hub size you will be putting the load of the car onto the wheel bolts and this is incorrect. If you have a rim with the wrong bore size you can get an adapter called a "spigot ring", to fit that will act as a shim between rim and hub so the weight is correctly held on the centre of the hub.

Unfortunately if you have rims with a smaller bore size than your hub there is little you can do. However, if you have to use a spacer you could get a custom made spacer that acts as a very thick spigot ring.

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The Peugeot 306 has a bore size of 65.1mm

Offset (ET)

The offset is simply the distance between the inner face that mates with your hub to the centre of the rim width (J). This is probably the one that confuses most people. The higher the number the further away in millimeters the face is from the rim width centre. The confusion tends to happen because in order to maintain the manufacturers centre line for the rim you need to know the ET of the vehicle. In the case of the Peugeot 306 it has an offset of ET13-18. Between 13mm and 18mm. So if you have a rim that is ET28 you would need a 10mm spacer, to bring the offset down to ET18 (28 - 10 = 18) in order to align the centre of the rim with the manufacturers intended centre for the hub. Typically spacers are available in 5, 10, 15, 20mm thickness, so you should use the closest match or have a custom thickness made.

The reason this gets difficult is because wider rims can cause problems with rubbing on the inner arches, even if you have the correct spacer for your offset. So in order to compensate you may have to ad 5 or 10mm to your spacer to stop it from rubbing, but your offset will then be wrong. Although wrong in terms of manufacturers spec, you're not going to see much of an issue unless you go way too far.

Pitch Centre Diameter (PCD)

Every wheel has to be held onto the hub somehow. The PCD is a guide as to how many fixings (bolts/studs) it uses and the diameter to the centre of those fixings. This is given in the format of "PCD 4 x 108". Where the 4 represents the number of fixings and the the 108 tells us they are arranged in 108mm diameter circle around the hub centre. So PCD 5 x 100 means 5 fixings at 100mm diameter. So to make sure the rim you choose fits the hub on your car, you need to make sure it has the same PCD.
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The Peugeot 306 has a PCD 4 x 108


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Now you know how to choose the right size spacer, the next thing you need to think about is back to the bore size notes. You can't just pad the rim out by 10mm and not consider the centre bore - remember, it's the bore that takes the vehicle load, so it's important! This is where you'll need what are called "Hub centric Spacers". A hub centric spacer is simply a spacer with a centre lip so it meets the hub centre lip and then replicates it on the outer edge for the rims to connect to.

With a 5mm spacer there may be enough of  the hubs rim protruding to not need a hub centric spacer. But once you go beyond 5mm, you must use a hub centric spacer.

Thicker spacers means you'll need longer wheel bolts. Again once you go above 5mm, things change. After 5mm the standard bolts don't have enough turns on them to hold the wheels on safely. So if you put on 10mm spacers, please also use 10mm longer bolts. Similarly 15mm spacer, 15mm longer bolts.

At around 20mm you may then prefer to look at spacers that bolt onto the hub and then the rim bolts onto the spacer using the existing standard wheel bolts. I'm always wary of using bolts that are overly long for your wheels. So bolt on spacers offer a great replacement and the cost can be offset by not having to buy new bolts.

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Another reason for using bolt on spacers could be to convert the PCD to match your new rim. Let's say your new rims have a PCD of 4 x 100, these obviously won't fit your 306 which has a PCD of 4 x 108. Well using a bolt on spacer you can use the PCD 4 x 108 to bolt onto the hub, but have the wheel fixings onto the spacer as PCD 4 x 100, or even 5 x 100 to convert the amount of fixings.

As an alternative to solve the longer bolts issue you can convert from using bolts by fitting studs into the hub and then using nuts to hold the wheels onto the studs. The added benefit of this is that you'll also be able to get your wheels on easier. No messing around trying to hold the wheel on and aligning the holes to put the bolts through.

Tyre Dimensions

Tread Width / Profile / Aspect Radial Rim Diameter Load Index Speed Rating
195 / 55 R 15 85 V

Tread width - The width of the tyre tread that contacts the road surface in millimeters.

Profile / Aspect - The height of the sidewall of the tyre in relation to it's tread width. So a 55 profile tyre has a sidewall height of 55% of it's tread width - in this case 55% of 195mm

Radial - The construction of the tyre is made of radial plys of banding inside the rubber construction. You may also see a ZR marking here, which simply indicates a high speed tyre rated beyond 149mph.

Rim Diameter - The diameter in inches on the inside fitting of the tyre. This is the part to match against your rims.

Load Index - Many tyres you're probably interested in are designed for cars and are made to carry only a specific weight. You need to make sure that if you're fitting a tyre to a larger, heavier vehicle, a pickup or MPV, that the tyre can carry the additional load. You can check this at many tyre sites but in this example a rating of 85 gives us a maximum load rating of 515kgs.

Speed Rating - This indicates the maximum speed for which the tyre is designed for. In the example a V indicates a maximum speed of 149mph. Again many tyre sites have tables to cross reference this with.

Choosing Tyres

As a general rule of thumb you should keep the outside diameter of a tyre exactly the same as the original tyres fitted by the manufacturer (OEM = Original Equipment Manufacturer). Keeping the diameter the same ensures your speedo reads correctly and that your gearing remains constant with the new tyres. But when you fit a bigger rim fitting the same width tyre with the same profile will result in a bigger overall diameter.

Generally speaking you can achieve this with some simple changes. For each inch of rim diameter you increase by, you should decrease the profile by 5%. So as standard 195/55R15 tyre becomes a 195/50R16 on a 16" rim and 194/45R17 on a 17" rim.

Similarly for tread width. If you fit a tyre with a wider width tread then for each 10mm added in width you should decrease the profile by 5% too. So a 195/55R15 becomes a 205/50R15 on a tread width of 205mm.

Conversely, if you fit smaller rims or narrower tread increase the profile.

Typically the Peugeot 306 comes with 195/55R15 tyres on Cyclone alloys. This means that changing the alloys for wider, bigger gives you the following choices.

16" Rims 195/50R16, 205/45R16
17" Rims 195/45R17, 205/40R17

A handy calculator for working out the differences in tyre widths, profiles and diameters can be found here: http://www.miata.net/garage/tirecalc.html

Further more detailed reference: http://www.carbibles.com/tyre_bible.html