What you need to know about wheels and tyres

Tyres

The GTi-6 and Rallye models were supplied with 7" x 15" Cyclone alloys, and 195/55 15 Pirelli P6000 tyres. The Pirelli's are very good on a dry surface but are not so impressive in wet conditions. Changing tyres is a relatively painless experience, but always ensure that new tyres are the same dimensions as those being replaced, and do not mix new and worn tyres.

The dimension information can be found on the sidewall of the tyre in the format P195/55R15 85

• P: Indicates passenger tyre.
• 195: Width of the tyre in millimetres.
• 55: Aspect ratio. An aspect ratio of 55 indicates a tyre is 55% as tall as it is wide. High performance tyres generally have aspect ratios less than 60 which results in a comparatively wide tyre with short sidewalls.
• R: Construction. R indicates radial construction.
• 15: Diameter of the wheel in inches.
• 85: Load Index Number used to indicate the weight capacity of the tyre at its maximum speed.

Yokohama, Toyo , Bridgestone and Goodyear all make excellent tryes for dry and wet conditions. As the GTi-6 tends to get through front tyres much faster than rears, it is not necessary to have the same quality of tyre on the rears, but avoid poor-quality tyres as they will exaggerate the risk of lift-off oversteer.

Wheels, on the other hand are a more complicated matter ...

Wheels

When replacing wheels, the two most important factors are size and offset.

Size

Smaller wheels tend to use high-sided tyres, which results in lateral instability as the rubber 'flexes' during cornering. The way around this, and to provide more cornering stability is to increase the size of the wheel and decrease the size of the rubber sidewall.

As a general rule, and to retain the car's original handling qualities, it is not recommended to increase the size of the wheel by more than 2 inches. In the case of the GTi-6, this brings the wheel size up to 17"

Fitting a wheel wider than 7" tends to cause the tyres to rub against the wheel arches on full-lock. This heartbreaking noise is called scrubbage, and for this reason it is best to stick to 7" wide even when fitting 18's.

There has always been a great deal of debate about changing wheel sizes, and the difference it makes to acceleration and top speed. To clear this up, see the table below.

Wheel Size    Tyre Size   Sidewall Height   Total Height   Circumference   Increase   True speed
15" (381mm)   195/55 15   107.25   595.50   1870    0.0%   60.00
15" (381mm)   195/50 15   97.50   576.00   1809    -3.39%   57.97
16" (406mm)   195/45 16   87.75   581.90   1828    -2.34%   58.60
17" (431mm)   205/40 17   82.00   595.80   1871    -0.05%   60.03
18" (457mm)   205/35 18   71.75   600.70   1887    0.87%   60.52

Now we've established the size issue, you have to decide on a style. Different manufacturers and different styles make a multitude of styles to choose from. The wheel simulator contains a handful of wheel types to help the process. The next concerning factor in choosing wheels is offset (see below), so ensure that you're choice is available in the right offset.

Offset

The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types.

Zero Offset

The hub mounting surface is even with the centerline of the wheel.

Positive Offset

The hub mounting surface is toward the outside of the wheel. If the offset is too high, the wheel will rub on the inside wheelarch liner or the strut. Positive offset wheels are generally found on front-wheel drive cars and newer rear-wheel drive cars.

Negative Offset

The hub mounting surface is toward the inside of the wheels centerline. If the offset is too low, the wheel will stick out from the wheelarch. "Deep dish" wheels are typically a negative offset.

If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. The offset required for the 306 is 15 - 18mm, and it comes very highly recommended that this is adhered to.