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What tyres are the right type for an 1100?
When Morris 1100s began rolling off the assembly lines in Sydney they did so on 12 inch cross-ply tyres. Olympic contoured Air-Ride C64s, to be exact. But a decade later, cross-ply tyres were out and steel belted radial tyres were in - and they've been "in" ever since.
To help define and differentiate the increasing number of tyres available, a naming convention of letters and numbers was adopted across the industry. It begins with the nominal tyre width in millimetres, then usually the letter 'R' to denote Radial, followed by the rim diameter in in inches.
So a radial tyre of equivalent size to the Olympic Air-Ride original would be denoted 155R12.
When low-profile tyres were introduced, the convention was extended by way of appending the aspect ratio to the end of the width.
The aspect ratio is the percentage of the tyre's height to the tread width - so 155/75R12 means the tyre height is 75% of the tread width. By the way, an aspect ratio of 80 is considered to be "standard" profile and as such the aspect ratio code is sometimes left out. For example: 155R12 is equivalent to 155/80R12 but modern convention is that car tyres must include the aspect ratio, commercial vehicle tyres (with thicker sidewalls) do not and are usually marked as "Light Truck Tyres".
Today there is a wide variety of radial tyres and the trend is toward large rims and "low-profile" tyres.
So where does this leave the 1100?
As the population of 70s Honda Civics, Ford Escorts, Datsun 1200s and 80s Daihatsu Charades & Suzukis has dwindled, so too has the demand for 12 inch tyres.
There are a couple of brands of 155R12 that continue to be available in Australia for the Light Truck market and they include GT Radial and Sime.