It describes the key dimensions and construction: 145 is the tyre width in millimetres, 70 is the sidewall height as a percentage of the width, R means radial construction, and 12 is the wheel (rim) diameter in inches. Keeping to these dimensions helps maintain correct clearance, handling balance and speedometer accuracy.
Match the load index and speed rating shown on the tyre sidewall already fitted, or the vehicle handbook/door placard specification. When buying, you can choose a higher rating, but do not go below the minimum required, as that can affect safety, legality and insurance.
Those alternatives change the rolling radius and can affect ride height, gearing and speedometer readings, and may also create clearance issues on full steering lock. Only change size if your vehicle manufacturer lists it as an approved fitment, and keep the correct load and speed ratings.
All-season patterns are available in this size, but the range can be narrower than more common modern sizes. If you want true winter capability, look for the 3PMSF symbol on the sidewall, and compare wet grip and noise ratings to suit UK conditions.
Most modern tyres in this size are tubeless, but whether you need a tube depends on your wheel design and condition. If the rim is not airtight or is designed for tubes, a tube may be required, so confirm with your fitter before fitting and replace valves at the same time.
