There are a number of quirky laws relating to motoring here in Spain, and a number of these relate specifically to the tyres that are fitted on a road vehicle and the condition that they are in. Tyres in Spain are not as cheap to buy as the UK and as such it is important that you understand the legality behind tyres and avoid the likelyhood that you will end up buying a set that does not meet with your exact requirements.
Anybody who has been resident in Spain for even the shortest period of time will be able to tell you the the Guardia can, and do, hold regular spot checks for drivers and their vehicles and tyre regulations are something that feature highly on their list of things to check - especially on older vehicles.
Driving with worn or non-matching tyres can attract on the spot fines of 150 euros, and the vehicle can even be taken from the driver immediately and impounded in cases where 3 or more of the tyres are found to be unsafe.
Yes, the specification of the tyres is crucial. If your 'Fichas Tecnicas' state that 225/70/R16/102-S* tyres are fitted, then you can not change them for something else, no matter how practical or economical they are. Your vehicle will not pass an ITV unless the tyres fitted match the tyres shown in the specification. Regardless of their condition.
* The final letter in the sequence denotes the speed rating of the tyre. The letter rating can be equal to, or greater than mentioned in the Fichas Tecnicas. For Example, where an "S" is fitted, it can be replaced with a "T", but not an "R".
All tyres fitted on a vehicle are require to be the of the same make/size/speed code etc... on every wheel - Don't forget the spare !
Motoring law stipulates that the tread depth on a tyre should be no less than 1.6mm (although many motoring organisations recommend that a tread depth of 3.00mm should be observed to ensure safe and optimum driving conditions are met.
Those who have brought a vehicle over to Spain from the UK often notice that their tyres seem to lose pressure and therefore need topping up with air more than they ever did in the UK, and can spend ages searching for slow punctures that just don't exist. The reason behind this is the significantly warmer temperatures seen here in most of Spain - especially if your car is generally kept outside or in direct sunlight for long periods of time. Sunlight and higher temperatures causes the rubber to perish and degrade, which results in the walls of the tyres crystalising and cracking and the air slowly escapes. If this does happen, you will have accept that the tyres will need replacing altogether sooner or later - regardless of the tread depth remaining.