I have 3 new tyres of size 155/70 R13 75H and 1 tyre of 155/70 R13 75S. Does it make any difference?
The letter determines the max payload of the tyre (Amount of max load at specified max speed)
It is recommended that you do not mix tyres with different payloads. Even if you do, it should be done in pairs.
God forbid, but during accident claims some insurance companies might create a mayhem if manufacture recommended tyres are not used.
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Technically you should be driving for the lowest rating of your tires, your S one is the lowest rating of the ones you have on (S=180kmh H=210kmh). Think about when you put up the spare tire because you have a flat, that one is the one telling you how to drive (obviously a spare is a spare and you are not supposed to keep it on the car forever). Rather than the suitability of the H tires for your car you should pay attention to the safety issue that is mixing tire rated speeds. While one could say to just adapt to the lowest rated speed, the fact is that tires made for different speeds are engineered in different ways, so now you have a tire that does not react to your driving in the same way the other three do and that can be cause of poor handling. That could be minor but there's always the risk that it will catch up on you at the worst moment.