This, the step up from 225/40R18 to 225/45R18 is about ~12.5mm on the tyres radius, or ~25mm (~1") on diameter (which then affects the tyres circumference)
It would make you speedo under read by about ~3.5%, so not the end of the world, it's about ~2.5mph at 70mph (ie your speedo says 70mph and you are doing ~72.5mph). Although all recomendations I've heard are to stick to <2.5% difference in size.
If you changed from the OEM tyre size you would have to inform your insurers.
There is about a 3.5% difference in overall diameter thus your speedo would be affected by that amount. If your speedo currently over reads by less than 3.5% (5% is normal) you would find your speed under reads with the 45 profile tyres which is illegal.
Think its fair to say Seat have spent countless millions developing the cars and the specs and any changes are unlikely to improve the car.
If you want 45 profile tyres you will need to change to 17" rims and still inform your insurers.
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