4 Answers

You could experience a ROUGH IDLE with the wrong spark plugs in your car. I would check your owners manual and make sure you installed the right ones.

The engine would run rough and "miss out" a lot...

The most serious problem is if it it too long and when piston reaches TDC (top dead center) it will push a hole through the top of the piston, not good.. :(

If it is too short, then you'll have a lot of fuel wastage as it will not combust fully or at all. It is best to stay with the manufacturers specs, also, they must be 'gapped' correctly to assure proper burn, plugs are rated hot or cold..  Some plugs do not need gapped.. some are 'fusion' Great plugs but also expensive

Loss of power.