Firstly, the basics: - A load rating index of 84 indicates a maximum of 500kg per tire - A load rating index of 87 indicates a maximum of 545kg per tire Basically it all means that if your car had weight perfectly distributed, with 87 rated tires your gross vehicle weight can be a maximum of 2180kg whereas with 84 rated tires, your gross vehicle weight can be a maximum of 2000kg. When they determine the load rating, they do give a little extra clearance, but it's not to say that you should go below it. Also during accelerating/braking/cornering, the load on each tire can vary so therefore weight is not always equally distributed. That all said, it's still illegal to have the incorrect load rated tires on your car and I would highly suggest to get them changed ASAP. As well as being illegal, should you get in an accident and your insurance company finds out about this, you will not be covered.