I dont think thats a big worry. My friend and i drove to orlando florida from miami and back in the car with a total weight exceeding 400 lbs and i didnt have any problems nor blowouts i dont think you should worry, however if you are unsure, better safe than sorry, it doesnt hurt to stop by the dealer and ask a service advisor to look into it before getting out of the car.

Just make sure though that you adjust your tire pressure accordingly to the amount of weight in the car, that should help considerably also.

However i've also wanted to know myself how they determine the load limit on vehicles.