Over weight? Does it bother you?

For those saying 10%, 20%, 30% over is OK, I'm wondering what engineering basis you are depending on? If you have researched the axle, spring, and tire ratings and weighed each corner, and are under in each case, you could say that after careful measurement, in this application, you are within ratings. But to depend on an unspoken and unknown ''safety factor" that the manufacturer hopefully has built into their product puts the full burden of failure on you. Perhaps that safety factor - if it exists - is intended to cover the odd owner who willfully violates the specs. More likely it is to cover variations in manufacturing quality, aging, etc. How do you know that your 5 year old tires aren't the 3 sigma case, just clearing the failure criteria by a hair?