Load ratings
I think you should just believe the posted load ratings for the tires you have. The manufacturer has already taken into account different road conditions when coming up with their numbers. The safety margin is built into the numbers. When measuring your own rigs weight at a weight station, your towed weight' tongue load needs to be added to the rear weight for a true rear weight load. Are you sure you are not carrying some gold bars for an emergency. They are very heavy.
__________________
|