My Stepson rides a 1500cc Kawasaki Vulcan bagger. He is replacing his rear tire only 8 months after putting a new one on previously. He is running tubeless tires. If you look at the mileage instead of age, he is getting about 6,000 miles a tire(rear). He and his wife ride 2up and usually both saddle bags full and luggage on the back rack. Is this normal tire wear ?