The only other brand I looked at was Pirelli and they were the same:
"For vehicles equipped with staggered size fitments (different tire sizes front and rear), the mileage warranty for the rear tires will be 50% of the stated mileage warranty for that line. Run Flat tires will have a mileage warranty of 50% of the stated mileage warranty for that line, not to exceed 30,000 miles, whichever is lower."
Just look under "Milage Warranty" for whatever brand you are looking at buying.