Here's a readout of how they determine it:
The Approved Emissions Modification Extended Warranty period for 2009-2014 model year vehicles will be the greater of:
(a) 4 years or 48,000 miles, whichever comes first, from date and mileage of implementing the Emissions Modification, except for vehicles offered for resale, in which case from the date and mileage of the first resale transaction after the modification to the first person who in good faith purchases the vehicle for purposes other than resale, or
(b) 10 years or 120,000 actual miles from the initial date of purchase (whichever comes first).
For 2015 model year vehicles, the warranty period will be the greater of:
(a) 11 years or 162,000 miles, whichever occurs first, from the vehicle’s original in-service date; OR
(b) 5 years or 60,000 miles, whichever occurs first, from the date and mileage of Phase 1 of the emissions modification. At the time of the subsequent Phase 2 modification, the extended warranty will be honored for 5 years or 60,000 miles, whichever occurs first, from the date and mileage of the completion of Phase 2.