I was looking in .../MMOCoreORB/bin/scripts/object/mobile/vehicle and it looks like the decay rate is set in the lua for each vehicle.
Example from speederbike:
object_mobile_vehicle_speederbike = object_mobile_vehicle_shared_speederbike:new {
templateType = VEHICLE,
decayRate = 25, -- Damage tick per decay cycle
decayCycle = 600 -- Time in seconds per cycle
}
ObjectTemplates:addTemplate(object_mobile_vehicle_speederbike, "object/mobile/vehicle/speederbike.iff")