I work for a small, not for profit rural ambulance company. And we base ours on budget, but obviously the highest mileage rig is the firstto be taken out of inventory and given to one of the FR units.
My agency buys 2 new ambulances each year so it's roughly 5-6 years of service per ambulance. As far as deciding which ones to get rid of, it's whichever one is least functional to our company. Most of our rigs now are type III's but we have a few type II's that still have fairly low miles on them. This year we got rid of 2 more type II's even though their in better shape than a couple mods due to the fact that working ALS in a van just doesn't work. Once we get rid of the last of the vans (no more are in service only backups) it will be a mix of age, mileage and mechanical condition which really go hand in hand since we rotate rigs to keep it even.
Ours is by age. Up until 3 years ago, my service was a private non-profit, from what im told, company. We have 3 trucks, an 05, 97, 90...the 97 has 45,000 more miles than the 90 but we are getting rid of the 90 just because of the age. It runs and drives good, it just needs a lot of help that we can't afford.