I am not affiliated with any fire department, however I work multiple shifts a week for a very busy ambulance service.  Ambulance miles are in my opinion like dog years....One ambulance mile is equal to many many normal wear miles. 

I assume fire trucks are similar.  Time is always a factor in responses, so hard starts and even harder stops are frequent. 

Fire trucks-especially wildland trucks see the roughest terrain in the absolute worst of conditions.  Smoke, soot and dust ingestion have to be much higher than the normal vehicle.  Couple that with large amounts of weight, add a little speed, and often adrenaline- and you have an absolute recipe for wear and tear.