20K miles for a 20 year old truck sounds about right. However, keep in mind that fire truck miles are like dog years! Also, many older fire trucks don't go very fast...55 or 60 might be top speed. One more thing is that a fire engine is designed to pump water while parked. So it may have a ton of hours compared to the miles it has driven. Fully loaded with water a truck from the 80's or earlier might not even be able to hit 55 mph.
One of the tankers here is an old Diamond-REO 5-ton Army truck...it will barely hit 55 empty.
It's definitely bringing up the rear when the tank is full.
The ladder trucks will sail up hills with the cars, though...pouring black smoke, and with the 12V71 Jimmy in Ladder 12 drowning out the siren spinning 2500RPM through its 5" straightpipes.
It's been overhauled and, uhh, turned up a little. (It's about 600HP according to one of the deputy chiefs.) Ladder 8 isn't quite as crazy, but its turbocharged 855 Cummins Big Cam is turned up pretty high (500HP, maybe). One of the volunteers is a retired truck mechanic & he overhauled them.
Both have finally had their duplex 15-speeds replaced with 13-speed Road Rangers in a nod to the fact that only three guys--none younger than 60--could drive them. They also got power steering (air-assist) around 1995, and Ladder 8 got big super single tires in front. (It got those before the power steering, which is something I do not even want to think about in a tight parking lot.)
Amazingly, the only automatic transmissions in the entire FD are the ambulances.