The five-litre straight-six petrol engines do 8-12mpg depending on whether you use four- or two-wheel drive and they'll top 60mph given time.
For fire-fighting there is a still-impressive 900-gallon per minute Sigmund fire pump and an additional Coventry Climax lightweight pump.
http://www.ayeready.org/green_goddess/greengoddess.html
http://www.channel4.com/4car/ft/feature/feature/210/1