I cant rightly remember the fuel comsumption on 631, but I would say somewhere between 10 to 15 gallons an hour.
As far as self loading top soil, or any material for that matter- unless you have down hill grade of about 8% or more, forget about getting a full load without a push cat. Sure you can get a half or less load with desent underfoot conditions, and a slight downhill, but throw in a little mud or soft ground, or a bit of a uphill, singles are useless without the help of a pusher.
A good rule of thumb for scrapper, or even truck production is 10 loads per hour. A 631 hauls 24 yards per trip (our companys estimate). Thats 2400 yards per hour. You would need 5 scrappers to meet your estimate, and work 10 hours a day minimum. With a 1 mile round trip, 10 is a very concervitive estimate. You also have to factor in haul road conditions, operator skill, push cat hook-up time and so on.
Hope this helps a little, Jason