If it's anything like my 520, the range varies greatly depending on how you're riding. When I'm just cruising along a logging road, I get about 30 mpg; when I'm at the track or really getting on it, my mileage drops almost in half.
I tend to take many day-long rides, so I bought the tank from the MXC. Bolts right on, and adds another gallon or so capacity. It seems to be only about an inch wider, too - not that big a deal.
I don't own a 4-stroker, but my dealer says you should be able to get around 60 miles of racing if jetted right.
But I've seen this question before and many seem to be topping out at around 50. I did a 55 mile loop with a friend on a 400. He was on fumes when we got to a gas station, I had at least a gallon left in my 250.
I guess both IMS and Clarke make big tanks, and the MXC is available too.
My 400exc can hit a 160km range on roads and highways. ( Converted to Motard ) Mileage does reduce by 20km if i ride it hard. Average comsumption is 21km per litre of fuel.