How many miles per gallon should I be getting with a 17 1/2 ft. bass tracker with a 70 hp evinrude engine..... It is about 10 years old and I hardly ever run it wot... I usually run it anywhere from 25-30mph with 2 people,.....Is there any other info needed to find this out?

It's GPH(gallons per hour), not MPG(miles per gallon), when you measure efficiency of gas usage of a boat. Your speed over water can vary greatly from one hour to the next when running on a given day, but your fuel usage at 4000 RPM will remain constant. SOW(speed over water) is not the same as SOG(speed over ground) unless all things are calm so learn what your unit burns in GPH and you will never run out of fuel.

How to find out your fuel consumption: simply run a course for one hour and refill the tank.

How to find speed over ground: a GPS will give you that assuming you can run in a straight line.

This is important to know and understand if for example you plan a longer trip with your boat. Wind and current will alter your MPG but not your GPH. Learning to caculate fuel consumption in a marine enviornment is important if you operate on bigger bodies of water.  

A general rule of thumb for outboard fuel consumption to 10% of rated horsepower per hour at WOT. So 10% of 70HP is 7 gallons per hour at WOT.  

And if you throttle back to 3/4 throttle, you will use a lot less than 3/4 the amount of gas than you would at full.  Bring it all the way to WOT, then throttle back.  Nice motor by the way.