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the mileage will get better as you put more miles on it. It is based on engine load vs RPM, at higher speeds, there is more load(wind drag, momentum) you will probably get the best mileage at a constant speed of around 60-70 mph

it should say in your car manual how long u have to break in your car. It will tell you how fast you should go within the first like 2000 miles or so... it should tell you. I def wouldn't be cruising at 80 when you have only 600 miles on it... that's too fast too early for that car. The RPMs are basically the gears you are in. That sounds about right. I would go read your car manual and see what it says. Good Luck

Your fuel economy is a rolling average. It is based simply upon the size of your gas tank and how far you can go on that "one" tank.

Considering that you only have 600 miles on it, you will see a slight improvement as everything "sets" (if left stock)

You will see a better increase if you put in a high flow air filter such as K&N or Airhog. I put in a generic high flow filter and gained 9 mpg on a 1992 Chrysler 5th Ave.

Take your base mpg, get your figures and then add in the K&N for a second set of numbers.

After all this, look for a high flow exhaust system. (IMHO somewhat pricey) but if you are going to keep the vehicle and/or are wanting to keep it's resale value, a definite plus.

One old friend of mine added in both the air filter and exhaust and got up to 27 mpg on a 1997 Chevy 2500 diesel.

RPM determines how much gas you use.

and fuel consumption may be better after a engine has been broken in for then it can work at max efficiency.

It is based on a variety of speeds and conditions and then averaged. fuel economy is immediate.