Now I'm no Einstein - many people will vouch for that!
And I'm certainly no expert when it comes to engines etc.
However, all you have done is changed/modified the air intake, you have not changed the gearing.
Therefore I do not see how all of a sudden you are gaining an extra 10mph (from 72mph - 82mph) @ 3000rpm. Sounds like you've miscounted your gears somewhere.
You have not changed the gearing or wheel size. If your stats are correct then your mpg will raise dramatically.
If this is the case I suggest you patent this and start your own business - you'll make a fortune with today's gas prices.
Working on your theory, we could add nitrous and at 3000rpm we would be making about 75hp more and be doing a speed of about 112mph.
I'm sorry, but something is wrong somewhere.
I don't care how many times you make that run, just adding more air (or fuel etc.) will not affect the speed at any rpm.