how much does a could air intack system effect your mpg?

It depends wether you start to drive with a heaver foot on the gas to hear the nice sound from a new intake. The first week my gas usage went up but it has gone back to 31-32 miles to a gallon now that the novelty of the sound has worn off.

I still get 30 mpg doing around 75-80 mph in a 5 speed.

i average 26 mpg on the highway and my xb is bone stock.