I agree w Doc. Stock tires are around 29" in diameter and if you put 33's on you are running 70 when your speedo reads 65 (or thereabouts). You would need a programmer to correct this and get the calculated mpg correct for the 'puter. Have you done the manual calculation? You would need to use a GPS to get actual miles travelled since the odometer would be off as well (I am assuming you have not re-programmed the vehicle).

Personally, I did not see a very big drop when I installed my 32's. I see huge variances with outside temp, especially over 95 deg F - so make sure that is not impacting your results as well.