That does seem very low mpg, around 20mpg if my maths are right.
As said though a small hole anywhere near either side of the lambda can do this, the reason why it can do it after the lambda is because exhausts also suck, in between each power pulse pressure wave there exists a low pressure vacuum pulse, I changed a car years ago with this problem a tiny 3-4mm hole about 1/2 foot after the lambda affected the mixture.
tabetha