would a faulty cat cause less MPG

Hi i beleive i may have a faulty cat im just wondering would this cause me to have less MPG? and ifso how many mpg would i be loosing?

The reason i think i have a faulty cat is that i had the exact same problem on a 4.0 v8 bmw where when i apply acceleration there is vibration and hear a dummin sound (i cant think of a word to describe the noise) and after spending £££ on diagnostics and changing cylinders etc.. it turned out to be the CAT. and i have the exact same problem on the land rover

?

If it is choked up yup it will cause less mpg , what model disco is it you got ?

Certainly would,had a scorpio with the cat so choked the engine wouldnt run....took a week to figure that one out.
If its a 200/300 tdi de cat it ASAP

mine is a 4.0 v8 Es (xreg 2000)
the reason i think the cats have gone is that when applying accelaration i got a vibration from the car and a humming vibration noise ( i had the exact same thing on my bmw 540 which turned out to be the cat)

It could also be messing with the O2 sensors post-cat if it's knackered which could be overfuelling the engine - if you decat later Camaro's you need o2 simulators else it'll send too much or too little fuel through.