Have you scanned the car for codes Yet? What exactly is the code?

 

You need to have it scanned to see what code comes up. If they put an aftermarket cat on it you may still get a P0430 cat efficiency code as most non factory cats do not have the same amount of the precious metals inside that make them work as efficient as the factory piece does. Walker direct fit cats usually do good at keeping the P0430 from resetting but I know people that had a "universal' cat installed & didnt get the light out.

That mostly depends on your driving style but a stock M6 is capable of well over 300-350 miles to a tank. I got over 250 out of a tank a few weeks ago mods in sig. mostly highway driving.

 

I'm sitting on 180-190 a tank all city in my A4. No AC widows down 100% or the time. I get 180 no matter what baby it or racing check Gages comes on around 180

 

I have a bone stock 01 TA with a hooker muffler and no cats. It has a minor tune from having a 6 speed transplanted in it and the rear 02s shut off as well as some odds and ends for emissions deletion. I averaged 28mpg with the A/C going 75% of the time and my 3.73 gears on a recent 1000 mile road trip. It was put-put driving mixed with some street racing but in all it averages out pretty well. My understanding is that having a CEL will throw off your gas mileage a bit. My car has 202k on the odo but I seafoam it every 2 months or so to keep it flowing smoothly. I never change the fuel filter but I emptied it out once and got clean clear gas out of it. That seafoam is some awesome ****. Hope my advice helps you out.

 

I'm pushing 400hp and still getting 29 hwy and 17 city, wot runs wreck me though and overall jacrabbit launches.