So previously I have been running Downpipe - Precat (soon to be cut out) - CAT - Centre silencer (straight through with rifle barrel style holes on inside) to straight pipe to tailpipes.

Car sounded pretty good except for epic drone at 70mph. Was getting around 48mpg average on a run as high as 52 average sometimes.

today I replaced the straight pipe with the standard backbox, car is now completely silent which is boring but practical.

But wierdly I can't get over 32mpg now. Even driving like a granny I get 31.9MPG.

How can a simple backbox have so much effect on MPG, almost 20 mpg down AVERAGE?

It is not even the computer but the actual dial drops rapidly the car guzzles fuel at this rate whereas before it was fine.

My only explanation is the backbox must be so packed full of baffles and what not that the exhaust gas hits a wall when it enters, so it must take more fuel to push the exhaust gas out?

Performance/lag etc don't seem changed.

I am wondering about getting a straight through silencer made and welded into the straight pipe, I measured the straight part and I only have about 10 inches available. Is this enough for a quiet but not-mpg/performance affecting exhaust?

Or could I open the backbox up somehow and take out some of the baffles or wadding to make it more freerer flowing? I wouldn't mind SOME noise but I don't want any drone.

HELP!