Depending on what kind of mufflers the car has, (sometimes they are replaced with muffler shop non diesel mufflers that are restrictive) I would throw them away. If noise is a concern, install a straight through diesel muffler. Other than clogged exhaust/dirty air filter, the single biggest thing that will affect mileage is the injection timming. More timming will net you better mileage. Assuming your injectors are not worn out. Time it at the top of the spectrum. You can go farther, though it will have more injection rattle, and I have found they idle a little rougher. I would try to get the timming between .90 to .95mm. If you have very high mileage, you may want to think about getting your injectors pop tested, or possibly rebuilt. 33 mpg isn't bad, depending on your commute. Is that a mix of city and highway, mostly highway? The auto will get good mileage on the highway, with the lockup converter. Around town is where the manual will show the mileage advantage, since the auto wont lock unless your going around 50 mph.
My auto has been getting around 30mpg, but that has been with me running the crap outa it, and blowing lots of smoke on the ricers around town...
Jason
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