Please go easy on me, I am a new comer to the diesel game, and still green when it comes to my Mercedes. I have had the 300SDL for almost two years(Sunday car till now but considering it as a daily driver), and the 300d 4 months(the current daily driver but have it for sale due to too many cars). I prefer the SDL due to it's size, luxury, look, and performance over the 300d but enjoy the 300d for what it is.

My questions are:
1) Can simple things be done to diesels like exhaust, intake, chip, injectors to improve mpg and/or performance like a gas car?

2) what are the things that are the most popular for performance and bang for the buck? & what brands or where to get them?

3)Totally subjective but, what do you recommend to get some noticable performance gains??

4) Can I do performance mods and not kill my mpg?

Thank you for your time!!
Hip

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2006 Jetta TDI DSG 320k miles
1997 Ford F150 325k miles 4.2L V6 "Work Truck"
2008 Tundra 225k miles 5.7L
1982 240D.....sold
1984 300D...Totaled OUCH!
1985 300D Turbo 222k miles "Dos" sold to 79Mercy
1986 300SDL 98K miles "The Beater"....sold
1987 190E 2.3 16v Euro spec 115K miles....sold