Do a back pressure test.
I did a back pressure test and found that at max RPM and power my truck was only building like 2psi of back pressure.
Stock 6.2L exhaust back pressure test - Diesel Place : Chevrolet and GMC Diesel Truck Forums
That was 2007, around 2009 I did the forward of the rear axle dump exhaust. Just felt like doing it one day. I changed the intake manifold and did the exhaust that weekend and started getting my best mpg numbers. I think 90% of that improvement was intake.
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1984 chevy suburban, custom made 6.5L diesel turbocharged with a Garrett T76 and Holset HE351VE, 22:1 compression 13psi of intercooled boost.
1989 firebird mostly stock. Aside from the 6-speed manual trans, corvette gen 5 front brakes, 1LE drive shaft, 4th Gen disc brake fbody rear end.
2011 leaf SL, white, portable 240v CHAdeMO, trailer hitch, new batt as of 2014.
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