Gas Mileage v. back pressure

[Poland1 <polands@xxxxxxxxxxxx>]

Having just installed full Stebro "free flow" stainless steel cat &
exhaust, (but not the "tip silencer" inserts yet), I have noticed a
curious mpg anomaly.  The around town mileage went down from around
17-18 prior to around 15 with this low back pressure system.  However on
a run from Portland to Seattle and back it easily did 23 mpg, which is
close to prior highway mileage.  (This requires staying within about
2mph of the posted speed).

So the question is whether it is logical that reduced back pressure at
low speeds could reduce fuel economy?  Or is it (more likely) that the
driver is either:
a. favoring rpm over 2k to get out of the resonant boomy mode; or
b. getting sucked in by the exotic sounds created as it comes on the cam
around 3k and ...

(guttural mild staccato reminiscent of a 1972 V.Polak 911 in turns 7-9
at PIR, or an E-Jag with no muffler once heard winding out up Portland's
Canyon Road... ahh nothing like a 6!)

(but Aaron Bohnen please tell me it kills the <2k boom to put in the
silencers when they arrive??)

Mr. Conforti you are truly a genius, and should be awarded the UUC
Roundel of Honor medal for your response recently re more efficient fuel
burning with your chip.  How could any S.O. object to that!

Steve Poland
'88 535is
Portland, Or BMW ACA, CCA

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