Big exhaust will hurt fuel economy if you accelerate slow and drive slow, and MIGHT help if you drive normal.
You want a perfectly tuned exhaust. Here's an example :
Exhaust tuning
you want the puffs of exhaust to line up at a perfect distance, so that you get the most torque/power at low rpms, where you need it most in good FE. Too small will create too much back pressure, too big will create not enough. Back pressure isnt always bad, it just seems like it because when you talk about eliminating back pressure, your usually talking about racing and high rpms, where exhaust needs to escape fast and unrestricted. Read the link to get the just of it.
Quote:
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In the lower RPMs, pulses are smaller, and further apart. When you rev up into the high RPMs, pulses get bigger and closer together. So we want to keep the small, spread out exhaust pulses in line in the low RPMs
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for low rpms, smaller exhaust is better for the smaller, faster exhaust pulses, they don't bounce around in a giant 3" exhaust, so they are in-line, and they can go faster cause the route is an easy one way, so no one (exhaust buddies) get confused
