Not really. Back pressure only becomes an issue when you, say, run a straight header. If even then. Back pressure is only a major concern with carbed systems because the simple carb cannot compensate for a rush of dense/extra/etc oxygen from reduced back pressure that creates a lean condition. Of course we all know lean means hot which means many issues can occur. His Jeep's ECM compensates for these changes via the o2 sensor. So it all boils down as to whether the glasspack sounds good or not. Toss a high flow cat on their too. If back pressure is so vital for good economy, why did I get a boost in mpg when I upgraded to a 2.5" mandrel exhaust, high flow cat, and flowmaster super 44 muffler?

I used ngk v power plugs gapped to .035, Accel 8.8mm wires, new coil, new cap and rotor, just did an oil change with a wix 51182 filter and fram 10 w30 full synthetic, checking all the rest of the fluids this weekend, gonna put a high flow cat and a flowmaster 40 when I get paid, im still only getting 16 mpg, the motors only got 150 k

Cowl/cold air intake, 4 port (dodge neon) injector, Header, bored out 63 mm throttle body, 2.5 exaust will all add up to a few extra mpg

Also check your sensors. O2 iac map

The guy who owned it before me had just replaced the map sensor and iac, gonna get a new O2, def want the 63mm throttle body, lovin the helpful info!

Take your doors off. Not even kidding, the two fronts are about 75lbs each and the rears are about 40 each. I get 15 with all 4 on, 18 with the fronts off, and 20 with all 4 off. And I drive hard all the time.

Yessir. I go up 2 jet sizes when installing headers and true duals for that very reason.
PCM's and O2 sensors keep the mix pretty accurate, even when the exhaust is a little bit stuffy and the air filter is dirty.

Cat... won't help mileage, won't hurt. But they do keep the air a bit cleaner which is a big plus.

A properly operating, efficient engine helps. Keeping your foot out of the gas helps. Low rolling resistance tires, stock size, helps. As thin a viscosity oil as you can safely run helps. Get rid of the roof rack. Proper alignment, no dragging brakes...
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City? Highway? That isn't bad for city mileage with a 4.0/AW4 combo if that's what you have. AW4 is not the most efficient transmission out there.

Ive read on here Relocating the IAT sensor helped too. I did it and I noticed a slight improvement. .5 - 1 more mpg

16 was a mix of freeway and city, It was also my first camping trip with it so some of it was loaded up headed up hill, I have a 4.0 with a 5 speed, not sure wich 5 speed, came with no roof rack