Sounds ignition related with the poor mileage and the smell of unburnt gas at the exhaust. The back firing is the raw gas burning in the hot exhaust, this is not good for the cat converter and can kill it. You said you placed the plugs and wire, did any of the plugs have oil on them? The valve cover seals can leak and oil can get into the spark plug tubes and cause the plugs to short creating a misfire. Maybe bad coil and you could start swapping one of the three out one at a time with a known good one and see if the problem improves. I had a similar problem where the engine would run terrible under load and would not get up to speed, but put it in neutral and it would rev just fine. Turned out I had some oil in the spark plug tubes and one of my plug wires was intermittently making the circuit incomplete. My mileage went way down and I would spew soot out the exhaust. New plugs and wires and the problem was gone. Maybe you got a bad plug or a bad wire in the ones you replaced? I used NGK plugs and Bosch OE wires (would have used NGK ones but they weren't available when I needed them), if you went the inexpensive route sometimes you get what you pay for. But even the expensive stuff can fail/DOA. HTH.
95 Trooper 3.2 SOHC Auto 220K, Spectre/DIY air intake, K&N air filter, front and rear poly sway bar bushings, Cooper Discover A/T3s LT265/75/16, Magnaflow cat, Flowmaster 50 series, 2.5" exhaust, removed factory exhaust restrictors, Superwinch hubs, fuel injectors serviced by RC Engineering, PCV catch can, SS brake lines, JET Performance ECU, Hayden electric fan, B&M trans cooler, 150 AMP Quality Power Alt, OME 919s, KYB Monomax shocks