13 isn't unrealistic if you're doing mostly in town driving and/or are heavy footed. Depends on gearing (mentioned), tires (ply, size, aggressiveness, etc.) and other factors.
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I use both tripometers. "A" for mileage checking, and "B" hasn't been reset since new. Lifetime for me is 15.9mpg. That includes about 50% in town driving (over the lifetime @ 38k) and the other 50% is made up of freeway and off-roading/4wd.
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I say this because you *should* be getting better. Something's not right. What gears are you running? (3.31s suck in town). Also, every vehicle sold in the US must, by federal mandate, be tuned to run on 87 octane. Running higher octane will generally cause the Nox sensors to allow the computer to advance the timing, and thus produce a bit better performance and mpgs. This all gets scrapped at elevations higher than about 4-5000ft. I usually run 87 (am tuned), but about 2-3 times a year I put Chevron 91 in. The Techron cleans things out. No reason not to these days with the price of gas so low...
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When was the last time the plugs were changed? The EBs are real sensitive to the condition of the spark plugs. Get ONLY the Motorcraft SP534 plugs and gap them to 0.030" (0.028" if tuned). You should be seeing 15-17 around town. I see 13-14, but I'm in stop-n-go a LOT, and have aggressive, oversized Load Range E tires, a camper shell and a 65hp tune (430 total). I mostly drive nice, but a guy's gotta play with a Maserati or two now and then... :elkgrin: