My wife's Expy has been getting terrible mileage, so I set out to rectify it. She drive an '04 EB with a 5.4 and has been getting 15ish w/ E10 gas. Back in the day with real gas we were in the 18-19 range on average with 50-60% city driving. Temp: 85F in the AM and 103F in the afternoon.. for the last 15 days or so Tire press is 42 psi all the way around with Mitch LTX tires. Otherwise the Expy is stock with 114k miles.
The last few days I have been driving it to work to see what I could get. 64 miles round trip with 30% in heavy stop and go rush hour traffic, 50% back country roads with a 60mph speed limit and the remainder highway speeds.
First 2 days I averaged 15.2 mpg.
Last weekend I installed a Spectre wet air filter and Superchips Mileage XS canned tune. I disconnected the negative cable just in case I did get better air flow with the filter. I topped off the tank and drove it in Monday morning. It was obvious from the start I was getting better mileage. The "Miles to empty" kept recalculating and I topped off at the same pump this evening to see.
Today I averaged 18.17 mpg
Is this purely scientific? No, but it works for me and I am happy.
Will everyone have the same results? I doubt it but I wanted to share what I found. It will take a while for the gas saved to pay for the tuner, but I wanted one anyway