Gas Mileage
 

I have a previous post where I was trying to find the cause of misfires in my 2004, 4.0L engine. No codes are present. Replaced all wires, plugs, PVC valve and fuel filter. This is not a hard misfire and and increasing idle to 1200-1300 RPM the misfire disappears or the speed of the misfire is self smoothing for the driver. So given the misfire history I decided to check my gas mileage as accurately as possible. The rear differential ratio is 3.55 which I believe to be the best for gas mileage as offered in 2004.
For my in town test and highway test, I always topped off the tank by putting as much fuel in it as possible. I refueled each time at the same place so if the station was not level for my tank, at least all refueling might be affected.
For the in town test, I started to lock out the O.D. but that is not the way I drive every day so I left it in place. Since the O.D. engages at 45 MPH, it rarely shifts into O.D. and if it does, it isn't for very long. All test were without AC. I had 2 separate in town tests, and the average test for the 2 was 13.2 MPG.

The highway tests started at the same location ( gas station ), traveled the same highway to the same destination point, and returned the same day to the starting point (gas station). Each of the 2 test traveled 227 miles(round trip) . My goal was to stay at 67 MPH as much as possible.I estimated that less than 18 miles per round trip was without O.D. The average of the two trips was 20.1 MPG. The in town MPG was slightly less than I expected ( cause by misfire?) and the highway was more than I expected.

Dump a can of Seafoam(or similar injector cleaner) in the gas tank
(I do this once a year on my fuel injected vehicles, I can tell the difference in idle stumbles after 10 months or so, vs 1 month after I run the cleaner)

See if misfires lessen over the next tank
Dirty injector tips don't provide a good air/fuel mix at lower RPMs, they drip instead of spray, at higher RPMs and higher air flow the mix gets better

Spark misfires would usually be steady so often set misfire codes, there may be pending codes in computer memory

Gas Mileage
 

I have used Techron, Seafoam, and Lucus and nothing worked. I will check for misfire codes again.

Use an injector cleaner that has PEA in it.

The Techron injector cleaner has PEA. I will try it again with a slightly higher ratio of Techron to gas.

I once had an '02 Chevy S10 4.3L V6 with the poppet injection that started limping badly one day. Took it to the shop. They deemed it as a bad set of injectors. 1200.00 fix.Truck only had 80k on it so I said thanks, and limped it home.
At the time, the truck had about an 1/8 of a tank in it. I dumped in a full bottle of injector cleaner and then took it out to a back road for a bunch of hot laps, then parked it overnight.
Next morning, after it soaked all night, I fire it up and it ran perfectly smooth. Filled the tank and drove on. I don't know if it was water in the tank or what, but it worked. Drove that truck for another 50k, trouble=free, before trading it in.
Some folks say injector cleaner is snake oil. I'm a believer.

Gumout and Redline have the highest amounts. I don't believe Techron has made available how much they put in it so it may require a higher amount. Do this every fill up for a few weeks, gas mileage should improve if it is dirty injectors.