I recently had my 4WD re-geared. From what I've read I should have seen a slight increase in my fuel economy, but instead it actually went down a bit. I have been running and ODB scanner on it for while to try to figure out what was wrong. The long term fuel trim numbers were running consistently high and from idle to 1500 rpm the would go down. From all I've read that usually means the computer thinks the engine is running lean and so it dumps more fuel in an attempt to get the mixture right. This usually is due to a vacuum leak somewhere. So I began replacing hoses and elbows and so forth in an attempt to be sure I fixed the leak. Ultimately, I traced a couple of the lines under the front end and found that they were not plugged in - probably an oversight from the re-gear. Aha! Maybe this is the source of my grief. So I plug them in and look at the ODB numbers. They are much much better. In fact, the go to 0 when you give it just a little gas. I had never seen them at zero. I button everything up and head to the gas station to fill up. All the way there the numbers look much much better. I was in the double digits and now it's running around 2-3. Here's the crazy part. When I start back up at the gas station, the numbers go back to bad and stay that way. When I got home I rechecked the lines and they are still plugged in. Any guesses as to why and how I saw a momentary improvement and then it was gone? Thanks guys.

 

Lets start with what year and the engine that you have.

Other then the re-gear, what has been done ?

What gears did you have and what gear ratio is in there now?

 

I don't know which way you went with the gear change but here is the Theory to consider.
If the gear change was numerically lower (to turn the engine less rpm), the engine is not making the same torque at the same road speed due to not using enough airflow as before the gear change to make the torque.
You may not realize you are adding more throttle to keep the truck moving after the change.
As the throttle is opened farther more fuel in metered to the motor. A 4 cycle engine is a variable efficiency machine that runs on air flow to make its power.
The Mass Air meter reacts to the higher airflow which has an effect on the long term fuel tables over time by moving them toward the rich side.
The Ox sensors read this difference and signal the PCM to richen up the LTFT tables.
Beyond this I can't tell you what you have.
Sometimes you go out of practical limits in any design and get into these effects by making changes without understanding the consequences.
Also the speedometer correction needs to be made in the computer if the truck has an electronic dash.
Or a speedo gear change in the older trucks.
If not and you use the mileage as a way to compute mileage it can be off s significant amount.
Good luck.

 

2001 5.4
Regear to 4.56
And running 35's with 3in lift

Oh, and thanks for the input.

 

I appreciate the input. I do know that I must do some calculations to figure actual miles in order to get the MPG having changed gears and running bigger than stock tires. I see your logic, but what I can't figure out is why it ran very differently (as far as the trim numbers) for just a few miles and then reverted immediately back to the old numbers. All this after find 2 huge vacuum leaks and fixing them. If you do a little research on the forums, or maybe friends, you'll find that most people with 35's, 4.56 gears and the 5.4 engine are getting around 14-15 MPG. I can barely scrape up 12 by driving like an old lady. I know something is wrong, I just haven't been able to put my finger on it. I was ecstatic when I found the 2 vacuum lines unplugged because this is exactly what the experts said was the problem with higher than normal long term fuel trim numbers. Maybe a little more time will tell. I wasn't able to drive the truck at all today, but I will put a few miles on it tomorrow. Thanks again for the info.

 

You need a programmer with tunes (preferably custom) to correct for the gears and tires. It's not just for the speedometer, the PCM needs the right info for proper engine and transmission operation.

 

Just bought a Superchips. Hopefully this will help. At least my speedometer will be correct again