Computer having issues with math

[Woob]

I think the computer in my truck is having issues with new math. It is 20 years old. Seems I am consistently getting readings of around 17 mpg on the overhead console mileage computer. BUT, according to good old fashioned math with paper, pencil, pad of paper and a gas receipt, I'm only getting a little over 11 mpg (OK. I did use a calculator to do the math). What gives?

 

[crazykid1994]

My 2017 1500 was never that far off. 6mpg seems like a huge difference. Is your truck stock? No lift. No performance parts. Stock size tires. Mine is usually only 1 mpg off but can be up to 2 mpg off at most.

 

[chopperman1]

Did you reset it at fill-up? If not, it's an average from last time it was reset.

 

[Travelin Ram]

If your math is correct, I’d check MAF, oxy sensors, and fuel trims. Maybe temp sensors.

How does the tailpipe look, is it really sooty?

Temp sensor errors can cause cold enrichment to be operating when it shouldn’t. MAF and OS control the fuel dose. If there’s sticking injectors or air/fuel measurement errors the fuel trims may reflect that.

 

[Woob]

Truck is stock. Yes, I always reset it. Today's numbers were 17.7mpg/computer and 11.6mpg/actual.

 

[Woob]

I recently replaced O2 sensors and MAP sensor. I have been having issues with LTFT being way off. Haven't figured out why. Replaced spark plugs a couple months ago. I'll check tailpipe and temp sensors.

 

[Woob]

The truck is running rich. Been chasing that problem for months now. About all I haven't done is replace injectors. I really don't want to do that. Grrrrr.

 

[Daw14]

Did you use OEM or aftermarket sensors ? Only use OEM sensors in any vehicle,others can cause issues .

 

[Dean2]

The truck is running rich. Been chasing that problem for months now. About all I haven't done is replace injectors. I really don't want to do that. Grrrrr.

You have identified the source of the discrepancy. Your truck computer is calculating what it should be getting. Due to running way rich you are getting maybe half that. The reasons for running that rich run the gamut from manifold gasket leaks, to issues with the computer, injectors, sensors and a raft of other things.

Once you fix the cause of running so rich your mileage will get a lot closer to the computer read out. One of the things about modern computer controlled vehicles, often the only indications of a problem is a large change in fuel consumption, even though you can't really feel the difference. At the price of gas it won't take long to recover repair costs.

 

[tron67j]

Wouldn't an engine running rich result in a lower MPG on the readout? I am not sure exactly the system works, but if it is basically fuel used against distance travelled, the number should be lower.

I would check the tire size in the truck versus the recommended size on the door jamb, if there is a mismatch that could account for some/all of the discrepancy.