Last 25 miles MPG different in M5 versus Drive

[clogan2]

2016 Tahoe LTZ, 145,000 miles

Recently I've been driving in M5 instead of D (for Drive), to prevent the engine from going into V-4 mode. Interesting thing is that my info screen read out for "MPG Last 25 miles" jumps from 16 or so when in D, to 19-20 when in M5. If I switch back to D, it reverts to the 16 mpg figure.

I don't really think my mpg is all that different in M5 versus D, but am puzzled as to why the computer thinks it is.

Any theories?

 

[B-train]

Depending on the mph you are driving, it is actually not working as hard as in 6th gear lugging along. More rpm isn't necessarily a bad thing

 

[clogan2]

Depending on the mph you are driving, it is actually not working as hard as in 6th gear lugging along. More rpm isn't necessarily a bad thing

I guess maybe I didn't explain it right (or maybe I'm just dense). Anyhow, my Tahoe display includes a readout of the rolling-average miles per gallon over the last 25 miles. I find that I can manipulate this reading by changing my gear selection lever from Drive to M5. (The actual gear in which I'm driving is likely the same with either selection, since I'm very rarely ever going over 50 mph). If I'm driving in D, the average shows as 16. If I change my lever from D to M5, the average jumps from 16 to 20. If I change the lever from M5 back to D, the readout changes from 20 back to 16.

That's what puzzles me: a rolling average that changes the instant the gear selection changes. If it's a true rolling average, an instantaneous change like that would be impossible.

 

[B-train]

You will probably have to math out 1 tank in M5 and see if the dream meter is correct or not

 

[clogan2]

My current theory is that, when in M5, the computer monitors miles driven and fuel consumption ONLY when the transmission is actually in gear 5. For example, if the lever is in M5, and you are sitting at a red light, the transmission is actually in gear 1. If my theory is correct, fuel consumed at the red light is ignored for the purposes of the rolling average, because the transmission is not in gear 5.

Conversely, when the lever is in D, the computer monitors all miles driven and fuel consumption, without regard to the actual transmission gear. It's a simple (miles driven ÷ fuel burned) computation.

 

[RST Dana]

My current theory is that, when in M5, the computer monitors miles driven and fuel consumption ONLY when the transmission is actually in gear 5. For example, if the lever is in M5, and you are sitting at a red light, the transmission is actually in gear 1. If my theory is correct, fuel consumed at the red light is ignored for the purposes of the rolling average, because the transmission is not in gear 5.

Conversely, when the lever is in D, the computer monitors all miles driven and fuel consumption, without regard to the actual transmission gear. It's a simple (miles driven ÷ fuel burned) computation.

You can’t really trust the computer for mpg in any model. I’ve been manually calculating for years for 08,13,16,17,20 and currently my 21. 90% of the time is the computer is off. Usually less for longer trips than city comparisons. Like someone already said, try a tank in 5th and one the same trip in 6 Th and post what you see from old school calculations. I will say, my 21 is the most accurate.

 

[clogan2]

Well, guys, thanks for the comments, I appreciate them.

But I'm not really interested in the accuracy of the figure; I'm more interested in the mechanics of the computation; what the programmer was thinking. Call it idle curiosity. I never put too much stock in the mpg read outs of any of my vehicles.

But thanks anyhow. Y'all have a Happy Thanksgiving!