Attempted a short economy run (5.3 vortec)

Pandabird · Aug 22, 2024

Went to visit the old folks the other day, roughly 200 miles back and forth. Went quite heavy on the pedal on the way there with the AC on full since it was a sunny day. Jumped a bit when I checked the average mpg (converted from L/100km for your convenience) it was at 14 MPG and I was like yikes! I've made a previous note about the AC because it makes a constant whiny noise when turned on. Anyhow I refused to believe i couldn't get it down to near 20mpg on my way home so I literally crawled it at ~50 mile speeds with very smooth pedaling and whatnot, AC turned off because the outside air was cool enough once it got dark. it went down to 20 alright. Actually ended up at almost 22mpg which is a really good result in my opinion.
Could of course have something to do with more uphill one way there and more downhill on the way back.

Anyways few notes since this is one of the first actual rides in my Tahoe not counting grocery runs.
Is ~500 rpm idling normal? Doesn't seem to struggle with it, it just looks low compared to what i'm used to.
Is that oil pressure gauge meant to be this active? it seems to idle right between the 0 and 40 but when I hit the gas it rises to 35 ish?
The automatic gearbox upgears downhill without touching the pedal, is this normal?
My cruise control is posessed by a demon, we have tons of speed cameras here so it's a no-no to use as is.
And does the AC truly add this much MPG? Why is it so whiny when in use though quiet when turned off (can't be a bearing since it still runs on that same belt right?)

RST Dana · Aug 22, 2024

Manual calculations?

Geotrash · Aug 22, 2024

Pandabird said:
Went to visit the old folks the other day, roughly 200 miles back and forth. went quite aggressively on the way there with the AC blaring since it was a sunny day and jumped a bit when I checked the average mpg (translated from L/100km for your convenience) it was at 14 MPG and I was like yikes! I've made a previous note about the AC because it makes the engine noise whiny when turned on so I had it turned off when showing off the engine to my friends. Anyhow I refused to believe i couldn't get it down to near 20mpg on my way home so I literally crawled it at ~50 mile speeds with very smooth pedaling and whatnot, AC turned off because the outside air was cool enough once it got dark. Results had me baffled. it went down to 20 alright. But it kept crawling down and down, little by little, fought some uphills, cursed at some traffic stops since they seemed to set me back. But once home I was almost at 22mpg.

Anyways few notes since this is one of the first actual rides in my Tahoe not counting grocerie runs.
Is ~500 rpm idling normal? Doesn't seem to struggle with it, it just looks low compared to what i'm used to.
Is that oil pressure gauge meant to be this active? it seems to idle right between the 0 and 40 but when I hit the gas it rises to 35 ish?
The thing upgears downhill without touching the pedal, no engine braking allowed
My cruise control is posessed by a demon, we have tons of speed cameras here so it's a no-no to use as is.
And does the AC truly add this much MPG? Why is it so whiny when in use though quiet when turned off (can't be a bearing since it still runs on that same belt right?)

Oil pressure gauge behavior is normal. A/C whine is not. The A/C clutch bearing turns when the clutch is disengaged and compressor shaft isn’t turning. But when the clutch engages, the shaft turns and a different set of bearings turns with it, along with the scroll inside the compressor.

My guess is you have a compressor on its way out.

tom3 · Aug 23, 2024

Could possibly be an idler pulley with the extra load of the compressor making it a bit noisy. More likely what the above post noted though. My older brother is always saying the biggest loss of gas mileage is using the brakes. You're driving technique on that return trip probably made a big difference.

swathdiver · Aug 24, 2024

Pandabird said:
Anyways few notes since this is one of the first actual rides in my Tahoe not counting grocery runs.
Is ~500 rpm idling normal? Doesn't seem to struggle with it, it just looks low compared to what i'm used to.
Is that oil pressure gauge meant to be this active? it seems to idle right between the 0 and 40 but when I hit the gas it rises to 35 ish?
The automatic gearbox upgears downhill without touching the pedal, is this normal?
My cruise control is posessed by a demon, we have tons of speed cameras here so it's a no-no to use as is.
And does the AC truly add this much MPG? Why is it so whiny when in use though quiet when turned off (can't be a bearing since it still runs on that same belt right?)

RPMs is normal for emissions compliance. Oil pressure fluctuations is normal as it's based on load/speed.

Transmission uses best gears for going up a hill or down a hill with cruise control on with varying precision depending on the model year.

Running cruise control makes the engine more fuel efficient.

My compressor started whining back in 2018 and still does to this day. My 2012 Sierra's compressor started whining about 2 years ago. Both blow cold and do not drag on the motor; so far. The '13s is quiet still. I hear them whining all the time at the gas station, other people's cars. Must be using the same bearings as the 1990s GM power steering pumps!

Attempted a short economy run (5.3 vortec)

Pandabird

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RST Dana

Geotrash

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tom3

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swathdiver

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