MPGs

On the RX

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We spent the holiday near Dallas this past week. I am convinced that the Texans sent their good drivers up here but thats a whole different story!
We took the 2011 rx450h down and were getting between 27 and 31 MPGs on mid grade fuel. We filled up the last tank just north of Amarillo with premium and suddenly the MPGs were low 20s. The entire time were drove around Dallas was this way. No change in driving technique. The only difference was the octane and elevation. I read on several different places when we bought this car that we could use mid grade at higher elevations, like where we live, but needed to use premium when at lower elevations. This is why I switched octane on the last fill up.
We ran though 3/4 of a tank while down there and I decided to go back to mid on the next tank and there after. Our trip back yesterday was the same low 20s until we filled up just outside of Raton. That was the turning point where the MPGs started to climb again.
All of new Mexico and most of the Texas driving on the way down we had good mileage. On the way back, all of that section of the trip was not good mileage. Does anyone have an explanation for this? There was an entire tank of gas expended in this section of the trip with an 8 to 9 MPG difference.
 

Johnny Utah

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I’ve noticed something similar when going out to Moab. Not as extreme. But after topping off in grand junction my fuel mileage seems to drop off considerably. Never really figured out why, always assumed it was me driving faster trying to get to Moab.
 

DaveInDenver

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I’ve noticed something similar when going out to Moab. Not as extreme. But after topping off in grand junction my fuel mileage seems to drop off considerably. Never really figured out why, always assumed it was me driving faster trying to get to Moab.
If you normalize tracking MPG on both legs GJ->Moab and Moab->GJ then you should be able to reasonably eliminate your things like driving habits and environment.

Like that it's usually a headwind between the state line and Cisco but going back eastbound should see a proportional increase that averages out to your real MPG.

There's a lot of possibilities here but one I've noticed over doing into my 3rd decade of regular trips back to Missouri that aren't fuel quality (different refineries and all) or octane related (I like more sea level States because base octane is 87 and not 85) but are still environmental. It's the air temperature and pressure and tire inflation.

Your atmospheric pressure is higher but the pressure inside your tires is the same as when you left. That means the relative pressure is lower and since tires aren't solid and can flex a little you will see a lower gauge pressure. My experience (observational) is that going from Denver to St. Louis results in a lower apparent pressure of about 5 psi.

I don't personally see an 8 to 9 MPG difference from being just 5 psi underinflated but then again I see nearly that much difference going from highway to city driving. So perhaps there's more than one factor in play, like doing a lot of short trips accumulated.

But to the tire pressure aspect, I do see a couple of MPG difference going from 35 psi to my normal of 42 psi. So it's been part of my routine for a long time to add 5 psi of tire pressure at a fuel stop in mid Kansas and check them the morning after arriving to get them right. Then for the return trip I burp 5 psi in Burlington on the way home so that they don't end up at 50+ psi in Denver. I can't say I've seen a definitive and abrupt one tank MPG correlation on my trips with respect to all of this monkeying around with air pressures. But once I thought about I couldn't neglect anymore. I do see the change in tire pressure, though.

If you want a visual image of what's happening look at potato chip bags or water bottles. If they are packed at low elevation and brought to Denver they puff up like balloons. The opposite will happen to a bag packed in Denver and then taken down in elevation.
 
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On the RX

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If you normalize tracking MPG on both legs GJ->Moab and Moab->GJ then you should be able to reasonably eliminate your things like driving habits and environment.

Like that it's usually a headwind between the state line and Cisco but going back eastbound should see a proportional increase that averages out to your real MPG.

There's a lot of possibilities here but one I've noticed over doing into my 3rd decade of regular trips back to Missouri that aren't fuel quality (different refineries and all) or octane related (I like more sea level States because base octane is 87 and not 85) but are still environmental. It's the air temperature and pressure and tire inflation.

Your atmospheric pressure is higher but the pressure inside your tires is the same as when you left. That means the relative pressure is lower and since tires aren't solid and can flex a little you will see a lower gauge pressure. My experience (observational) is that going from Denver to St. Louis results in a lower apparent pressure of about 5 psi.

I don't personally see an 8 to 9 MPG difference from being just 5 psi underinflated but then again I see nearly that much difference going from highway to city driving. So perhaps there's more than one factor in play, like doing a lot of short trips accumulated.

But to the tire pressure aspect, I do see a couple of MPG difference going from 35 psi to my normal of 42 psi. So it's been part of my routine for a long time to add 5 psi of tire pressure at a fuel stop in mid Kansas and check them the morning after arriving to get them right. Then for the return trip I burp 5 psi in Burlington on the way home so that they don't end up at 50+ psi in Denver. I can't say I've seen a definitive and abrupt one tank MPG correlation on my trips with respect to all of this monkeying around with air pressures. But once I thought about I couldn't neglect anymore. I do see the change in tire pressure, though.

If you want a visual image of what's happening look at potato chip bags or water bottles. If they are packed at low elevation and brought to Denver they puff up like balloons. The opposite will happen to a bag packed in Denver and then taken down in elevation.
I did take psi into consideration and forgot to mention that part. I added 6 pounds to each tire at the last fuel stop (they had free air on the pump islands!). I do need to go back out and remove some air now that we are back home!
 

DaveInDenver

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I did take psi into consideration and forgot to mention that part. I added 6 pounds to each tire at the last fuel stop (they had free air on the pump islands!). I do need to go back out and remove some air now that we are back home!
Great minds, or something like that eh?

Anyway, that's a big swing in MPG so I wonder if it's got to do with the VVTi adjusting to both octane and air density?

FWIW, I can't find the source again at the moment but I've read that with modern engines the recommendation that you can use lower octane (e.g. 85 instead of 87) isn't as true as it once was. The basis for that was cars many years ago didn't need as much octane to prevent detonation with thinner air (which means reduced compression) but EFI and smarter ECUs mean the engine can deal with it so by using lower octane all you do is leave performance on the table.

So I generally always use 87 (minimum per the owner's manual) in my 1GR-FE both here and at lower elevations. You also notice that having 85 as the baseline isn't true of every state with high mountains, so CO and NM (the only places where it's common place I've seen) kind of getting the shaft in this regard. As I understand the range of octanes depends on elevation and 4,000' is apparently is the cut-off set decades ago but there's no reason in 2021 that still needs to be the case.

Oh, also, meant to ask are you sure both Texas and Colorado measure octane the same way and perhaps Texas wasn't using ethanol blended fuel? I'd have thought 100% gasoline would improve MPG (gasoline having more energy per unit than alcohol) but in any case throwing too many changes at the ECU might mean it was hunting around its fuel map trying to figure things out. So you have bad mileage (or power) until everything settles down.

ETA:
Found the source. It's Chevron, so not just any random one. Although one might wonder sarcastically why an oil company might suggest you should be buying midgrade instead...

https://web.archive.org/web/2014032...ucts/documents/69083_MotorGas_Tech_Review.pdf

Screen Shot 2021-11-28 at 1.31.00 PM.png
 
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On the RX

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FYI, regular gas was sub $3.00 over the holiday. My uncle thought that $2.89 was really high😬
 

Rzeppa

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Denver air pressure is 12.0 or so PSIA, sea level is 14.7 PSIA.

EtOH in the gas can make a huge difference though, it packs considerably less energy/gal than straight gasoline.
 

On the RX

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Be honest, you weren’t driving “a little” faster in those wide open spaces?
Ha! Nope. I was running 83mph everywhere but in NM where the speed limit was 70 so I was set at 77 / 78mph. I wanted to really rip it but need to maintain a clean record to keep my company truck. That rx really wants to run free.
 
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