Gas mileage

[Darksteel165]

Yup. its recommended for optimum performance and fuel economy, but is designed to work with 87.

Yes you can run other grades of fuel.
The person I responded to posted this picture of an different engine which recommended 87, and said to NOT use 89.
Currently they recomend 89, but say you can use other grades.

 

[HSKR R/T]

Yup. its recommended for optimum performance and fuel economy, but is designed to work with 87.

Due to sensors it's designed to allow people to run 87, mainly because 89 isn't available everywhere. It's tuned for 89 octane, but you can get by with 87 because the PCM will pull timing to adjust for it

 

[HSKR R/T]

Yes you can run other grades of fuel.
The person I responded to posted this picture of an different engine which recommended 87, and said to NOT use 89.
Currently they recomend 89, but say you can use other grades.

View attachment 169891

FWIW, back in early 2000s before custom tuning was available for the Dakotas and Rams of the time, I had a Mopar performance PCM in my 99 Dakota R/T(5.9l V8). Mopar recommended 91+ octane with it. Myself, and many other owners, found out the trucks actually ran better and had more power running 89 octane with the Mopar PCM. Was at least 1/10 second faster in the 1/4. The Mopar PCM has a "safe" tune that ran slightly rich with 91 or 93. The Hemi isn't the same.

 

[6of36]

I find their tests are fairly accurate for the specific conditions they rate and they apply to my summer conditions. But it may not be the same conditions that you drive under.

I've compared all of my vehicles with highway runs and city driving and I'm able to match the EPA estimates without much trouble. For instance, I did a pure highway run to prove that it's possible to get over 21mpg cruising on the highway with 3.92 gears, and that exceeds my window sticker which is 21 on the highway (personal best was 23.6 but average is 21-22 in the summer). So RAM didn't lie or fudge the numbers. Window sticker says 15 in the city, and I get 17 in my area with few stops and stretches of 30-40mph zones. In Los Angeles traffic I'm sure I'd get 1-2mpg.


I got the 3.92's specifically for frequent towing. While the axle ratio is shorter, it only looses about 1mpg on the highway as long as you keep it under 2,000rpm or about 68mph. If I run at 75+mph mileage drops like a stone due to VVT kicking in and I would get about 18% less mileage or 3-4mpg less. Up here the posted limit is 62mph so cruising at 65-68 works fine and I get decent mileage consistently.

It all depends on how you drive. Here, speed limit is 75, in the upper half of the state, so average highway speed is closer to 80.

 

[6of36]

18% difference in gear is not equal to 18% difference in mpg. It's more complex than that.

The driveshaft has to turn 18% more times, to go the same distance. That's physics.

 

[6of36]

That was from the last generation of engine. The current 5.7 is from 2003 forward. They don't even recommend 87 anymore and recommend 89 octane for example.

The current engine is actually 2009 and newer. The parts are not interchangeable, due to a major redesign.

 

[Idahoktm]

Yes you can run other grades of fuel.
The person I responded to posted this picture of an different engine which recommended 87, and said to NOT use 89.
Currently they recomend 89, but say you can use other grades.

View attachment 169891

The pic he posted says nothing about 89 octane. As far as we know, they are recommending that you don't use 93 octane. Currently, Ram only recommends 89 for optimum performance and fuel economy.

 

[CalvinC]

FWIW, back in early 2000s before custom tuning was available for the Dakotas and Rams of the time, I had a Mopar performance PCM in my 99 Dakota R/T(5.9l V8). Mopar recommended 91+ octane with it. Myself, and many other owners, found out the trucks actually ran better and had more power running 89 octane with the Mopar PCM. Was at least 1/10 second faster in the 1/4. The Mopar PCM has a "safe" tune that ran slightly rich with 91 or 93. The Hemi isn't the same.

That's really interesting. Reminds me of the old JET Chips I'd run which I'm fairly convinced were placebo.

But now comes hair splitting - if the tune can detect (infer I should say, via KR) differences in octane, and then select a different map, then it's possible one map isn't as good as another.

My conjecture is mostly theory, as modern tunes - both stock and custom - shift among a vast range of parameters and "maps" in real time. But apples to apples, on any given specific map, I would still maintain that running a higher octane than called for wouldn't ever loose you any power or efficiency.

The fundamentals just aren't adding up in my mind.
Higher octane doesn't change the combustion properties that would be experienced at the same timing and compression that a lower octane also optimally burns at. It simply allows the latitude for more timing or compression that a lower octane might not support due to pre-ignition.

E.g. energy density is the same, octane doesn't change that. Engine parameters simply manipulate what they can recover of that energy via timing etc.
The "gas go boom" part would be the same regardless of octane?

Maybe my density is the problem...

 

[6of36]

That's really interesting. Reminds me of the old JET Chips I'd run which I'm fairly convinced were placebo.

But now comes hair splitting - if the tune can detect (infer I should say, via KR) differences in octane, and then select a different map, then it's possible one map isn't as good as another.

My conjecture is mostly theory, as modern tunes - both stock and custom - shift among a vast range of parameters and "maps" in real time. But apples to apples, on any given specific map, I would still maintain that running a higher octane than called for wouldn't ever loose you any power or efficiency.

The fundamentals just aren't adding up in my mind.
Higher octane doesn't change the combustion properties that would be experienced at the same timing and compression that a lower octane also optimally burns at. It simply allows the latitude for more timing or compression that a lower octane might not support due to pre-ignition.

E.g. energy density is the same, octane doesn't change that. Engine parameters simply manipulate what they can recover of that energy via timing etc.
The "gas go boom" part would be the same regardless of octane?

Maybe my density is the problem...

Too much octane CAN reduce performance, there have been tests. I don't know if he is right about that on the 5.7s, but that is definitely the way the Demon 170s adjust for octane. That is how they get over 1,000 HP, on alcohol, but lower, on premium unleaded.

 

[Darksteel165]

The pic he posted says nothing about 89 octane. As far as we know, they are recommending that you don't use 93 octane. Currently, Ram only recommends 89 for optimum performance and fuel economy.

The pic he posted said to use 87.
On current gen 5.7 engine it says to use 89.

 

[6of36]

The pic he posted said to use 87.
On current gen 5.7 engine it says to use 89.

It does say recommended, that pt is alright to use 87. But yes, that tag is not from a Hemi.

 

[HSKR R/T]

Too much octane CAN reduce performance, there have been tests. I don't know if he is right about that on the 5.7s, but that is definitely the way the Demon 170s adjust for octane. That is how they get over 1,000 HP, on alcohol, but lower, on premium unleaded.

The Demon adjusting it's tune based on fuel isn't the same thing. Granted, if you try to put 110 race fuel in your street car meant to run on 87, the results won't be good. It will struggle to ignite the A/F mixture because the spark isn't hot enough.

 

[GMetal]

The tune will be different from use. No engine has a sensor to "know" what octane it is running. Also every gas station may not have the octane rating they claim to be selling within reason. The only thing the engine and PCM knows is if it sees knock on the sensors it will reduce timing. Say you are at 20 degree BTDC at idle and 36BTDC at 3600 and it sees knock it will reduce the timing event based on load and cylinder airmass tables. Most tuners run more timing which can increase HP to a certain amount. When dyno tuning engines you can drive yourself crazy adding or subtracting timing to pick up little numbers here and there. I highly doubt on a chassis dyno you will see any number differences between regular and supreme unleaded. Weather and altitude will affect power more on a stock tune than changing gas IMO.

 

[GMetal]

Here is a look at a LS3 main spark vs airmass vs RPM WOT spark tables with low octane vs high octane. If you look the numbers are very close if not the exact same. Download HP tuners and look at some stock Hemi tune numbers in the editor and you will see something similar.

 

[GMetal]

higher rpm view

 

[HSKR R/T]

The tune will be different from use. No engine has a sensor to "know" what octane it is running.

That is wrong. They have sensors for flex fuel vehicles to know what alcohol percentage is in the vehicle. Which is how they can be tuned for anything from 87 octane non-ethsnol to E85 pump gas. And we all know, once you get passed E10 blends the octane starts going up. Which is why E15 is 88 octane and not 87 octane.

 

[Wirerat]

The driveshaft has to turn 18% more times, to go the same distance. That's physics.

18% more torque available at the wheels means less throttle to move the truck.

I've owned both. Don't really care to argue it. But there isn't 18% difference in mpg in my usage. Maybe +/-1mpg depending how bad traffic is. Both trucks perform great.

 

[Eighty]

I ain't got a dog in this fight, but here's something to add to the conversation. The "work" that an engine does to move a 6000-lb vehicle from point A to point B is the same, regardless of axle ratio. THAT is physics.
The engine can either work faster (and slightly easier), or slower (and slightly harder). But the work is the same.

 

[RVTRKN]

I didn't buy my truck for the MPG except this thing called a Window sticker, it has words and statements that are provided as fact
I am lucky if I get 17 mpg on the highway when my truck was bone stock with EXACTLY what is listed on this document.
I just filled up and got mostly city miles this tank and I got 12.5mpg (I only care what highway miles show as there should be no variation on highway miles as there are no stoplights and I don't drive in traffic)
View attachment 169881

You say you have a Toyota Camry that gets 36 mpg?
Yes that checks out per the window sticker, there is no lying here just legit numbers that mean something.
How would you feel if you got 27 highway mpg when that sticker shows 39?
View attachment 169882

The MPG is based on the truck before any options are added, and indeed the 3:92 is an option and is more than likely the reason for MPG performing under the sticker.

 

[RVTRKN]

Does any one have Dyno numbers to prove or disprove 87 vrs 91, that add or takeaway performance. I'm more likely to believe the Dyno, but it doesn't prove or disprove the cleaning quality of high octane. So the old practice I have of burning a tank premium, with a high quality injector cleaner, on long trips that require a minimum of two tanks or more.