87 octane vs 89 octane, fuel economy differences

[freebird_78]

Ethanol is roughly 30% less energy dense than gasoline. That's straight ethanol, not E-85. Doing the math, E-85 would be 25.5% less energy dense. No one runs straight E-85. Hell, most E-85 isn't 85% ethanol (often less than 85%). E10, meaning 10% ethanol would be 3% less energy dense than "E0". I doubt anyone would "see" 3% change outside of noise in the data.

People claiming 10% by going to ethanol free just aren't looking at the math.

And, that's not even accounting for change in octane rating. Where I'm at, they sell 86 and 87. You can literally log short term knock retard on either of those and see the ECU pulling timing. My ECU was pulling 4 degrees out, not a trivial amount. A little splash of ethanol (straight ethanol has an effective octane rating of 113), a bump in effective octane (E15 is about 88), and no more short term knock retard... less power loss. Less power loss = more efficiency. Much more so than the 3% energy density difference.

 

[MEGA HO]

Or just drive a diesel and forget all this mambo-jambo.

 

[Rockcrawlindude]

Or just drive a diesel and forget all this mambo-jambo.

As if the diesel guys don’t chat about additives and such in the same way

 

[MEGA HO]

As if the diesel guys don’t chat about additives and such in the same way

Was more referring to the post right above me reply.

 

[Wanderlust073]

Was more referring to the post right above me reply.

As if the diesel guys don’t chat about additives and such in the same way

If I learned anything from this forum it is never buy a diesel. Seems like they are either in the shop or giving people ulcers worrying about regens.

Filled up with 87 octane. We only get 85/87/91 around me. Have been using 91 since I bought the truck. So far no diff in mpg or performance.

 

[Crusty old shellback]

Good luck finding ethanol free gas out on the west coast. Since I'm so close to the border, our gas comes from Komieforina so ethanol gas.

 

[freebird_78]

Just ran another inadvertent octane test. Was traveling, and only had access to what was marked as 87 octane (which is the minimum stated requirement). Sure as hell, it started pulling timing (ST Knock Retard in Alfa) after this. Not a LOT. It was pulling 4 degrees with 86, on 87 it was pulling 1.5 degrees under load.

Added 5 gallons of E85 making about a E23 blend @ 90 octane, and within a few miles, it's back down to pulling no timing at all. Probably could have gotten away with 3 Gal of E85 and still bumped octane enough to eliminate ST Knock Retard.

 

[Michael5705]

I've been running 89 in my 2021 RAM Power Wagon 75th Anniversary and averages vary. In/around city will average around 12-14 per gallon, and highway (running 65-70) it will average close to 16 to 16.5. The only modification I've done is to put dual exhaust on it. Before dual exhaust, I was averaging round 11-13 city and around 14-15 highway. So, small gain; however, over time the savings has made up for the cost of the dual exhaust.

 

[Dave01]

I've been running 89 in my 2021 RAM Power Wagon 75th Anniversary and averages vary. In/around city will average around 12-14 per gallon, and highway (running 65-70) it will average close to 16 to 16.5. The only modification I've done is to put dual exhaust on it. Before dual exhaust, I was averaging round 11-13 city and around 14-15 highway. So, small gain; however, over time the savings has made up for the cost of the dual exhaust.

That wouldn't exactly be a small gain - going from 14 to 16 mpg is a 14% gain. And has to be something else going on, there's no way just a switch to dual exhaust can gain that much. IMO anyway.

 

[Riccochet]

I don't know if it's placebo or what, but my engine started running more efficient at around 30k miles. Meaning I saw a jump in fuel efficiency. All other things remaining the same (oil type, tire pressures and always running 89 octane). On a recent 1200 mile round trip I saw 19.2 MPG on the highway (hand calculated) averaging 65-70 MPH using cruise as much as possible.

I have 46k on the clock now. She's running great.

 

[Rebeltaz83]

I don't know if it's placebo or what, but my engine started running more efficient at around 30k miles. Meaning I saw a jump in fuel efficiency. All other things remaining the same (oil type, tire pressures and always running 89 octane). On a recent 1200 mile round trip I saw 19.2 MPG on the highway (hand calculated) averaging 65-70 MPH using cruise as much as possible.

I have 46k on the clock now. She's running great.

A “broke in” engine will be more efficient than a “New” engine. As the tolerances in the bearings wear there is less friction on the moving parts allowing it spin more freely.

 

[HarryN]

Isn't this engine a variable cam design?

The first vehicle that we owned with that feature was a 93 bmw 525i. It tuned the engine real time to vary both the spark and valve timing to get maximum efficiency from the octane rating of the fuel over quite a range.

 

[Crusty old shellback]

So I took a drive last week. Still using base 87.
Kept the speed between 70 and 73 MPH.
Got 16.8 MPG.
Did another trip at my regular speeds of 80 to 85 MPH. Got my normal 12.8 MPG.
So speed does make a big difference in MPG, more so that the octane.

 

[Dave01]

So I took a drive last week. Still using base 87.
Kept the speed between 70 and 73 MPH.
Got 16.8 MPG.
Did another trip at my regular speeds of 80 to 85 MPH. Got my normal 12.8 MPG.
So speed does make a big difference in MPG, more so that the octane.

TOTALLY agree!

 

[MAKwagon]

Speed makes the bigger difference with these trucks, so does head wind haha Its not the most aerodynamic piece of machinery in the world, although its length can help. I have a cap on my 21 PW 75th with 37s, and the daily mileage didn't suffer at all but highway mileage took a small hit because of the giant vacuum it's creating. The thing that has affected my mileage (and braking) the most are the 37x12.5R17 Toyo Open Country M/T's, those suckers are 84 Lbs a piece! I weighed the wheel and tires on last tire rotation and they are 125 Lbs each. For reference I had 37x12.5R17 BFG KO2s before hand and they only weighed 64 lbs each, but mounted they are an inch smaller over all than the Toyos.

 

[HarryN]

So I took a drive last week. Still using base 87.
Kept the speed between 70 and 73 MPH.
Got 16.8 MPG.
Did another trip at my regular speeds of 80 to 85 MPH. Got my normal 12.8 MPG.
So speed does make a big difference in MPG, more so that the octane.

If you have time, try the lower speed test using a high end 93 octane gas and see if it makes a difference.

Something like shell or union 76, not costco / discount stuff.

It may not matter, but supposedly, those two also have the best detergents to help keep injectors clean.

You might not need to use it all of the time, but for instance if you need more range for a trip.

 

[Deleted member 47449]

Our nearest 1 pump gas station sells 85 Octane regular gas and diesel, only. No 87, 89, 91, or 93 available near our home. 85 is what our 2016 Ram 2500 4x4 PW 6.4L has always got with 129K miles on the engine with zero issues. I'd do the same with a 4.5 gen 6.4L due to filling up with 40 cents higher priced 87 octane fuel hasn't shown any improvement on the MPG or performance. 89 is not available in our region, but that's what our 2016 owners manual recommends.

Our 4.5 gen Ram 3500 4x4 HO Cummins is much more powerful while towing than our 6.4L at our high elevation with the biggest increase jump on the MPG. Glad that I didn't choose another 6.4L. The 6.4L is a great engine when I'm not towing 7,500 lbs up our local highway 10% grade at above 8K feet in elevation. It's floored screaming, whereas, the 6.7L takes it lazily.

 

[Riccochet]

Our nearest 1 pump gas station sells 85 Octane regular gas and diesel, only. No 87, 89, 91, or 93 available near our home. 85 is what our 2016 Ram 2500 4x4 PW 6.4L has always got with 129K miles on the engine with zero issues. I'd do the same with a 4.5 gen 6.4L due to filling up with 40 cents higher priced 87 octane fuel hasn't shown any improvement on the MPG or performance. 89 is not available in our region, but that's what our 2016 owners manual recommends.

Our 4.5 gen Ram 3500 4x4 HO Cummins is much more powerful while towing than our 6.4L at our high elevation with the biggest increase jump on the MPG. Glad that I didn't choose another 6.4L. The 6.4L is a great engine when I'm not towing 7,500 lbs up our local highway 10% grade at above 8K feet in elevation. It's floored screaming, whereas, the 6.7L takes it lazily.

Forced induction is a must at altitude. Gas or diesel. You lose about 3% of power for every 1000 feet of elevation with a naturally aspirated engine. At 10,000 feet that's a 30% loss of power. No wonder it was screaming on those grades.

Down here in the foothills in NC and through the NC/TN mountains I saw nearly no loss of power in my truck, and fuel economy was the same between my gasser and using my neighbors 6.7 F350 towing my travel trailer.

 

[Deleted member 47449]

Forced induction is a must at altitude. Gas or diesel. You lose about 3% of power for every 1000 feet of elevation with a naturally aspirated engine. At 10,000 feet that's a 30% loss of power. No wonder it was screaming on those grades.

The lowest elevation our 2016 Ram 2500 4x4 6.4L with 4.10-1 gear ratio ever gets down to is 4,646 feet in elevation in Grand Junction, Colorado. It will be driven this week to as high as 11,542 feet over Hoosier Pass. I felt the power loss with it only when towing and attempting to pass across the flat terrain highways of South Park which is around 10,000 feet in elevation. Higher 87 octane fuel doesn't compensate for the power loss.

Our 2024 Ram 3500 4x4 HO Cummins with 3.73-1 gear ratio was the right choice for here. 8% plus downgrades with the exhaust brake are no longer such a concern.