91 vs 87 octane towing performance

[Johnny_H]

Just had a really interesting trip towing my boat. I tow a 4000 pound pontoon - so say 5-6000 pounds loaded with gear gas and trailer - but the main issue is it’s a giant wind sail. 3.92 limited with e torque.

Anyway I left town, full tank of Costco 91 gas (because it’s cheaper than most other 89). Towed around 400km and averaged around 23L/100 (10 us mpg). Doing 110 the whole way, truck was almost always in 7th - sometimes even 8 - and no wind. I was pretty happy with this as my 17 was usually around 26-28 towing.

Stopped for gas in some ****hole town and only had 87 so I filled up anyway as it was shell at least. Within 5 minutes I noticed the truck wouldn’t go into 7 and was pretty much riding in 6 even 5 rest of the trip. Computer tells me I’m doing 28.5L/100 (8mpg). Stayed constant whole trip. Similar geography. No wind. Truck felt just weak and definitely used more gas. I can’t think of any other real variables that changed. I topped up with premium at around half a tank and it seems to be running better again but I’m only about 50k into this tank.

Anyone else notice this? Does 89/91 really perform this much better than 87 - especially towing?


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[SD Rebel]

The truck is optimized for 89, but 87 is fine. However, you are putting more strain when towing, I would highly recommend you go with 89 or Costco 91 (since cheaper than regular 87) if you tow or are looking at the best performance for your engine.

I would run 87 if I was cruising along on the highway, but towing, go with 89 at least.

 

[Richard320]

Anyone else notice this? Does 89/91 really perform this much better than 87 - especially towing?

Firsthand experience - no. Understanding why this is - yes.

The knock sensors detected pinging so they retarded the ignition timing. Probably a lot. Retarded timing will cut the horsepower. Less horsepower means lower gears to move the load.

 

[IvoryHemi]

Yes, 89+ is the way to go for towing

 

[theblet]

Yup 89 for towing heavy loads, gives you the max HP/TQ. 87 is fine for normal driving

 

[silver billet]

89 is the way to go all the time

Just for that reason. It's easier to spot the difference when you're working it hard, but that difference in power will be there if you need to pass somebody as well. I just don't see the point in spending $50,000 on a truck and trying to save 3 bucks every fillup.

And if you have a hemi that pings like mine, the more you can reduce pinging the better. It will ping even on mild acceleration with 87.

I just don't see the point. Our hemis crave 89+, regardless of what the manual says, that much is obvious.

 

[Bt10]

I tried 91 octane and got about 8-9 mpg pulling about 6000lbs total load. I could see 2mpg at WOT on the freeway accelerating tho!

 

[Quint]

I run 89 most of the time because I seem to be towing most weekends, or even tonight when I'm going fishing on the Mississippi with friends.

Like you said, pontoons are like pulling a sail and cause a lot of wind drag. It's not the weight at the wheel at that point but the additional power required to deal with the drag. When towing I sometimes run into the stations that only sell two types of gas - 87 octane with ethanol or 91 octane (non-oxy). I'll fill up enough to get to the next station and then get some 89 in there.

 

[Orion10182011]

Even with 89 I'd bet it's knocking and pulling timing. When Towing I'd be putting 91-93 in.

 

[Quint]

I am not an engine expert, so don't take this as argumentative. However, don't you think Ram would advise owners to use 91/93 when towing if they felt that would extend the life of their product? I thought that to properly utilize higher octane fuels, the engine needed to be tuned for a higher detonation point.

It probably also depends on the load. There is certainly a difference between not towing, towing small boats/trailers, and towing close to the max towing capacity.

 

[Orion10182011]

10.9:1 compression. That's a lot for 89 octane under stress IMHO.

 

[RAM Patriot]

The HEMI is 10.5:1 compression ratio.

89 octane fuel is recommended for optimum performance and fuel economy by Mopar.

Fuel Based On Compression Ratio

87 Octane
Gasoline with this octane number is proper for engines with compression ratios of 7:1 to 9:1.

89-90 Octane
Gasoline with this octane number is good for engines with compression ratios of 9:1 to 10:1.

93 Octane
Gasoline with this octane number is advisable for car engines with compression ratios of 10:1 to 11:1.

Based on this information our trucks will run optimally with 89-93 Octane fuel. Without having to advance or retard the timing.

 

[ALT_F13]

From my experience, I have +1 MPG with Costco 91 comparing to 89 at Safeway or Arco. Since price is the same, Costco 91 is a no brainer.

When using any 87, I can hear a tick (not loud, but noticeable) when engine switches to 4 cylinder mode ("eco"), but it quickly disappears after filling the tank with Costco 91.

And yes, I really want to save ~$10 on every fillup (30-40 cents difference between 87 and 91 at Costco, filling 30 gallons into 33 gallon tank), but then I remind myself that it is a truck, it needs a lot of gas anyway, and I have Tesla Model Y in my garage for daily driving that consumes $9 of electricity for 300 miles of range.

 

[Johnny_H]

You guys are convincing me to stick with 91! I save almost $30 a tank filling with Costco 91 vs shell 89 so that’s a no brainer, but next time I’m out on the road pulling I’ll spend the extra $10 for 91 instead of 89. It’s a 90k truck I really don’t mind spending $20-30 a month to keep it running better, and really if it even gets 1mpg better it’s basically a wash. If I really wanted to save money I could cut my speed by 10kph and save money that way. Or buy a Tesla.

Fun fact when I filled my truck it cost $120…..then I filled my boat for $400. Makes you care less about the 89 to 91 cost difference.


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[silver billet]

I am not an engine expert, so don't take this as argumentative. However, don't you think Ram would advise owners to use 91/93 when towing if they felt that would extend the life of their product? I thought that to properly utilize higher octane fuels, the engine needed to be tuned for a higher detonation point.

It probably also depends on the load. There is certainly a difference between not towing, towing small boats/trailers, and towing close to the max towing capacity.

Sadly it appears the answer is "no". FCA has made decisions that benefit it over us, the other major one being oil weight. We need to be running 5w-30 in our hemis but they need to get their corporate MPG up so they picked a watery oil to eek out a little better MPG.

Sometimes the politics or bean counters get in the way. No doubt the engineers would tell us what is best if they could, sometimes you need to read between the lines or look at past history/decisions or talk to other engineers etc.

 

[Redfour5]

I tried 91 octane and got about 8-9 mpg pulling about 6000lbs total load. I could see 2mpg at WOT on the freeway accelerating tho!

My mileage 91 octane with 3.21 and 6600 travel trailer was 8 on the nose mountain driving 300 mile trip 70 mph. BUT, the 3.21 gears did great for all you doubters. Plenty of power The only weakness was the Falkens...too soft "wallowing" been there done that before and KO2'S solve that and only 900 bucks...on sale. Anyone want a deal on 20 inch falken AT's... great tire if you don't tow.

 

[Johnny_H]

I was a huge believer in the 3.92 with my 2010 Ram and the 5 (really 4) speed transmission. With an 8 it’s basically obsolete and you’re just losing the extra overdrive of the 3.21 and the truck will find a gear to match any load between second and seventh with either diff. Nobody is struggling to pull anything in the first few gears that really necessitates the 3.92 anymore. I would have taken the 3.21 if it was available for me but it wasn’t a deal breaker either way. Just my opinion, but the 3.92 has been obsolete since the new transmission.


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[Bt10]

My wife asked if something was wrong when we stopped on a hill at a stop sign. When we started to go with the 6000lbs loaded trailer, the tires squealed for quite a few feet as I pulled away gently. See sig line.

 

[Bt10]

Seriously, my kid has a 3.21 and gets 20mpg driving very conservative and I have the 3.92 and drive aggressive and get 18mpg. I had hoped Ram would've put more spread in the total ratio, and would've eliminated some axle choices to save cost, but they didn't. I haven't needed 4 low offroad yet, and with a ecu reflash, it should get better.

 

[jvcnova73]

I have 2020 with etorque 3.92. I get 10.5 pulling my 33‘ TT that weighs 6300 dry. I get 18 driving around Indianapolis. I only use 87. Never heard any pings. Overall pretty happy