Gasoline mileage

We were on a road trip with interstate driving. I pulled into a Shell station and there was a note on the pump that the only fuel available was the premium 93 octane. I pumped in 10 gallons.of the 93 octane. Our gasoline mileage went up from a little over 25 mpg to over 30 mpg. On the next gasoline stop, I pumped in 10 gallons of 87 octane and the mileage dropped back to 25 mpg.
While I liked the higher mileage on premium 93 octane, the price difference over 87 octane doesn’t justify the cost. The owner’s manual for the Sienna allows for the use of 87 octane. My other concern is that using 93 octane may get the Sienna spoiled. Mrs. Triedaq bought expensive food for our dog and now the dog turns up his nose and won’t eat the house brand we had been feeding him. I don’t want to have the same problem with the Sienna.


My car allows 87 but recommends 93, but if non-alcoholic, 90 octane is the same price or less, it gets that on occasion.

My dog considers being fed any dog food as animal abuse😁


@Purebred Before we were married and Mrs. Triedaq and I were going together, I was buying dog food for the dog I had. The dog thrived on the house brand and while we were in the grocery store, I pulled the house brand off the shelf. The future Mrs. Triedaq said, “You feed the dog the inexpensive food?”. “Yes”, I replied. “He is just a little cheap dog I got an the pound”.
Some woman customer overheard me and really gave me hell and accused me of animal abuse. That has made me so paranoid that when I fill up my.vehicles, I pull up to the farthest pump so nobody can see what I am pumping into the gas tank. I don’t want some motorist to yell at me and say “You put that cheap stuff in your gas tank?”. I don’t want to be accused of vehicle abuse


I don’t think advancing and retarding the timing has this much of an effect, so I think you have a measurement error here. I’d be curious to see what you observe if you run a few tankfuls of premium.


@lion9car I just noted the readout on the mileage indicated on the dashboard. I was going the same direction on the interstate and did not reset the mileage indicator on the dashboard. I find the dashboard indicator on the Toyota Sienna is very close to what I calculate by dividing the distance traveled by the gallons of gasoline used.
Some years ago, I tuned up the Ford Maverick I owned and set the timing a little too advanced. I was making a road trip and was getting spark knock. At the first gas stop, I filled the tank with premium. That took car of the spark knock. The miles per gallon increased about 3 mpg and the performance was better. After the trip, I retarded the spark because most of my driving was around town.
The Model T Ford had a spark advance control on the steering column. Today’s computerized spark advance is better than the good old days.

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How much more is 93 octane over 87 octane where you live? Here, it’s $0.50, so that mpg increase would pay for itself as long as 87 octane cost $2.50 or more.

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Perhaps the 93 fuel had less or no ethanol.


+1. That was my first thought too.

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Well, the pumps do state up to 10 %.

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Academic questions:

Other than the cost difference, is there anything that higher octane would do to damage an engine designed for 87 octane, such as valves, rings, etc, or result in higher temperature, more wear, speeding tickets…etc?

Are there any negative effects on the catalytic converter from running 93 octane when not specifically needed?

Is it easier starting in extreme cold conditions (think North Dakota or Minnesota), or could it harm an engine running in extremely hot weather on sustained uphill?

ALWAYS enjoy your posts! Thanks for the clever humor!

Academic Question - Was this supposed to a joke post ?

The owners manual and service manual state: “87 octane or higher”, 93 octane is acceptable. This engine has a 10.8 to 1 compression ratio.

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When it comes to reading @Triedaq 's posts, all I can say is your mileage may vary. :laughing:

Our Audi is flex fuel with an ethanol sensor. The car is premium fuel only and I consistantly read 9% ethanol whenever I check it, FWIW.

I’m no expert, but I’ve never heard of any such issues caused by using a higher octane than necessary.

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Back in the late 1960s, one of my colleagues owned a VW microbus. He had to have a valve job when the microbus was relatively new and did not have many miles. He blamed the problem on the Clark gasoline he was using. Clark gasoline only had one grade and advertised that it sold premium gasoline for regular gasoline price. My colleague claimed that the gasoline burned too hot for the engine and burned the valves.
I couldn’t buy this explanation. The higher octane has more resistance to burning to prevent spark knock and therefore wouldn’t cause the valves to burn out. My theory was that the valves were out of adjustment and weren’t closing all the way which caused them to burn out.

There was lead in most gas (except Amoco, as far as I know, and white marine gas for boats) in the 60’s and maybe Clark’s did not have lead. I believe the lead helped prevent valve burning. Please correct me, politely, if I am wrong.

All car gas pre-unleaded had lead after 1950 or so I think.

Not quite wrong; not fully right.
I believe lead content started falling in the early 60s, about the time of the Clean air act.
At the same time engine makers started using hardened valve seats.
In the late '60s I believe Amoco/American premium was the only lead free gas readily available to consumers.
I doubt the owner of a microbus would pay the high price of $0.41 per gallon (in my area).
Plus, the air cooled VW needs frequent valve adjustments, every 5,000 miles recommended IIRC.
One of my college roommates learned that the hard way back in the late '70s.