An old argument, and time to give my input. Here I sit in the local Toyota dealership getting my first oil change as well as installing window tinting on the front side windows. Purchased my '09 GX470 back in mid September and decided to do a little "test" of gasoline grades. For the first 1,300 miles, I filled with premium fuel, (from Costco). This was from purchase date to early November. At that time, I switched to regular fuel, (also from Costco), for the next 2,955 miles, (to current).

My driving style is “mild”. I never ‘jack rabbit’ start from a green light or stop sign, and usually keep to the speed limit, or slightly under. Hardly ever over. Let my foot off the accelerator pedal when I see a red light or stop sign ahead. Mix of driving is mostly on the prairie, (few stops, mild speeds, (35-45 mph)). Maybe 20% urban driving, but rarely heavy traffic. Some freeway driving, maybe a once a week for a few miles. Two monthly freeway trips to Drill, (~50 miles one-way, mostly freeway). Here are my results:

Premium gasoline. From odometer 165591 to 166901. 1,310.2 miles with 74.861 gallons yielding 17.5 mpg average. Autumn weather, so mild temps. Car display mpg optimistic by about 1.5-2.0 mpg.

Regular gasoline. From odometer 166901 to 169814. 2,955.2 miles with 176.385 gallons yielding 16.8 mpg average, about 0.7 mpg less. From early November to now, so temperatures much less. Some snow on the roadways, and many temps at freezing, or thereabouts. Car display mpg the same, (i.e., 1.5-2.0 mpg less than actual).

Conclusion: It may be possible that the colder temperatures are the reason for the lesser mpg numbers. Price difference between premium and regular at the same station is about 50 cents a gallon. No discernible difference in car performance. No knocking. No difference in power, (again, I’m not an aggressive driver and haven’t towed my Aliner camper since starting the test). For now, I’m sticking with regular fuel.

 

Octane is not the only thing "in" gasoline.
What % ethanol?
Weather plays a role.

When it's colder outside the engine itself runs more fuel efficient, however, driveline grease takes that efficiency gain negative.

As long as the only change in the gas is octane, you need to run the test for the same months, so something like Jan1 to Jul1, then wait until end of year to run it again using the other gas.
Gasoline will be changing though, it's not exactly the same stuff being delivered. I don't know Costco gas too well but many places will buy off the wholesale market and the actual refinery may vary.

Summary: The 0.7MPG diff can be due to many things, not just the octane you pumped in.

I run 87 in my 470.

 

Fuel is also different with a winter blend formulation in colder months. Winter blend has lower mpg

 

No idea if Idaho runs a winter blend, but given their snowfall - I would guess they do.

OP - can you repeat your test this summer to eliminate that?

Chip H.

 

I have had a 2004 and now an 09, I have tracked the difference a few times I’ve the years…definitely get maybe 1 mpg more when I use 93 octane. That’s it…I suspect when using the recommended higher octane you get the HP that the truck is rated at…lower octane might retard the timing and decrease HP…just a guess. I know my Mazda CX-5 has two different HP ratings for 87 and 93 octane.

 

I can certainly do that. That is if I still own the vehicle. I have my eye out for a late model 460 and if I snag one with the exact configuration I want, I'll be getting rid of the 470.