So I recently took a trip down to Buffalo and filled up on 93 Octane (Premium) since the price was so much cheaper than Canada. I found that my mileage for that tank got better. I had been averaging a fairly consistent 37 mpg but the tank of 93 Octane yielded 39mpg. Then, back in Canada, I filled up on 91 Octane (which has no Ethanol) for another tank and ended up getting 40.6mpg! I know it's a small sample of 2 tanks but it's very interesting to note. Plus, filling up on premium actually made my on-board trip computer accurate! I used to get fuel economy figures from the trip computer which overestimated by about 6-9% (basically says fuel economy was 6-9% lower than the actual). But the last 2 fill-ups, it was almost spot on. Anyone tried/experience this?

 

When I had a loaner HS250h at different times of the year I got an average calculated 40.3 mpg avg with regular unleaded 87 octane. The onboard computer registered higher at 40.8. This occurred on long distance trips from LA to SF last year.

 

When I purchased my HS the salesman told me to always use 87 octane because the car was designed to run on it. Later I found on Lexus' own web site a foot note under specifications, that stated the car would run on 87 octane, however the EPA mileage and the 0-60 MPH time were based on 91 octane and higher. I confronted the salesman with a printscreen of the note and he had no good explanation. Reading these forums, you can find owners of the HS claiming to get 50, even 60 MPG. Personally I have never observed more than 34.7 MPG. I think I have noticed a slightly higher mileage using premium gas, but I am not sure - the difference apparently is not that great. I measure my mileage by always waiting until the tank is approaching empty, always using the same pump at the same station and always fill the tank to the same level by stopping when the pump "clicks off" automatically. Only then do I attempt to calculate the mileage. And I still find differences that have run between 28.5 and 34.7 MPG. All of my calculations near the 34 MPG level (my highest) were achieved during the winter. In the last month the weather has been very hot and the calculations have reveled lower mileage - as low as 28.5 MPG. I attribute that to using the AC. I don't believe I will ever see mileages above the rated EPA estimates.

 

I have often tried to convince myself that premium gas would give me better mileage but haven't been too successful with that. I find that nothing has a greater impact on gas mileage even tank to tank than driving style. I can see it varying by up to 10 mpg just by how and where I drive on a tank. I no longer buy 91 octane just 87 and 89.

Dave Mac

 

FWIW, the July, 2011, Consumer Reports has an article about the best ways to save gas. One of their recommendations is to avoid premium gas:"If your car is designed to run on regular gas, as most vehicles are, don't waste your money on premium. It won't make your engine run any better, and the only real difference you're likely to see is about 20 cents more per gallon."

Here in Winnipeg premium gas is 20 cents a litre more than regular (if I've done my math correctly, that translates to a difference of roughly 75 cents per US gallon!). I filled the car with about 40 litres this morning. If I had used premium, that fill up would have cost me an extra $8. I'll continue follow Lexus' recommendation to use regular gas.

 

i've been documenting my gas mileage with regular and premium and my results have been inconclusive at best, although i, too, would like to believe there is a difference. quite frankly, if there is anything i perceive, is that the car tends to pick up better under 3/4 full to full throttle acceleration, and less 'kick' when the ice kicks in from electric mode. but this could be just my imagination.

 

I believe this is the deal, but I am not auto engineer. The engine is designed to maximize the energy of combustion by ensuring the combustion power curve occurs in a particular manner after TDC. Unfortunately combustion is not instantaneous relative to the speed of the engine and the spark needs to be initiated slightly before TDC in order to maximize efficiency. As long as the HS uses 91 octane, knock sensors have free reign to accurately set the beginning of combustion as needed for maximum power. When an octane of less than 91 is used the gas will ignite uncontrollably before TDC. This can be countered by advancing the spark, the sum result being that all gasolines will run at almost the same efficiency. There is no difference in the heating value of the different octane gasoilines, only the ignition points. The knock sensors can counter for precombustion, but the thing is - precombustion of low octane gasolines is not constant. It occurrs at different points every cycle and the knock sensors cannot correct that fast - they take a series of readings and make a correction, so the correction is not perfect, hence the 91 octane gasolines which will never precombust will always have the more accurate spark point and out preform the lower octane gasolines somewhat.

I think!

 

In everything I've heard over the years, premium will have no discernible advantage over regular, unless the engine requires it. I've also tried it in the past and did not detect any difference in mileage or power.

 

But it must have an effect. Why would Toyota put such a noticeable disclaimers in their sales literature:

http://www.lexus.com/configurator/#/...41075&group=HS

HS 250h Premium

* Combined Fuel Economy (City/Hwy): 35
* 2.4-Liter Hybrid Inline-4
* 187 hp (Total Output)[1]

[1]Ratings achieved using the required premium unleaded gasoline with an octane rating of 91 or higher. If premium fuel is not used, performance will decrease.

I have not been able to detect the difference between premium and regular either, but that doesn't mean it isn't a factor. I make every attempt to drive green and produce accurate mileage data, yet I see differences in my results within a range of 15% regularly. It is very hard for me to detect any single mileage robbing factor (other than driving habits) except for hot weather - when I turn on the AC my mileage drops every time.

 

So to give an update, I've had a few more tanks of various 87 and 91 octanes. Basically, I think it comes down to ethanol content. In Toronto, Regular (87) contains up to 10% ethanol while Premium (91) does not contain any ethanol. Premium is getting me 39.0-41.2mpg. Regular is getting me very consistently around 36.8-37.7mpg. So the difference is roughly 5-8% better fuel economy with premium. Premium also definitely makes my on-board trip computer accurate, whereas regular (and ethanol content) makes that figure overstated by about 6-9%. Performance wise, I haven't noticed a difference at all.

 

it is interesting to me that you get better gas mileage during a winter. Myself I have always lost approximately 2 miles per gallon turn on winter driving and have attributed that to the different chemicals added or not added to the gasoline by the company's producing the gasoline depending on the season

 

I have only owned my 20/20 gx460 for a very short time and have only used premium gas in it and the onboard computer is way off. It reads higher than I am calculating manually. However I have not tried regular gasoline in it to see if the computer is even less accurate because people are saying that premium gas produces an accurate computer estimate

 

Yes, I have also noticed this and it was a head scratcher. Your comment about the ethanol rang a bell in my head!
ethanol has a lower energy density than refined petroleum fuel. According to ChatGPT, there's 30% less energy per unit of volume, so with a 10% ethanol blend, you'd expect a compound energy density drop of about 3%. That would show up immediately in the volume/distance calculation.

This means to me, if pure unleaded costs less than 3% more than E10, it's a good value!

thanks OP for mentioning this. I thought I was insane for thinking there's a difference in how the fuels would burn.