2nd tank of gas with Chevron Techron 87-octane: huge hit on the fuel economy

What was your DTE right after you filled the tank. That number gives you a far better estimation of true mpg over a full tank. As far as the grade of the gas on mpg, I have read someone getting better mpg on highway speed drive on the tank of high octane gas, but most of the others reported not much difference. I have used Ethanol-free Premium 93 gas on my last driving mostly on a highway on my 2020 PP, just because I happened to have a gas can full of them in my garage to use on my generator. No obvious difference from E10 regular 87 gas.

 

What is your DTE after fill-up? | PriusChat

 

No, it changes every time you fill-up. I had as low as 430 miles and as high as 580 miles. Someone reported as high as 644 miles. I don't know how you are estimating your mpg, but if you drive any distance on the EV mode, your number is off. Also, the mpg displayed on the dash is off by anywhere from 5% to 15% on the optimistic side. I have done enough times of driving 2017PP a full tank without charging the traction battery, and that is the almost only way to get semi-accurate average HV only mpg. And that number matches very well to the estimation made by DTE.

 

There is no inherent reason why higher octanes should produce higher mpgs. In fact, decades ago when my refinery relatives were new in the fuel blending business, lower octanes from their refineries would produce slightly higher mpgs, at least in the engines that could safely burn it, due to slightly higher energy density. Octane is about resistance to detonation and knocking, not about energy density. But fuel blending requirements have changed since then, and you live in a special 'CARB' zone which now imposes additional fuel requirements, so results could be different today.

When testing different fuels, beware of seasonal blending requirements, and undisclosed batch-to-batch differences. These are likely to swamp the factors you are trying to look for, and are likely to cause spurious results.

And also: has the weather changed? Colder temperatures, or any precipitation or road wetness, will decrease mpg.

Did a breeze direction or speed change? At these rarefied mpg numbers, small air drag changes produce outsized appearances.

 

Also: exactly how "huge" is your observed change?

Our American mpg scale is very deceptive, greatly magnifying smaller fuel consumption differences on high mpgs. The Euro style of fuel consumption, translated from metric to our units as gallons per 100 miles, is more descriptive.

E.g:
80 mpg -> 1.25 gal/100mi
70 mpg -> 1.43 gal/100mi, except that the metric folks keep only 1 decimal place, not 2.
Difference = 0.18 gal/100miles.

For contrast, this is about the same fuel difference as the shift from 35 to 33 mpg, not something that folks in that level would describe as "huge":
35 mpg -> 2.86 gal/100mi
33 mpg -> 3.03 gal/100mi
difference -> 0.17 gal/100mi

 

The first tank from the dealer is tricky. It may not be completely full or it may be more than full and still shows the full tank on the gauge. When you are estimating HV only mpg, it is almost impossible to use any assumed EV efficiency to calculate semi-accurate the true HV only mpg. The mpg on display is wildly off (almost always on the optimistic side), and EV efficiency can also vary a lot for each drive. I still think the only way to estimate any meaningful mpg on PP is to do old fashion way of full-tank calculation. You need to fill the tank to the auto-stop without any topping off at a specified gas pump after EV range has been depleted and drive at least a quarter (more the better) tank of gas without charging the battery thus 100% HV mode only drives. Fill the tank again at the same pump with the same grade gas. And do hand calculate mpg from the gas used and distance driven (from ODO reading or from tripmeter). With many times experiments, I have found the DTE estimation to be always very close to this hand calculated HV only mpg. However, even with this method, there are still other uncontrollable variables that affect the outcome.

 

I don't want "abouts", I want more specific and concrete figures.

I showed a computation for a 12.5% loss, a 10 mpg loss from 80 to 70, showing how even that isn't "huge". It only looks big because of our American backwards fuel economy scale. Even small changes, such as reversing a breeze direction, can cause that much difference.

 

I don't think you can (nor should you) establish a trend based on tank to tank variance in MPG, with ANY vehicle. Also, it is good to know that there aren't bigger worries in your life right now! Congrats.

 

When I purchase a new car, I calculate a fuel consumption baseline based on several gas station visits. This helps me understand how much the car actually uses vs. how much it says it does.

My old Gen4 was about 4.5% off (that is, the MFD was 4.5% more optimistic than the actual real value.)
This calculation needs several months to smooth out errors at different pumps (what actually lands in the tank vs. what they tell you they sold you), errors of the odometer (which should be pretty accurate but never perfect), and so on.
Even the speedometer can be off by quite a bit - my gen4 was about 6% off (i.e., speed reported was 6% faster than actual GPS speed); on my Gen3, it was almost 10%.

This is all to say that MFD values tend to be optimistic. The error is actually quite significant. Considering all other variables like temperature, terrain, driving style, and so on, what you consume with one tank vs. the next cannot be used reliably.
How much one type of gas/brand will be better/worse than the previous one cannot be determined unless using a very scientific and controlled approach. And that is very impractical.

You manage to determine, though (if you keep a good log of all data provided by the MFD, miles driven, and fuel pumped) the average MFD error and other trends. It will help you build a good enough statistic to say with confidence that "my car does 45mpg average" and "that gas station always pumps less than what you pay for" when talking to friends. (You have no idea how many gas stations out there cheat just enough to go unnoticed by the average user vs. being outright illegal. Prius users logging fuel consumption know best )

Most people don't really look at fuel consumption, let alone calculate it by hand. They are worried about car color, how much is a full tank of gas in $ (and maybe how often), Car Play, speed, and "oomph." That is not what most Prius users do. We tend to check, measure, and log.
IMHO, gas stations make bucks from that not attentive behavior of the average user.
As a Prius driver, you end up automatically reducing in your head the too optimist mpg value the MFD reports. You end up knowing which gas station to avoid, what effect (on average) the seasons have on your mpg, and so on.

Regarding the DTE (= Distance to Empty): I always considered it a gimmick rather than useful. It is based on estimates and grossly approximate calculations, IMHO.
DTE depends on how you have driven, but it cannot predict the future.

I ended up making a table (see example below for my Gen4 based on L and km) to know how many miles/km I had left. It is based on how much I had driven thus far, how much the current tank's average mpg is, and the MFD's measured approximation errors. I kept the printout in my glove box (especially the first few months - after a while, you get a sense of what your car does).
I also estimated that the Gen4 DTE was a solid 15% off (i.e., optimistic). On a Prius with low fuel consumption, 15% off means several tens of mi/km difference between what DTE says and what you can actually do with the remaining fuel in the tank.

So, for example (see image below), if I had already driven 700km, and the current average MFD reported fuel consumption was 4L/100km, I knew I had at very least 259km to go (assuming I kept driving the same way) before I would hit the 5L reserve. 5L was my comfort zone: considering that it is effortless with a Prius to get 20km/L, a 5L reserve would have given me at least a good 100km before ending up dry. I never ran out of gas, and my estimations were always spot on. Never a surprise at the pump of how much or how little it would go in.

 

You are correct that DTE is based on how you have driven. It is not very useful for the future drive, but I have found it to be extremely useful in estimating the HV mode only mpg from the last tank on Prius Prime since PP does not display HV mode only mpg if you drive EV mode from the wall charge.
Please see this thread: What is your DTE after fill-up? | PriusChat

 

Where can I get me some of this generic, ethanol-free, dealership gas??

 

Winter blend gas and other factors affect MPG in winter and more so on Hybrids and EVs. Maybe one of the reason Ford is delaying the release of the Mustang Mach-E and staggering deliveries for so called quality checks. Their customer base could be up in arms for poor MPG-e.

More info on EPA site regarding poor MPG in winter.

Fuel Economy in Cold Weather

 

Fuel economy in winter goes down because heating a cabin with the heat produced by a gas engine is the most inefficient way to do it. This is why the Prime has a more expensive, but way more efficient, heat pump to avoid using the engine in most cold weathers (for extreme cold, it still has to use the ICE).

It also depends where you drive most the car in winter. With constant speed on a flat highways (less effects from the cold) or in city traffic (where the engine sits idle and spends most of the time trying to keep warm, let alone heating the cabin)?

Moreover, rain and snow, winter tires, as well as darker days that require longer usage of headlights, have all a negative effect on the fuel consumption. I am not too sure that "winter blends" (are there really winter blends for gas? I know there are for diesel, but not for gasoline) are the cause of any significant drop in mpg vs. the other above mentioned causes.

As for the Mustang Mach-E owners, they already get abysmal fuel consumption on the regular Mustang. I don't really know how much attention will they give to the e-miles they get from their new electric thing, as long as it makes noise and crushes them on the seat when they accelerate to the next red light. I don't think Mustang owners ever gave much attention to fuel efficiency. You don't buy a Mustang to be fuel efficient and environmentally conscious. It is the wrong car to do that.

 

Yes there are winter blends of gasoline, forced by both driveability concerns and by EPA evaporative emissions requirements.

See Reid vapor pressure:
"The matter of vapor pressure is important relating to the function and operation of gasoline-powered, especially carbureted, vehicles and is also important for many other reasons. High levels of vaporization are desirable for winter starting and operation and lower levels are desirable in avoiding vapor lock during summer heat. Fuel cannot be pumped when there is vapor in the fuel line (summer) and winter starting will be more difficult when liquid gasoline in the combustion chambers has not vaporized. Thus, oil refineries manipulate the Reid Vapor Pressure seasonally specifically to maintain gasoline engine reliability."

According to a now-retired refinery relative who had blending duties in her early years, these seasonal blends have different densities, thus also different energy contents, thus different MPGs. (Energy content is essentially proportional to hydrocarbon mass, not volume.) The summer blends are heavier than the winter blends. The difference is enough that one year when her refinery didn't remind the tanker truck drivers of the seasonal density change, a number of drivers accustomed to filling up by volume, not by weight, were hit with overweight penalties at a nearby weigh station.