SO how do I drive this thing?

How many miles per year do you drive? Any guesstimate on freeway vs city driving? Gasoline has a shelf life of between three to six months. If you don't fill up that often, I would definitely use a good stabilizer.

Note: Ethanol breaks down in as little as one to three months. Not sure if gas with ethanol breaks down faster than pure gas.

 

If you fill up every two weeks or even every two months, you should be fine.

 

Maybe it’s a comprehension threshold thing. I’ve a personal analogy:

Purchased a MK I Canon 5D (digital single lens reflex camera), spent a few hours reading the manual, played with the menus and controls, customized a few things for convenience, got comfortable with it.

A few years later, upgraded to MK III version, which had loads more “stuff”. The camera’s menu had evolved from a simple scroll, to a multi-page rabbit warren, the controls and complexity increased as well, and I kinda hit a wall. It’s still usable, but I spend inordinate time going back to the manual, just too much to retain, and I end up eschewing a lot of the features.

either that, or my brain calcification is progressing, who knows. Either way, the new one was an ill fit.

 

While the manual indicates the car will demand that you use half a tank a year, I am planning to force use of half a tank every three months if I have not done so from regular use.

My use pattern is primarily EV but once a month or so I usually find a reason to do 80-100 freeway miles. That should translate into using half a tank every three months, although I may fill it after each highway excursion to keep the tank full. Supposedly a full tank ages better.

If I find I haven't used HV mode in a while, I plan to let the SOC (state of charge) go to ----% and charge to 80% using the HV Charging mode. I calculated this should use about a gallon of gas, and I am curious to see how fast it works also.

 

For peace of mind, I would add some stabilizer to the fuel then and not worry about it.

 

That advice is basically correct.
Don't use more than the recommended amount of stabilizer; more is not better.

Non-ethanol gas will stay "good" a bit longer.
IF you can use most of the tankful in 3 months, Ethanol should be fine. No stabilizer needed.

IF you stretch it more than that, E0 would be better with stabilizer added.
The general consensus is that you should not try to stretch it longer than 6 months though,
stabilizer or no.

 

Read that gas loses about an octane point a month. That's in the basic plastic can used to fill the lawnmower.

The fuel tank of a car is more engineered and better sealed than that simple gas can. The exchange of air in the head space of the tank is much slower; less oxygen and humidity is getting to the gas. The loss of volatiles in the fuel to the atmosphere is a fraction what is seen in that gas can and lawnmower. The fuel tank and evap control system of a low emission car like the Prius is even better.

The engineers were aware of the potential for the gas to go stale in a PHEV. Being overcautious is part of the reason the gen1 Volt called for premium. These cars track the age of the fuel in their tanks. They will take steps to use that fuel up, and get fresh gas added to the tank. The PiP and Volt were quite 'vocal' about doing so.

I do not delve into every subforum here, but the PiP came out about 10 years ago. If gas going stale in a Toyota PHEV caused problems, it would have come to my notice like the EGR, head gasket, and coolant leak issues. This is not something to worry about.

 

I believe I read an EPA survey of PHEV drivers reported 50% EV mode use on average, which may be why Toyota doesn't mention fuel stabilizer in the owners manual, but never-the-less adding fuel stab sounds as scary as old gas to me.

 

Fuel stabilizer isn't going to hurt your Prius but as a general rule don't take advice from the dealership as they often say the wackiest things. For instance here, unless the Prius is in storage long-term - the car will automatically run a maintenance cycle so that fuel will not get stale.

 

These are not full EVs. The Internal Combustion Engine is meant to be ran somewhat regularly. If you don't want stale gas, just keep the tank at 1/4 full and drive a few miles here and there on HV mode.

 

With fuel injected engines, the difference between the blends is mostly about evaporative emissions outside of regions with severe cold. Warm weather leads to more evaporation, so summer blend has less volatiles. Winter has more, but that is likely more important for carbureted engines that rely on a more passive method of getting fuel into the air. Today, it is more about letting refineries use up the butane stocks that built up over the summer.

This could be a case of accumulated data leading changes in design. With more lab tests and the miles driven on PiPs, it maybe that the fuel doesn't age as quickly as originally thought. Then add improvements to fuel tank design, and the engineers are confidant in letting the car go longer between requiring fresh gas being added.

 

Be sure to include a "What Am I Going To Do With A Flat Tire?" plan.

 

Where exactly did you read that ?
Seems to me like the first things to go would be the most
highly volatile components.
And loss of them might actually make the octane rating go UP, not down.

 

Several articles on stale gas mention the octane dropping. The point a month came from one talking about the Volt when it was new, IIRC.
Gas isn't just losing volatiles as it ages. It is also oxidizing, and that will degrade the high octane compounds in the mix.