How to run out of gas

I've only ran out of gas one time in my life -- it was my first car, a 65 Impala, and I let it run to melt the ice, so I didn't even run out of gas on the road, and it was an easy fix.

That said, I *always* drove my 2010 Prius until I had 0 miles left according to the gas gauge, yet I never put in more than 11 gallons. I don't know if I'll do the same with my Prime, but it's kind of likely -- I hate taking the time to fill up any more than I have to. And now with my Prime, I know if I can get it home, I can "fill" up with EV and not have any sort of problem getting to an open filling station.

 

'Safe' means different things to different people. Airline pilots would not tolerate the fuel gauging uncertainty that is common in passenger cars.

The first car I actually owned, the same one in which I experienced that fuel extortion attempt, had a range of about 250 miles. I had previously run it dry, with the needle still above 'E', before learning that a disproportionate amount of its tank capacity was on the top side of its gauge.

A later car was very much the opposite, with a very disproportionate amount of its fuel on the bottom side of the gauge, much of that even below 'E'. At the same gauge point where that first one went empty, this one easily had more than 3 hours of range remaining, and I needed that to get past the high price traps on my common route. But many other drivers who don't characterize their tanks would feel very 'unsafe' because the needle was getting low, and end up paying tourist trap prices to top off long before necessary.

Fortunately, my current cars have better fuel gauges. But old planning habits don't die.

Some members here (e.g. Mendel) live in areas (e.g. BC) with little price disparity -- every station in town has the same price, and adjacent town are within a couples cents of each other. So there is essentially no cost penalty to refilling early and often. But I live in a state with very wide price spreads -- where I filled up today, $0.60/gallon spread in a single block, and $1.00/gallon within two miles (Costco @ 2.39, 76 @ 2.99, Shell at 3.39). My long trips often have comparatively few places to refill, so I will plan those stops as much as 500 miles ahead. Or harvest good prices much earlier.

 

I get the academic exercise of determining when we might expect to run out on each new vehicle. It's nice to know what happens in this vehicle if you do. Beyond that, I never have myself. I know I've got to fill up every now and again anyway so why not do it while there's still 1/4 or 1/8th of a tank left. Running out for real is not worth the hassle. That's just me and everyone lives life differently.

I'll never forget what I saw when I thought the electric fuel pump inside the gas tank in my 1972 Vega was bad. Had to empty and drop the tank. Pulled the pump out and the neoprene sock over the pump intake was completely coated with water. The water surface tension on the fine mesh was stronger than the pump's ability to suck adequate gas through the sock. Removed sock and blew it out with compressed air. No more problem.

Water is heavier than gas and will accumulate on the bottom of the tank, If you run the tank low, any water in the bottom could cause what I described above. Every since then, I don't let it go way low, especially after the car is less new and has had more gas run through it.

 

I just got my first fillup and the gas mileage reported by the car is very slightly less than my calculated mileage -- that's quite a bit different than my Gen3, which was always 4 or more MPG less than calculated. That's a good thing to know. fwiw, it was 87MPG, which is a bit higher than I calculated it would get given the Prime's specs and my commute.

 

Some station pumps click off sooner or later than others. So even if you stopped filling exactly the same way each time, there's bound to be some variation in computed MPG and displayed. Some of my Toyotas have consistently run 10-12% off either way, while others have only been in the 2-3% range. I've suspected sampling rate frequency may be different on the different types of Toyotas I've owned.

 

Could be a vagary of the Prime? Still, I'd strongly suspect it's a "first fill-up" aberation, and next tank it'll be back to normal, that is: the dash displaying 5~10 percent optimistic. There's somethings in life you can always count on.

 

We'll see in a couple+ weeks, but I can't think of any aberration on the first tank that would do that.

 

I try to fill up before the low fuel warning comes on, and treat the last few gallons as an emergency reserve. I'm both paranoid about running out of gas, but also try to get as many miles out of a tank as possible. My cars spend the majority of their time above a 1/4 tank. I used to have a Chevy Cavalier, and it's fuel pump failed at about 80K miles.

My wife used to have a Mazda3, and when the fuel light came on she would start thinking about when to fill it up. That car's fuel pump was still original when we traded it in at 120K. Ironically, this was the car with a note in the owner's manual cautioning against running below 1/4 tank!

Anecdotal evidence says fuel pump failure (or not) has little to do with driving while low on gas, but rather has to do with whether the manufacturer was trying to squeeze the last few cents of costs out of the part.

 

They filled it up to the neck maybe? Or it was filled after a trip meter reset, or whatever. I'll put money on it being an aberration, lol.

 

I've found that after 241 fill ups in my 2012 v the reported mpg's to be 5.31% optimistic, 51.6 reported vs 48.86 calculated. Interestingly in my Prime, after 7 fill ups, it's 5.49%, 114.8 reported vs 108.5 calculated.

 

This is a point I must emphasize: because of that very early experience with a car that had no reserve at all, it went dry with the needle still above E, I refuse to take it on faith that there is any emergency reserve at all. That is why I specifically test the lowest reaches of the fuel gauge, at times and under conditions of my choosing.

I've intentionally run only two cars to actual fuel starvation. The others were run progressively lower, beyond the last gauge ticks and warnings, to get close enough to their advertised refill capacity to satisfy my curiosity. And all these tests were done at my chosen convenience, not at times when they would complicate other adverse circumstances.

I don't run really low as a standard practice, only just enough to remain confident that I could do so when needed.

 

For me it'd be sufficient to just fill up at a moderately low level, then extrapolate what empty would be, from capacity spec and how much was required to fill. One vagarie would be how full it is at their spec volume. Maybe first shut off?

 

After that early 'empty while still above E' experience, I refuse to extrapolate anything below 1/8 tank without having actually tested it. I.e. if I haven't used it before, then it is presumed to not be there.

 

Sounds like that experience understandably scarred you for life Do you remember if when you ran out with the needle above E, if it took the full rated capacity of the tank to fill it? In other words, was the fuel gauge wrong? Or was there some other issue such as the fuel pickup not in the bottom of the tank leaving a few gallons stranded? The latter is the more devious problem, because you'll only know about it after the engine sputters and dies.

 

Interesting! I'll see if mine is the same later, but there is one thing better with my Prime, the reported gas mileage is almost 10MPG higher than my 2010 (in HV only). My 2010 was a V, so it had OEM 17" wheels, and I think that hurt MPG more than just a little.

In any case, I could have gone a full 900 miles on my first tank, so the old 2010 isn't even in the same ballpark.

 

Our 2010 is like US V, 17" wheels. Makes you try harder, lol.

I like the steering feel better: we test drove both versions

 

Some questions about how the Prime figures out the "miles left to go on gas"...

(1) The "miles left to go with gas" constantly changes/adjusts, as the car notices the current MPG you're getting, correct? (As opposed to always giving you the same formula regardless of how the MPG has been so far)?
(2) If so, then the MPG history that the car looks at to figure your "miles left"... does it reset each time you fill up? Reset the trip odometer? Or does it take into account all your MPG from Day One?
(3) And I assume when figuring it out, it only takes into account the MPG while in HV mode (not EV mode?)

Wondering, because here's what happened yesterday:

The "miles left to go with gas" indicator on the Prime showed 119 miles left when driving home (about 40 miles yesterday, all HV). The first 30 miles of it were in heavy traffic (generally moving, but 15-35mph) and after driving 27 miles in that kind of traffic, it only deducted 18 miles from the indicator (showing 101 miles left). But then for the last stretch where the traffic was moving fast (65-68mph), it started ticking down faster/normally.

So now it shows 90 miles left.

(4) It looks like if my next trip were to also be in better-MPG speed traffic (35mph say, as opposed to 70mph) I would probably get over 90. But if my next trip were to be at 65mph instead (all 65mph, but not over 65mph), would I be "guaranteed" at least the 90 miles, or could it be less, like 70 or 80?

In other words, could this (pretend) scenario happen: My car is still relatively new. Pretend ALL the HV mode driving I've done to this point has been at better-MPG speeds (ie, 35-45mph), never 65mph. And the display shows "90 miles left on gas". Say I want to take a 90 mile trip -- but for the first time now, will be able to drive at 65mph because it's late at night with no traffic. Would the car actually do the 90 miles, or would it only get 70 or so before it shows "0" (because I'd be driving at less-efficient speed than has been the case up to then)?

I know the "miles to go" figures are just guides, not precise, but I'm curious as to how it figures out that approx. range...

Thanks!

 

I've never ran out of gas in any vehicle... with that said I'm definitely the 'I know my car' type.

 

+1

 

Me too on both counts. I once put 11.6 gallons in my 2012 v's 12 gallon tank. No worries.