2012 Prius Model Four - first tank MPG

Be careful assuming how many miles you have left. Some people made similar assumptions and were left stranded. Others were luckier.

Congrats on the first tank!

 

I hear ya. I start acting like I think I know how much I have left and set off to drive another 100miles then quickly chicken out after 20 or 30 miles and hit the next gas station.

 

Are the bars fairly accurate?

I just filled up this morning for the first time with one bar left. I got 9.3 gallons in. From my math, that is only 78% of the tank... so one bar is 22% of a tank left? That doesn't really make sense to me since there seem to be 10 bars (or is it 8??), I am mighty confused!

 

The hatchback Prius gas tank actually holds more than 11.9 gallons. True capacity
when filled up to the filler neck about 13.5 gal's.

 

I guess I will just have to test it more to see! I am surprised by how little I got in, but it said I only have about 30 cruising miles left. I have a 40 mile drive to work with no gas stations (rural routes) so I didn't want to risk running out.

 

Assuming MID equals what the car is telling you, that's suprisingly close to calculated. Maybe Toyo finally caved, decided the truth isn't that bad?

 

That sounds about right. When the bar starts flashing, you have about 2 gallons left in the tank. The bars don't necessarily correlate with a volume. And cars have had a fuel reserve capacity for decades to prevent us drivers from driving until there's no gas left (it goes to "E" and you still have some gas left to get to a filling station).

Assuming you have had a proper fill up (filled up and it wasn't artificially underfilled), you can easily get away with multiplying your displayed value by 10 and you will find that you should be able to add about 10.5 gallons of gas. Thus, if your displayed mileage is 50.0 miles per gallon, you should easily be able to go 500 miles on that tank (note this will adjust as you drive). If your displayed average is 50.0 miles per gallon and you've driven 500 miles, when you fill up, you should add about 10.5 gallons of gas. You'd still have about 1.5 gallons (and about a 70 mile range).

11x the displayed value is very close to running out of gas. If your display was reading 50.0 mpg and you had 550 miles on the tank you better be close to a gas station, very close.

 

My only worry is getting an underfilled tank without realizing it. I know some gas stations (for some reason or another) have an auto-shut off well before being full.

Case in point, I filled up at a Shell gas station about two weeks ago. I was surprised how little fuel I had to put into my Prius, but every time I tried to "top it off" it would click off almost immediately. I was like okay... that's fine. Not more than 10 miles away from the gas station and my first pip of gas was already gone, despite a Cons. on my display showing roughly 46mpg at the time.

This also screwed up my MPG hand calculations, as I noticed a 1-2% difference between hand calculation and display when I had the underfilled tank. When I went to go fill up again the difference was almost 10%.

 

Precisely why you should keep records and expectations of fuel ups (Fuelly is a good resource for this). Once I got comfortable with my car and understood my displayed vs calculated differential, I could predict within +/- 0.1 gallons how much gas I would put it. If it shut off sooner than my target value range, I knew it was likely underfilled. I get more panicky when it adds an additional 0.3+ gallons to my tank, because of the fear of the tank overflowing and dumping gas onto the ground.

The pumps can vary with handling and damage. I've had some pumps that refuse to put gas in while others fail to recognize the tank is nearly full and overfill it. But must of the pumps I've used work well. I do try to limit the variability by going to the same gas station and trying (but not always succeeding) in using the same pump.

This is why I always try to remember to qualify my "10x" rule with the assumption that you had a normal/complete fill up. I'd hate to have someone blindly follow it after getting an incomplete fill up and unknowingly put themselves at risk of running out of gas. Still, with a 1.4 gallon reserve, it has to be quite the underfill.