Ran my car out of gas yesterday. Here's what I observed. - GreenHybrid

foo monkey · #1 (**permalink**) 09-21-2005, 07:03 AM

I ran it out at 534 miles, with the display reading 47.0 MPG. If you do the math, you see either:
-The display is incorrect. On a full tank (11.9 gal), 534 miles equates to 44.9 MPG. At 47 MPG, I could have gone another 25 miles.
-The last time I filled up my tank, I somehow missed 1/2 gallon, which is a fair amount of gas.

When the low fuel light went off, I was able to go another 64.5 miles.

When the ICE cut out and I was on electric only, I was only able to go another mile or so. I could have nursed it a bit further, but the point is, you can't go far on electric-only.

I'm disappointed by the discrepency between the displayed and actual fuel milage. the 2.1 MPG/ one-half gallon difference is not insignificant. Any thoughts on why this happened?

tcampb01 · #2 (**permalink**) 09-21-2005, 07:39 AM

The car is probably not wrong. Each time you fill your tank there's a fair margin of error. You never really run your tank down to bone-dry. In fact even when you ran out of gas all that means is that the remaining fuel in the tank wasn't able to be slurped up by the car's fuel-pump, but I'll bet there was still some in it -- and I'm not talking about a tablespoon or so, I'm talking like maybe a whole pint... SURELY at least a half-pint. There's no drain on the bottom of a tank.
The pump system always mounts from an opening in the top-side of the tank (otherwise the tank might leak gasoline on the ground) -- and it either sucks the fuel up like a straw in a soda can. Since the bottom of the tank is a lot bigger than a soda can, a mere 1/8" of fuel covering the bottom of your tank is actually a fair number of ounces.

Same is true when you fill. The gas pumps cut off when the fuel splashes back at it. The splash back is caused by the fuel not being able to leave the filler-neck fast enough. I don't own a Prius, but if the filler is on the left (lets just say it is to make this easier) and the car is not sitting level but leans slightly to the left, then there's going to be an air-bubble on the right side of the tank -- probably a good pint of fuel. If the car leans the opposite way then you'll get that extra pint to fill. So there's a quart of fuel (2 pints) and that's by rough estimates only.

The filler neck actually holds a pretty fair volume. The tank capacity is NOT supposed to include the volume in the filler neck. But you might stuff another quart of fuel in that neck. Depending on the sensitivity of the gas-pump's cut-off sensor, you'll get some margin of error there as well.

If only the tank was a giant glass graduated cylender (like a chemistry lab or a measuring cup) then you could be sure that each tank was filled to precisely the same level.

Point is, you can't really trust the measurement of a single fill-up to be accurate and this is why it will disagree with the car's computer. If you average many many tanks over time (knowing that some tanks will error on the high side and some on the low side and in the long run it should balance out) then you should get a number that's closer to the car's computer.

That's my guess as to why your computer and tank fill-up disagree with each other.

Best Regards,
Tim

Schwa · #3 (**permalink**) 09-21-2005, 08:22 AM

The MFD display is a bit slow, and I suspect that this causes it to 'miss' some fuel consumption, so at best it's still an estimate. Most people find that it's a little on the optimistic side on average, but it can slip into the pessimistic, just depends what it "sees" and what it "misses" since it's just sampling data from the car's network of devices every so often.

zadscmc · #4 (**permalink**) 09-21-2005, 12:53 PM

Good analysis, Tim.

Monkey · #5 (**permalink**) 09-21-2005, 04:02 PM

Yeap. For those who have kept track of both calculated and screen for a year or so reported the difference being like .1 or .2 mpg. It is rare to get the full 11.9 gallons in (at least when you stop at the first auto shut-off). In 44 tanks I have not put in 11.9 gallons (I have gotten close 2x). Some on other boards recommend a 10X screen mileage to estimate how many miles on a tank in the summer and 9x screen mileage to estimate the miles on a winter tank.

galaxee · #6 (**permalink**) 09-21-2005, 04:42 PM

keep in mind calibration at the gas pump. they're always going to tell you they're selling you more gas than you really pumped into the tank.

i personally trust toyota a lot more than the gas station.

kirby · #7 (**permalink**) 09-22-2005, 08:41 PM

I don't understand why Prius owners have this thing about trying to drive on the last three teaspoons of gas. Why not just fill it up when it gets down to two bars -- then you will never have a problem.

Schwa · #8 (**permalink**) 09-22-2005, 11:36 PM

It's a psychological thing, people "compete" to see how far a tank of gas will take them. It is silly, but somewhat understandable.

coz6 · #9 (**permalink**) 09-23-2005, 12:10 PM

Glad I am not the only one to run out of Gas. When I ran out I still was only able to get 9.8 Gal in. So I gave up trying to calculate Gas myself and just trust the computer. I only got 16 Miles on my Flashing Empty light... So just be careful. As another user said you have to know your terrain, if you go up a hill the car will say you are out of gas sooner then if you are on a nice flat road.