fuel gauge, mpg vs mfd

[JBL]

I feel like this has to have been asked before in some way, and I've searched through these forums looking, but I'm still confused. I've had my Prius for about a month now and I just filled it up for the second time.

I was running at 460 miles on the tank, according to the MFD I was getting 49.2mpg since I reset it when I filled up the tank last time. I figured I could probably make it to 490 easy as that'd be 10 gallons. Right then, though, the last pip on the fuel gauge started blinking, having read here that that means "get gas now," I got off the highway and filled up. Much to my surprise, the pump stopped at 8.78 gallons (52.39mpg) which means there seemed to be a lot more gas in the tank than it seemed to think or the pump was being overly conservative.

I've read about the bladder in the tank and how it's hard to know exactly how much gas you've got left. I thought, hey, I'll drive it for a bit and then try to fill it again and see what happens, so 18 miles later (I live in Northern California, so that's a short drive), I filled up at a different gas station and only got 0.122 gallons in before it shut off (that's ~140mpg which seems ridiculous) but suggests to me that the tank actually was full.

On my first tank, the MFD MPG calculation and mine based on miles traveled / fuel pumped were spot on, so this is leaving me a bit confused. I'm wondering the following:

1/ Is the general consensus to trust the MFD or how much gas you're actually putting in?

2/ My guess is the answer to #1 is to trust the MFD, but that means I had plenty of gas left when the blinking pip started, which means maybe I could have gone a bit longer. I know the gauge is notoriously inaccurate, but this seems a bit much.

Anyway, any help un-confusing me would be appreciated. Thanks,

-jbl

[JanTheMan]

I have been asking the same question since owning my prius. The book says that the gas tank should be 11.9 gallons. I drove till the last PIP lit and just started to blink, and as I was pumping it stopped at 8.2 gallons, but then I slowly pumped in more a little at a time till it came up to the top and saw the gas and I put in a total of 11.5 gallons of gas, I did 482.2 miles and that gave me 42.104 mpg, but my MFD was telling me that I was getting 46 mpg, so now I know not to rely on the MFD as it is not accurate and misleading and of course if you do all that mileage and only put in the 8 or 9 gallons you're going to believe that your getting all this high mpg just because a computer is telling you so. Since all the questioning and controversy I now have my prius back at the dealership to make sure that everything is working the way everyone else says it should. I even made copies of my gas fill ups and told them to do the math. Hopefully I will have an answer tomorrow from the experts.

[ceric]

Several things to consider here:
- pumps of different stations click off at slightly different point. The click-off method to measure MPG is only accurate over several tanks, not just one tank unless it is the same pump. (Let it click off twice to be sure.)
- MPG measured by the MFD (actually through OBDII monitoring) can be off by 5% also.
If you buy a ScanGuageII for example, it asks you to calibrate it so that you are seeing the "realistic" mpg instantly. That means OBDII measurement is relatively accurate but not absolutely.
Just my two cents.

[qbee42]

There are probably about a thousand posts on PriusChat relating to the exact questions you ask. It may be the number one question, followed by "what is B mode?" Since it has been answered so many times before, I won't post a long answer, but I'll give you the short version:

1) Fuel tank capacity - The North American Prius models come equipped with an internal fuel bladder, which reduces evaporative emissions. This bladder reduces the 11.9 gallon gross capacity of the fuel tank down to something around 10 gallons, give or take a gallon. The actual capacity varies with tank temperature, age, and gas station fuel pump. Try to forget that you ever read about the 11.9 gallon capacity and you will be a lot happier.

2) MFD verses calculated fuel mileage - For the same reason that you never actually know the real capacity of the fuel tank, you never really know how much fuel you burned in the last tank. Because of this, tank to tank manual calculations are meaningless. For short term mileage, trust the MFD. It's pretty good, and usually reads within one or two mpg of long term calculations. If you want to do manual calculations, you need to do them over a large number of tanks to reduce the uncertainty.

Tom

[a priori]

Tom has given you a very solid summary. I suppose we could say there are too many threads started on this topic, but the obvious reason is that there is so much confusion about the bladder and its "changing" size.

Here is a great thread from a few months back: MGP Variance Between Actual and Computer. (I think the OP's bias shows up a bit in the title -- my view is that the MFD is the more "actual" of the calculations.) In this thread there was a great discussion about how the manual calculation of MPGs tends, over time, to get much closer to the MFD numbers. It also appears that the MFD typically shows between 1 and 2 MPGs higher than the hand calculation.

As far as my numbers are concerned, at about 15,000 miles my MFD calculations add up to a lifetime MPG of 51.8, while the hand calculations show 50.4 -- a difference of 1.4 MPG. Just a couple of thousand miles back, the difference was 1.5 MPG. Perhaps by next year it will be closer to 1 MPG difference?

[JimboK]

[Deleted -- spam report, fixed by mods.]

[autoxic]

The MFD MPG is close enough. After 127,000 miles, I calculate 48.4 true MPG, while the MFD has averaged around 49.4. The only problem is that some gas pumps shut off early, throwing off your calculations. Over time, the averages converge.

You might be able to drive a while after the last pip starts to blink. I have gone 60 miles past that point. DO NOT RISK IT, unless you carry spare gas in your Prius.

[sleonardelli]

I've noticed many things about my tank fills. I usually pump at Sam's Club unless I'm travelling. If I don't pump beyond the first click, my calculated mpg is much higher than my MFD. If I pump another click or two; it's close to the MFD.

[a priori]

Originally Posted by JBL

. . . . I know the gauge is notoriously inaccurate, but this seems a bit much.

Anyway, any help un-confusing me would be appreciated. Thanks,

-jbl

Are you finding the information you've been seeking?

[KandyRedCoi]

IMO, because of the bladder expanding/contracting possibility, Prius owners are filling up slow at the pumps, maybe it will get you a more accurate fill up

and to my knowledge as far as MFD VS actual calculated MPG, the old fashione way is a bit more accurate