Prius C tank capacity more than expected...

Mr Ztanos and I do the same thing. I live out West and he's back East. Our MPG's are almost the same.
I know driving habit has a lot to do with it, but I've noticed we stay pretty close with the Fuelly data.

 

Your Fuelly mpg is identical to what the car says? Interesting. I don't see how adding more fuel to the tank will change the accuracy.

I use stock tires, size, and pressure. I never really tested my speedometer against a GPS one. I'm sure it's off by a bit as it usually is for most cars.

 

Yes!!!!

 

Then please explain how this works.

 

Because, as of right now you don't know how much fuel you are actually using in your car. You have a close approximation, but there is just enough incorrect information that you are getting ~1-3 mpgs difference from the car. The car calculates from the fuel pump, concerning how much fuel is being used.

 

Gas pumps shut off at different points. Even the same nozzle will operate differently from one week to the next. The only absolute way to know how much fuel you are putting into these cars are to have a rod that measures fuel volume, or to top it off so that you can visually mark the same spot every time.

 

I understand that I myself can't calculate my MPG perfectly because my (by my I mean fuelly, as in manual calculation) calculations are based on an assumption that whatever fuel I use up is replaced accurately when in fact it may not be since the fuel pump apparently fills up different amounts each time.

So I take it you mean the car is doing the same thing, more or less? I figured the car has more precise measurements of fuel in the tank.

I'll go ahead and try topping off next time I refuel and see what sort of results I end up with, though it seems I will need to do this more than once to get the accuracy.

 

You will need to do it for 2 tanks. One to put the fuel in and then the second to get the fuel to the exact same point for measurement. But like I said earlier, I neither promote this or will dissuade you from doing this. It's in the owners manual saying to not do it. *Note: just covering my nice person.

 

The car measures mileage vs fuel used. Not fuel in the tank. It will be almost exact. *Theoretically... as far as the software guys knew their maths correctly*

 

Yea that's fine. I'm just hoping that after I try this, I will see a pattern of precision as to the inaccuracy of the calculations that is insignificant enough to where I can actually correct for the way the pump shuts off by itself and not have to top off anymore.

 

Unfortunately, you won't be able to. Every pump is different. Even pumps at the same gas station will be different due to different maintenance cycles bringing in different manufacturers nozzles based on availability at the time. Honestly, I don't worry about it. I know the car knows what's going on, even when I don't. My calculations are close enough to the car that I don't worry about it.

 

Topped off the tank yesterday. The pump stopped at about 8.6 gallons, and I was able to go further until I reached 9.724 gallons. That's a very large difference, much more than I anticipated.

 

Topped off my tank 3 times in a row. Each time I was able to take on nearly a gallon of extra fuel. Thing is, my calculations are more or the less the same as before. Second topped off tank showed a 2.1 mpg difference from what the car calculated. Third showed a 3 mpg difference.

I can see this being beneficial for if you're trying to measure how much fuel you use before you run out, but beyond capacity testing, what other benefit is there? It doesn't seem to be helping my mileage calculation.

 

It should be much closer. Not sure why you are getting that kind of disparity. Mine is off less than 1 mpg whenever I do it. I have only done it 2 times though, so can't really say it's a great test group. To me it doesn't matter enough for that 1-2 mpg accuracy to keep doing it and putting the effort into it. Pump till it stops, fill to the next dollar.

 

Could you not do something similar but different? Why not get a 5 gallon gas can and fill it up visually and consistently, and simply add 5 gallons or so every time you need gas.. How far you've gone on 5 gallons of gas is a pretty good indication of how efficiently you're driving.. Perhaps I'm just overthinking things.. ?

 

This would work if and only if the lines and tank were empty before hand. Otherwise you wouldn't know if you were using just the 5 gallons, or a little of what is in the tank already.

 

I'm going to stop topping off my tank from now on. Results have been the same, still off by 2 or 3 mpg as usual.

The interesting thing is, I've noticed that the difference in gallons between when the pump first shuts off (different pumps, different stations) and how much it takes to get fuel to sit at the same point in the filler neck where I can see it has so far been the same. 1 to 1.1 gallons. Therefore, topping off is of no advantage to me except giving me a bit extra range.

 

That's interesting that you are seeing different results than everyone else. I have never had a top off that is close to a previous top off. Most gas nozzles don't even click off at the same point for me.

 

Maybe there's some sort of law or regulation in my region governing how the pumps work. Otherwise, no idea. At least I got to try it.