Calculated MPG vs. Indicated MPG of the c - Please post your results

Forgive me if this was discussed earlier in this thread (or a million times elsewhere in these forums), but...

People keep talking about using the calculated mpg to gauge how "accurate" the display is, or talking about how the display is "off" by however much. Isn't it more likely that the display is much more accurate than our calculations? I haven't looked into what technology is used to calculate the displayed mileage, but I imagine the car has a way of knowing how much gas was actually used. Whereas when we calculate it ourselves, there are so many variables that could cause it to be inaccurate. The hand calculations seem like a good, close estimate, particularly if the car doesn't calculate it for you. But I just can't believe that the car doesn't have a more accurate way of measuring mileage than our basic math. This would also explain why the difference between display and calculation isn't always the same.

However, I'm willing to be proven wrong, if there's a reason we doubt the accuracy of the displayed mileage.

 

No, so long as you use a couple tank weighted average and fill the tank up to 'full' (first off-click) each time.
The odometer is well within 1% accuracy, probably 99.5% accurate
The fuel station pump has 3 sig digits, implying a 1/10,000 accuracy (99.99%) for a 10 gallon fillup.

I don't know about the Prius 'c', but the G3 Prius meter is ~ 4 - 5% optimistic.

 

But don't pumps vary with regards to when exactly they click off? So even if you stop at the click each time, you might get more or less gas each time you fill? Or is this negligible? I did come across another thread where the consensus seemed to be that the display is more accurate for a single tank, but the calculation is more accurate long-term. Mine haven't been that far different, so it isn't all that important...just curious

 

Yep; that is why advice is given to combine a few consecutive tanks together. In effect this makes a BIG tank, and only one click to off variation for imprecision.

Have a look at ~ 4000 miles of consecutive fill-ups in my CTh. Average calculated to meter inaccuracy is 4.7%, while individual tanks vary from ~ 3.7 - 5.7% off. If you take any four consecutive tanks, you get quite close to the expected 4.7%.

Put another way: Keep track of ~ 40 gallons of fuel before you calculate, and your accuracy will be excellent. Start with a 'full' * tank, and end with a full* tank. The addition amounts in between do not have to be to 'full', just keep track of the amount.

* 'full' -- meaning first click to off.

 

Using pure gas (no ethanol), I've achieved my best tank yet in spite of triple digit heat and a rather liberal use of the A/C as a result:

Date: 6-26-12
Miles driven: 600.1
Gallons Pumped: 9.52
Hand-Calculated MPG: 63.0
Displayed MPG: 63.4
Average Speed: 27
Tire Size / Wheel size: 15
Tire Brand / Model: stock
Tire PSI (Front / Back): 42/40
Seasonal Conditions: Dry and hot, A/C usage as already stated

The cost premium of ethanol-free fuel is not worth the increase in mileage (though it might have been even greater if not for the arid heat wave). This is my second 600-mile tank, and it looks like I had, at the very least, half a GALLON in the tank remaining. This time, displayed versus calculated were very close. I am not using the same pump on any fillups, and I am overfilling it only once on each fillup (i.e. stopping after the second pump release). Yes, a rather low average speed, but there was minimal highway driving, so it worked in my favor.

 

erm, I meant half a gallon.

& thanks.

 

Yea, but your numbers still look better than mine, if only just.

I don't have a GPS installed at the moment on my C2 (I had assumed the display was also 2 mph faster than actual). If they have finally released a dash kit (so I can properly install my JVC head unit), I can jump on that opportunity to verify.

 

lol, well, would you laugh if I said I don't own one? I am going eco(nomical) in that dept as well by owning a cheapie pay-as-you-go phone. In the meantime, someone else could surely fill in.

 

I use a motorola RAZR. I'm not sure how old it is because I bought it used

Since it is for the most part used for short conversations at work, I buy per minute plans through the Verizon MVNO called pageplus at 4 cents a minute. I end up paying about $5/month.

 

Does the Prius save and calculate "total mpg" over time, or does the trip mpg get deleted everytime we turn the car off? Seems like a cummulative mpg would be easy to calculate and store?

 

Very frugal, Sage. Our govt. (both parties) would be wise to achieve similar frugality. Won't hold my breath, though.

Digilles, the only cumulative mpg it stores is via the trip function. I have left trip A untouched since shortly after purchase to get a near lifetime MPG reading to see how it stacks up against the calculations.

After 2808 miles, it claims an MPG of 63.5 mpg.

After calculating for 2866 miles, Fuelly shows the actual is 61.0.

This amounts to a 4% difference between displayed and calculated favoring the former, not accounting for the first 58 miles unaccounted for by trip A. If anything, they would likely bring the average down since the first fillup yielded the poorest mpg (59.6) of all so far.

 

I just filled up for the very first time. 47.4mpg on fuelly vs. 46.7 by the car. We are just leaning how to "glide" and all these Prii terms but we also used the A/c the whole time as its 90* + here in NorCal. And on the trip home have 2, 1 mile long hills at a 8% grade. I live in a very hilly area though. We did drive across town to Roseville ca, about 40 min from my home and we averaged 68.7mpg for that trip. 53 mpg on the way back. Our trip to SF and back got us 50.3 with the a/c on the whole way doing 65 to 70 and 1 hour of rush hour SF traffic. No ac needed in SF. It was a wonderful 60* on the coast.

 

before a stupid tech accidentally reset BOTH my trip meters for my last maintenance..

i had my trip B reading about 59mpg for the 10022 miles i had on it..
manually calculating over that time... hmm about 58 mpg.

where as most people think the vehicle's numbers are slightly optimistic... (typically from tank to tank)... after seeing how close those numbers were.... you know... i think the car knows what it's doing for the most part.. lol.

 

Do you know what the techs are doing that leads to them resetting the trip meters? Is there some sort of info they get from the display screen? And then they hit(/hold) the wrong buttons? Just curious.

 

the process of reseting the maintenance data requires you to push the trip meter 3 times and hold it the third time... it's a timing thing when you first start up the car... it's explained in the owner's manual.

he messed up.... obviously more than once..