Calculating MPG's

The MFD screen is best for short term mileage, for exactly the reason you surmised.

For long term (life-time average, for example), calculating mpg based on gallons of gas purchased is probably easiest and most accurate.

 

Also, bear in mind that you don't actually know how much gas was in the tank when you got it.

Best practice seems to be: Pick a gas station, and pick a pump.
Always use the same one.
Pump the gas at the same speed (I use the slowest), and when it clicks off, do NOT top off. Just remove the nozzle, and note how much gas it was.
Keep a notebook, and every 5 tanks or so, do an average calculation of MPG.

The reason for this is the gas tank bladder. If you do a search on "guess gauge" you'll see that the bladder can make every tank a slightly different size!

 

<div class='quotetop'>QUOTE(mgipe @ Aug 31 2007, 04:19 PM) [snapback]505117[/snapback]</div>

Let me emphasize that for long-term you use the total miles and the total gallons. The longer the term, the lower the error.

For short term, the MFD does a good job.

Tom

 

I have now pumped twice, actually since I live in Jersey someone pumped for me (state law, I don't know why, but that is another discussion) Both times the mileage did not equal the screen. the first time the screen said 46 and I got 48. the second it said 49 and I got 43. I long for the 55 average, but that is another discussion too. Long story short, I am thinking the screen is not that precise but it does get in the ball park, Just my opinion.

 

<div class='quotetop'>QUOTE(GreenRev @ Sep 1 2007, 09:49 AM) [snapback]505391[/snapback]</div>

The screen is very precise, and quite consistent. Two fillings is nowhere near enough to average out the error introduced by the fuel tank bladder. You will need somewhere near 40 fills to get close to the MFD results.

Tom

 

<div class='quotetop'>QUOTE(qbee42 @ Sep 1 2007, 11:33 AM) [snapback]505419[/snapback]</div>

Tom, you're saying then to not reset the MFD when refueling? I've only filled up twice and have had the same fluctuations with mpg.

 

I agree, MFD for short term, calculated for long term. Over 20,000 miles I find my MFD reads about 4% high, or about a +2.1 MPG in error to the upside. However, that is no where near the error you can see from fill-up to fill-up because of the difficulties in filling the Prius tank.

So, short term best = MFD.
Long term best = calculated.

I also reset the MFD every time, because I only use it for a short-term 'heads up'. I use only the odometer for miles traveled, and calculate long-term mileage based on the odometer and gallons used.

 

At 15K+ miles, my 2006 Prius has a lifetime average of 44 MPG, not too far from the new EPA ratings. That's calculated by averaging the display MPG and the manual calculator mode. My highest MPG for a trip was 55.6 MPG and the lowest was 37 MPG. I do run the AC all the time, and try to use the Consumption screen, trying to find the Sweet Spot to keep the right hand display as high as possible.

So far, nothing has broken on the car. I'm due for the 15K service. Regular oil, and tire pressure at 42/40 lbs. The battery charge is something I want to discuss with the dealer. I now have wide swings from the green area to the purple, often overnight. This was not an issue for the first 6-8K miles. The battery swing is most evident when the outside temps are high, and I'm driving at a higher altitude.

I love the car, 95% of the time. There are a few dumb things Toyota needs to address, such as seats that can be adjusted in heigth, the glare in the outside mirrors from the grey plastic trim on the AC vents, the hard to learn Nav system, and the mediocre audio quality, even with the JPL high-end system. A sub woofer helped somewhat, though.

I'm looking forward to the 2009 models.......

 

<div class='quotetop'>QUOTE(GreenRev @ Sep 1 2007, 08:49 AM) [snapback]505391[/snapback]</div>

How many miles on each tank?

You might find that if you average the two tanks together, the lifetime average gets pretty close to the screen. It's your individual tank calculations that are "not that precise" because of variations in how much fuel you fit into the tank. Over time the variations in the hand calculations have less affect on the lifetime average.

As has been repeated here multiple times now. The screen is more reliable for any individual tank calculation, and total miles traveled divided by total gallons pumped is more reliable for long term calculations.

 

<div class='quotetop'>QUOTE(jwe8f @ Sep 6 2007, 10:15 PM) [snapback]508324[/snapback]</div>

Glad to see that you are not the only one with a consistent variation. Of course, I've only driven 1200 miles on the car, but I consistently have the MFD at between 53 and 55 (reset with each fill, and I've only had four "fills" of 3, 3, 8 and 8 gals.) My manual calculation is showing a "lifetime" mpg of roughly 49, with a lifetime MFD of just over 53. So, you wouldn't see a +4 difference as anything to be concerned about?

The concern is that the odometer/speedometer is off, despite the dealer's assurance that the techs tested it over a 3 mile course.


<div class='quotetop'>QUOTE(Danny Hamilton @ Sep 11 2007, 11:08 AM) [snapback]510614[/snapback]</div>

Danny, I'll take this statement as a further assurance that my lifetime calculations based on odometer reading divided by gallons purchased should get closer and closer to the MFD readings?

 

<div class='quotetop'>QUOTE(jarhead's prius @ Sep 6 2007, 11:03 PM) [snapback]508316[/snapback]</div>

No, I didn't mean to imply that. What I meant was if you want to accurately calculate mileage from fill-ups, you need to keep a log of the total miles driven and the total fuel used. You can use a piece of paper or any other means. Use this formula:

Average MPG = (total miles driven) / (total fuel used)

The miles figure should be good since it comes from the odometer (unless you have changed wheels or tires). The fuel used will have a fixed amount of uncertainty. For the sake of argument, let's say that the typical Prius has about two gallons of uncertainty in the fuel tank. In other words, a tank reading empty will take anywhere from 10 to 12 gallons to fill. Hopefully you don't run your tank all of the way dry, so let's say a typical fill up will take 8 to 10 gallons. Now let's look at what happens when you try to calculate mileage on your first fill up:

What actually happens:
You have used 9 gallons of gas to go 495 miles. Mileage = (495 miles) / (9 gallons) = 55 MPG

What you see, case 1:
The fuel tank bladder is a little tight and only takes 8 gallons to fill.
Mileage = (495 miles) / (8 gallons) = 61.9 MPG (WooHoo!)

What you see, case 2:
The fuel tank bladder has warmed up and stretched, taking 10 gallons to fill.
Mileage = (495 miles) / (10 gallons) = 49.5 MPG (crud, what's wrong with my mileage?)

When you fill your tank the next time, you still have 2 gallons of uncertainty. After the thousandth fill, it's still two gallons. In other words, the uncertainty does not accumulate, but stays the same. Let's see what happens after ten fills:

What actually happens:
You have used 90 gallons of gas to drive 4950 miles. Mileage = (4950 miles) / (90 gallons) = 55 MPG

What you see, case 1:
Tight bladder, so total fuel is 89 gallons.
Mileage = (4950 miles) / (89 gallons) = 55.6 MPG

What you see, case 2:
Loose bladder, so total fuel is 91 gallons.
Mileage = (4950 miles) / (91 gallons) = 54.4 MPG

If you run the same calculations for 100 fills, you get 55.06 MPG and 54.94 MPG.

See how small the error has become? Without changing the fuel tank, you will always have some uncertainty, but by spreading the uncertainty over a large number of miles you can reduce its effect.

Tom

 

If you want an example of the differences between MFD and pump/distance calculations, you can see my log at: http://scrye.com/~tkil/prius/gas.html

Over about 40k miles, the MFD shows 47.6mpg while pump works out to 46.1mpg.

The table also contains the delta for each tank (gallons pumped in, vs. what the MFD claims I consumed); it varies from -1.5gal to +1gal. My source data (a plain text file) has other info that I haven't used yet: elevation, temp, etc.

Hope this helps!

 

How does the Consumption Display calculate MPG while in EV mode?

In my commute home, I can put the shuttlecraft in EV mode, and between EV, regen, and coasting downhill, I get nearly two miles before the ICE kicks in. My MFD show two car icons, but no yellow bar for the first two minutes.
Looking at the MPG figures, it seems like it is not calculating that at all, hence I think my MPG for that run is low.

I will do a reset before I come home tonight, and see what I get.

 

The miles per gallon in EV mode is infinite. The MFD still uses the same calculation, but displaying an infinite value presents a problem. The instantaneous value is displayed at 99.9, but 99.9 is not used in the overall calculation. Overall mileage is figured as total miles verses total fuel; the individual bars are not used.

Tom

 

Hmm, seems like maybe your hand calculations might be off a bit.

miles per gallon = miles divided by gallons.
gallons = 8+8+3+3 = 22
miles = 1200
1200/22 = 54.5455

Car indicates a lifetime of just over 53 mpg. Manual calculations indicate half way between 54 and 55.

If we assume for a moment that the manual calculations are accurate, you are looking at a MFD error of 2.7%

I suspect your speedometer is within the requirements of the law. It may be off a bit, but not any more than any average car. If you have changed the size of your tires, you may have affected your speedometer or odometer a bit, but if you haven't and your manual MPG calculations are the only indicator you have of a possible error, I wouldn't worry about it.

The measurement of amount of fuel consumed is probably where most of the error in mpg calculations occurs (both in manual calculations and in the MFD displayed mpg).

 

We also can't assume that gas pumps are perfectly accurate. The law allows them an error of a few percent, and gas pump owners have an economic incentive to make them pump less than they display (that is, display more than was actually pumped). That discrepancy will tend make the manually calculated MPGs less than the MFD MPGs.

 

I got a good EV run in today that confirms this. No yellow bars, but overall MPG calculated to include the EV miles.