Calculating MPG's

[EarthFriendly]

In calculating mpg's, what is more reliable: using the value on the consumption screen or using the old fashion method of miles driven/gallons of gas pumped. I ask because I just bought my first tank of gas for our new 2007 Touring, and the consumption screen displayed around 47 mpg average (we never reset it during the first tank of gas). When I pumped gas and did the math, it came out to 56 mpg (475 miles traveled/8.4 gallons pumped). My concern is that perhaps I did not fill the tank all the way given the bladder style tank issues, although 8.4 gallons pumped with two bubs left on my gas gauge seems to make sense. Any insights?

[mgipe]

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.

[Rae Vynn]

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!

[qbee42]

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

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.
[/b]

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

[greenrev]

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.

[qbee42]

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

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.
[/b]

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

[jarhead&#39;s prius]

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

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.

[jwe8f]

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.

[lytthans]

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.......

[Danny Hamilton]

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

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.
[/b]

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.