MFD shows 49.6, calculation says 52.4

Yea, that's about right. Just today I had like one of my worst fill-ups ever, MFD said 63.4 MPG and I calculated it to be 56.85 MPG or so. I can say that my previous fill-up resulted in 65.8 MPG while the MFD said 63.8 MPG. I figure that I ended up not putting in as much gas as I actually used on the last fill-up. The average for the two tanks is like 60.49 MPG or so, and that'd be about right for each tank anyways.

The only other time I've been off by more than 3-4 MPG was when I used a broken pump on one of my fill-ups. Made me so mad.

Unless you see huge discrepancies I wouldn't worry too much about it.

 

Temps in Central New York got into the high 80s last week, and I filled up when the gauge read one pip. Tank took 9.0 gallons on first click. Calculated average mileage was 46 compared with 48 of the MFD. Best to date on a 200 mile Interstate round-trip was 52.2 mpg. In extreme cold weather, tank fill ups are around 7.5 gallons - and average mpg are wickedly lower. Sounds about right for the Prius in this part of the country. No one I know who drives a Prius can (or would) brag about getting 60-70 mpg, even if they P&G and stealth and do all those other somewhat dubious, if not outright unsafe, things.

 

My MFD said 47.8mpg but when I refilled the tank after going 468 miles, I could only get 6.5 gallons in. That comes out to 71.9mpg.

The bladder's awesome.

And it's even been repaired by the dealer once already. :blink:

 

I have had my 07 Prius for a little over three months and have not reset the MFD mileage since new. At my last fill-up the mileage shown was 46.1 and my actual calculated lifetime mileage for the first 2,334 miles was 44.07.

On a couple of fill-ups the calculated mileage for a given tank was more than the MFD figure. I always fill-up with either two or three pips left on the gage, and the average amount of gas is about 7.5 gal.

I use pulse and glide when it is safe to do so, and on the freeways or highways drive at the speed limit to keep from getting run over and still get passed by at least half of the cars. My tires are inflated about two pounds over the recommended pressure. Since the last fill-up the MFD has climbed to 46.4 so maybe the next calculated mileage will be a little higher.

 

Oooh,

Last tank, if I stopped after the auto-off with the clip in use, I would have exceeded 50 MPG, but I "squoze" the handle until it clicked, got in another 0.3 gallons and just missed the big 5-0!

My MFD estimate of MPG was within one of the calculated value. It hasn't been off by more than 2 or 3.

What I like on the Consumption graph is the feedback that shows you when you are driving efficiently and when you aren't. It makes you look at traffic jams (slow-and-go, not bumper-to-bumper) a whole different way! Also, it gives you a real incentive to learn to Pulse & Glide and find places to do it.

 

I had a 6mpg difference in my last fill-up. I calculated 60mpg and MFD said 54mpg. However, after averaging in my other calculated values and mfd values from the past the difference between calculated and mfd were marginal... somthing like .5mpg difference.

 

<div class='quotetop'>QUOTE(kdk84 @ Jun 5 2007, 12:33 PM) [snapback]455737[/snapback]</div>

I do very low mileage, less than 1,000 monthly, so I think this is what's happening.

The MFD is showing MPG when the ICE is running. When you're coasting (ice is off) yet still doing mileage, your mileage counter is increasing, but your average stays the same.

 

<div class='quotetop'>QUOTE(ystasino @ Jun 5 2007, 01:17 AM) [snapback]455504[/snapback]</div>

The difference between 49.6 and 52.4 MPG over 404 miles is less than half a gallon of gas or a couple seconds or so of pumping. Regardless of the bladder, you can't assume that different pumps, or even the same pump, will click off at exacty the same fill level every time.

 

I've chosen to not perform manual calcs. I just use the MFD. My reasoning is that there is less consistency in manual calcs due to OAT swings (common in Colorado) and differences in pump shutoff sensitivity. The temp swings are particularly bad here in the winter. My first winter with the car I went from filling up in 60 deg weather to 0 for the next fill. Needless to say the calc'd and mfd numbers were way different. The next time I filled up I had the same problem the other way.

 

My advice regarding calculated vs MFD is that over time both will even out very close.
Individual tank fillings can be surprisingly high OR low so take each with awareness that the next fill-up will likely go the other way.

After 42 tanks, my MFD has shown higher 32 times and calculated higher 10 times.
And after three years now my MFD has been higher by an average of 1.37 MPG--thus my thoughts both being similar.

 

Regarding 'break in performance' question for ystasino, As mileage increases I still have some tanks with higher MFD and other tanks with higher calculated.

As other threads have noted, after break-in time, MPG should increase. For me it was around 10K.

 

<div class='quotetop'>QUOTE(ystasino @ Jun 5 2007, 01:17 AM) [snapback]455504[/snapback]</div>

When I filled up Tuesday, I also put in 7.773 gallons (cue TZ music), but after 420 miles. Calculated was 54.0, MFD was 51.1.

<div class='quotetop'>QUOTE(singingtiger @ Jun 5 2007, 01:04 PM) [snapback]455767[/snapback]</div>

After 34 tanks, my MFD has been higher 19 times, calculated has been higher 14 times, and once was a tie.

My lifetime averages are 48.3 calculated and 48.8 per MFD. Close enough for me.

 

<div class='quotetop'>QUOTE(chuck kershner @ Jun 5 2007, 05:07 AM) [snapback]455563[/snapback]</div>

hobbit gets 60-70mpg. Much depends on your terrain. Everyone on this thread is describing normal operation.

 

I just filled up tonight. MFD showed 54.1 mpg over 364 miles. But I put in 7.9 gallons, which works out to 46 mpg calc. Seems like a rather large discrepancy? My first thought was that my previous fill up didn't really fill the tank, but I remember filling until it clicked and then going 1 more click. And for that tank, the MFD was within 1 mpg of the calculated value. I'd like to believe the MFD, but I fear I'm closer to the calc'd value.

-Sean

 

Has Toyota told anyone how exactly they do their mpg calculations? Might explain the mpg discrepancies. What mileage number does the Prius calculation use? Could this also be an issue of individual cars being biased high or low on how the Prius does the mpg calculation?

My Prius calculated 55.4 mpg, and my hand calculations (odometer reading for the tank divided by gallons paid for) gave 53.1 mpg (about 4.3% error, the Prius is calculating that I used ~3 gal less gas than I paid for). This is over the course of 11 tanks of gas from March through June, filling up at different stations, with WI weights and measures certifying accuracy of the pumps, not overfilling (fill to first click), filling slowly, and pro-rating the Prius mpg calculations for the proportion of miles driven on that tank of gas. The odometer and fuel calculator are both reset at the same time when I restart the car after filling. The Prius underestimated the mpg 4x vs. overestimating it 7x. Biggest discrepancy was +10.7 mpg, lowest was +0.4 mpg

The salesman that I purchased the car from said that the Prius calculation was the right mpg because the fuel use is that measured at the #1 injector, and that the miles were prone to error ("just wheel rotations"). Ok, so, what number of miles other than the same odometer reading I'm using for calculations would the Prius be using?

Even if it was an issue of density of the gasoline as the weather warms up, the Prius should be calculating worse mileage on warmer fuel because the amount of energy in the same volume of fuel would be less (yes, that doesn't address what temperature the engine works better at).

More thoughts on this? Many thanks.