MFD way off.

The answer to this is actually quite simple: even though you put 6.7 gallons in your car, chances are you didn't actually burn 6.7 gallons going the 262 miles. Because the (model year 2004-2009) Prius uses a bladder inside the fuel tank to reduce emissions, you can never be quite sure what the size of your tank is at any given time. This means that sometimes you might fit 10 gallons of gas in your tank and sometimes you might fit 8 gallons of gas.

So, when you fill up, you don't know for sure that the amount of gas you are able to put in is the amount of gas you burned since you last filled up. The short version: trust the MFD; most people have found it to be correct to well within 1 MPG.

 

Nothing to worry about. I've tracked the difference between MFD mileage and calculated mileage for a year and a half. The two have been off by as much as 15% on a per-tank basis.

 

Jimbo,

After a year and a half of tanks (which is probably only about a dozen for you!) what is your percent difference between MFD and fill? Guessing somewhere in the range of 2%?

I just got my Prius in late March so I don't have alot of date to run mpg comparisons but others I have seen are in the 1.5-2% optimistic range for the MFD (meaning 60 mpg on the MFD is probably around 58.8 mpg).

Thanks!

Matt

 

OK. I'm not Jimbo -- and I won't even play Jimbo on PriusChat. Still, I have a fair amount of data to apply to this question.

I've owned by 2007 for slightly more that 21 months. I have driven a little more than 29,000 miles and filled the tank 75 times. The first tanks showed huge differences between the MFD readings and my "calculated" MPGs. (I use the parentheses because the bladder really makes these calculations quite inaccurate.) I've calculated a lifetime variance between the MFD and "calculated" MPGs this entire time.

I have noticed some seasonal variation that occurs, most notably in the Spring and Fall. Nonetheless, it appears the variance (in a lifetime reading) is steadying down, as would be expected as the numbers grow larger.



To simplify and summarize:

• My lifetime MFD mileage shows at 52.1 MPG
• My lifetime "calculated" mileage comes to 50.8
• The actual difference is just below 1.3 MPG, which is about 2.5% (measured as MFD above "calculated")
• Looking at the past 12 months of data, allowing seasonal differences NOT to outweigh things, the variance is slightly smaller. MFD: 53.3; Calc: 52.2; Var: 2.1%

What accounts for this? Here is what I see:

• When I record the information, I do not use tenths of miles. This could affect things, because the total miles may not add up correctly. Still, it is a very small change once you've gone 29,000 miles.
• I don't use the same pump each time when I purchase gasoline. Heck! I don't use the same station. In fact, I travel around a few states and see many different stations, pumps and fuel mixtures.
• I would trust the MFD readings on a regular basis, and I believe those numbers are more reliable than the gallons (to the thousandths!?!) as shown on the gas pumps.

I think many people are in a similar situation, though my variance may be greater than others experience. Still, I think most people see about a 1 MPG difference between the MFD and "calculated". The longer the time period and the greater the number of fills, the more reliable the data.

 

I've asked the same MFD vs. fill mpg comparison question to about 5-6 people and the answer has usually been 1.5-2.5% optimistic on the MFD (which over the course of a tank is right around 1 mpg).

I like to keep detailed fuel logs and obviously the Prius presents some challenges in reagrds to tracking mpg. I've been going with 2% as my standard deviation from MFD when calculating my mileage on each tank. I do record actual fill as well so after a year or so I'll have my own variance to report.

In only 5 tanks (2500 miles), my MFD is about 1.3% optimistic but the range in fill vs. MFD has been from MFD 10% optimistic to fill 4.2% optimistic (quite a range)...only time will tell where it settles.

 

Wait until you meet COLD weather! I've had some very, very large descrepancies between MFD and calculated. The big ones come from problems filling the tank, and the greatest Up or Down inevitably is followed by the inverse.

My biggest difference was an MFD reading that was 22 MPGs lower (30%) than the calculated MPGs. It was followed by a tank with an MFD reading 14 MPGs better (14%) than the calculated mileage.

Lesson: Don't be worried about it on a tank-by-tank basis. Trust the MFD reading to be highly representative of what your mileage is.

 

Until I read the posts on priuschat about the fuel bladder I was stumped by my calc'd mpg...glad I read it after two tanks so I didn't drive myself crazy. I'll be interested to see how everything works out over the course of a year's tanks (guessing I'll be in the same 1.5-2.5% MFD optimisitc like the others).

The 55 mpg you see in my sigs is after taking the 2% off my lifetime mpg (using the MFD calc's). I'm running at about 63 mpg on my MFD on the current tank after 275 miles so things are going well in the new buggy (800 miles and two tanks worth of highway driving last week killed my lifetime average but I'll have it over 60 mpg in a few tanks).

I use myhbridcar for the lifetime sig and ecomodder for the 90 day mpg and % over EPA sig (since the ecomodder sig doesn't represent lifetime mpg). After 90 days from my first fillup the two sigs will show different mpg as they don't track the same thing (lifetime vs. 90 day).

 

Or maybe lots of gas stations have slightly shortchanging pumps. They all have the same incentive to do it.

 

Pretty close -- it's about 2.6% currently. FYI, attached is my latest spreadsheet.

 

Jimbo,

What are you analyzing to get difference, weighted difference and cumulative difference in your numbers and why?

Thanks!

Matt

 

The Weighted Difference column adjusts each tank difference for miles driven so that tanks with more miles count proportionately more toward the cumulative difference. The Cumulative Difference then averages all the values from the Weighted Difference column.

Are you familiar with Excel formulas? If so, you can inspect the formulas to see the actual math. If not, I'll be glad to explain the calculations.

 

Jimbo,

That is sickeningly great mileage!

What I love is that we have the same numbers -- not mileage, just the percent difference between MFD and calculated!

 

Thanks. It comes from having a sickening obsession with fuel economy.