Large discrepancy btwn MFD MPG and calculated

inconsistent fillups and the bladder.

the MFD calculates using the amount of fuel fed to the injectors and miles driven on the tank. i'd trust that first just because you don't know how much gas is left in the tank when you fill up, you don't know how low you ran it down and if you really used all the gas you pumped in, etc. it's hard to get good accuracy when calculating on your own.

plus there's the whole pump calibration issue- some places take it upon themselves to say they're giving you more gas than you're really getting. that's the story behind those weights and measures certification stickers on the gas pumps.

 

Acousticbiker,

I had the same thing happen to me on my 2nd fillup. The MFD showed 44.5 and the calculated mpg were 51! This is my second Prius. My first 2005 always calculated less than the MFD. I don't know what is going on. I'm running Michelin X-Ice tires and like you live in Colorado and drive up lots of hills.

 

run a spread sheet on your mileage and fuel used and temps and station if you gas in a local area and I include the price and the mfd reading. After a year or so you'll see all the trends, summer to winter, hot to cold, when the wife uses the car or the kids etc. I also put in the oil changes, tire rotations, air filter changes and such. When people harrass me about my mileage they get a hard copy. All have shut up and moved on.

 

I've pondered the affect of how the car calculates mpg when you're sitting still but idling.

For example, my wife asked me to get Wendys for lunch today and while I was waiting for my food and wondering what quotient of correct to non-correct order I'd be getting.... ICE had been on for about a minute, then I coasted into the parking lot in EV for about 2 minutes, then sat in the line for a minute or so. My consumption screen said 100mpg for that time, which I know I didn't really get (thinking of the time the ICE had been on).

I have to think that there is some estimating the MPG calculator does, just based on situations like this that I've seen.

 

From tank to tank my readings vary wildly from the MFD calculations. But I've kept track of everything for the first 5000 miles and when averaging everything together the MFD said I averaged 46.0mpg and the actual average was 45.2mpg. So, at least on my car, the MFD is pretty close to reality. The bladder in the tank makes individual tank manual calculations pretty much worthless.

 

I just put in my third tank. I waited until the last bar started flashing (just to see what happens) at 429 miles. My MFD showed 46.5.

I expected the tank would hold around nine gallons, but it clicked off at 7.7 (I wanted more to go in, I resisted topping it off, well, maybe a couple of extra clicks...). I calculated 55.7 based on that amount.

Since 46.5 is my average over the first 1000 miles, I guess it's possible the current tack at 55.7 reflects my improving driving techniques! (Or, it's than #%$@ bladder....)

fd

 

yes

yes

doesnt matter the size of the tires. since your elapsed distance is only taken from one source. whether the tires roll 10% less or 10% more, how would the car know this?? the odometer is your only point of reference and im sure that is also how the MFD determines it mpg's

yes you should. you admit to not tracking your first tank. i have had many tanks just as far off as yours. look at my signature. eventually, your overall variance will be about the same

 

acousticbiker, Pria, Jeff, sseres and frankie:

Please see a handful of posts by me and galaxee over on a thread entitled "Help! my wife...", mostly as part of a conversation with cgraham, on this topic. I'm sure it'll help.

There are some off-topic posts to plow through, but I think you'll find it worthwhile.

 

heck i go through warmup after turning the car on and can get home from the gas station mostly in electric. i know for a fact that i'm averaging over 99 mpg, but that's what the screen says.

when the engine is idling it doesn't require copious amounts of gas.

your calculated now may be high, and your next calculation will probably be low to even it out.

 

Yes, I have the same thing after 3 tanks. I got 7 MPG more on paper than what the computer said. I fill the tank the exact same way each time for consistancy. Last fillup was 59.2 actual vs 52.5 computer. No complaints here.

 

If you have a 2006 Prius, you need to be aware that the average MPG in the display does not reset when you fill the car. The miles in the display resets ater you drive 100 to 200 yards, but not the MPG. So in addition to the variability of the bladder, your average MPG will be the average from when the reset button was last used. If you have never hit reset, it is the lifetime average MPG.

So if you have a tank that is much better or much worse than your current average, it will only slowly drage the average MPG up or down. Why this change happened on the 2006 is anybody's guess. I have verified this on two 2006's and compared back to a 2005.

- Tom