Is this possible, the MFD being off this much on MPG?

Just picked up our new 08-2. First fill showed 43 mpg on computer, 53 mpg manually. Since I don't know exactly how much gas dealer put in, I must assume I didn't reach that level. So I'll wait for the average. Second fillup, today, computer showed 48 mpg (more air in tires, slower speeds). I clicked off at 7+ gallons, decided to squeeze in some more, went over 9 gallons. Pulled the nozzle out and got the Prius Puke. So mileage was into the high 30s by my calculations. Again, just have to take the average.

 

The pump may have shut off too soon the second time too. I usually have to put in about 8 gallons at two pips.

 

Situation normal. My last tank showed 64.3mpg on the MFD but calculated at 50.36mpg. This is disappointing enough but it was the tank for the rally. I estimate this should be good for next-to-last place. Sometimes it's high, sometimes low, usually close, but can be off ~20%.

 

I started tracking late (I'm missing at least one gas receipt) but here is what I've got:

 

My varies from my calculated on paper mpg everytime. Sometimes higher sometimes lower. I always fill up at the same gas station. So its not that.

 

Hello:

Not only is it possible, but it is an absolute fact. I have over 6300 miles on my 08 Prius and the MFD has never been right. It can be off anywhere from .5 MPG to close to 10 MPG in my experience so far. Yesterday I filled up and the MFD said I was getting 47.8 MPG, but in reality I got 52.31. This of course is to my advantage, but should it be off by 4.5 MPG either way? I think the MFD really needs tweaked. However, for the record I want to state that I LOVE my Prius! I just took a 3,975.1 mile road trip in 9 days and I averaged exactly 47 MPG and this was going highway speeds which is anywhere from 65-75 MPH. 3 adults in the car and the car fully loaded. I am VERY impressed. Thanks for reading.

 

But how do you know yours is the correct calculation? I don't know if the car's computer is dead on, but to get an accurate reading either way, the tank has to be filled precisely each time. Not a Prius thing; that's the way any car's mileage is affected.

 

Record ten (or a hundred) tanks with volume and miles driven. Then you get a "real' average. Do the same recording of the mfd mpg. I suspect that after that number of tanks you will find it is within 1%, well within the range of accuracy of pumps, mfd etc. Over any given tank, or even a few tanks the variances of temp will have a huge effect.

Or, stop being obsessive and drive!

Icarus

 

I have driven my 2008 for 2217 miles on 6 tanks of gas.

Amount of gas purchased totalled 43.44 gallons which works out to 51.04 MPG.

I also recorded the MFD indicated MPG at the end of each tank and then re-set it for the new tank. If I divide the MFD MPG into the miles driven I can get "indicated" gallons of gas consumed.

Totalling up indicated gallons consumed, I get 43.11 gallons. That means the lifetime MFD indication is running 51.43. This is less than 1% error.

On a single tank, I have had the MFD be off by 11% (both high and low) on a single tank. I attribute that to fill up differences on any given tank.

If my last tank was totally empty when I hit the pump and I only added 8.31 gallons (what I bought), then the lifetime mileage at worst would be 47.27 (based on a full tank being 11.8 gallons). That would put the MFD error a a maximum of 8.8%.

ICARUS is right. You probably need to buy 115 gallons of gas (10 full tanks) to wipe out the fill up error in the manual calculations and really be able to say how accurate the MFD MPG reading is.

 

"ICARUS is right. You probably need to buy 115 gallons of gas (10 full tanks) to wipe out the fill up error in the manual calculations and really be able to say how accurate the MFD MPG reading is."

You would doubt me!!!! I smite thee! LOL

Icarus

 

After my first two trips to the gas station, I've concluded the computer mpg is the only reliable one. First 140 miles: computer 48.0 mpg, calculated 65.0 mpg. Second 140 miles (different gas station): computer 51.5 mpg, calculated 36.6 mpg.
Seems like the total gallons from the two fillups might be accurate, since the average calculated, 50.8 mpg, is very close to the computer average value, 49.8 mpg.

I'm not sure what this means yet. Maybe need to fill up at 250 miles or so a few times. But the computer mpg is certainly accurate, in my view.

 

There's only a one gallon difference in your numbers. (48 MPG would be 9.1 gallons) That's about 6 to 12 seconds of pumping. You can't be sure that your fill level is exactly the same as your last one. Particularly when you state that the pump kept stopping.

 

All it means is that there is uncertainty in the capacity of the Prius fuel tank. This forum is full of posts about that topic. Search for "bladder" and "guess gauge".

Tom

 

It seems to be commonly agreed that the MFD is very accurate and consistent, probably within 1-2% and usually reads slightly high. As has been mentioned you really can't tell anything from how much gas you put in, because there are way too many variables. The Prius adds to those variables by having an expandable fuel bladder to reduce fuel evaporation & the associated HC emissions. Since the volume/flexibility of the bladder varies with temperature, pump pressure/sensitivity, phase of the moon, you really never know exactly how much fuel is going to go in it from fillup to fillup. That makes manual calculation very inaccurate, unless you average it over a lot of tanks.

The MFD pretty much has to be that good, as its just displaying the information gathered by the engine ECU on injector timing, flow etc. If it were not accurate all of the emissions control info would be off and the EPA would be very unhappy, not to mention the fuel/air mix would be all over the place and the car would run poorly. Since evaporative emissions are minimal, the only place for the fuel to go is through the injectors so the accuracy should be very good.

Rob

 

I don't understand why people are so quick to say that the MFD must be wrong when it doesn't agree with your manual calculation. This is a pretty smart car.

As others have noted, the problem with using the miles you have driven since your last fill-up divided by the number of gallons you just put into the tank is that you are assuming that you replace EXACTLY the amount of gas that you used over those miles. Over time this approach smooths itself out and you can use your cumulative numbers to get a good estimate, but it is not an accurate measure over one tank of gas.

I don't know how the MFD calculates the MPG but I suspect that it is able to actually track how much gas has been consumed over the number of miles that have been driven.

 

Just like others have said, when I am at home, I fill up at the same station. I have gotten used to how much gas this car takes. The MFD is off as I said anywhere from .5 - 10 MPG either way. Are there variables? naturally! However, once you know what it takes to run out of gas, I speak from expeience, you get to know what is about right in gallons per fillup. I averaged between 8-8.5 gallons per fillup. I reset the MFD at the gas station everytime. I reiterate, I LOVE my Prius, but the MFD should not vary this much consistently. In case I failed to say it clearly, I LOVE my Prius and am not complaining. Just stating fact.

 

It is not the MFD that is wrong it is the variation in level filled to. Watch your average over several tanks. After 20,000 my MFD average error is 3%. It has highs and lows from -10% to +10% (about one galon) but the total error after 400+ miles is 12.5 gallons. This is about 1.5 MPG for me. Most of my tanks are in this area with the occassional large difference on two tanks, one high/one low.

I think if you are truly averaging a large error you should talk to your service rep.