calculated vs. computer MPG - Please post your results

Would you be able to state that as a percent difference? In other words, "The MID readout of fuel economy is ___ % higher than the calcuated fuel economy (based on the amount of gasoline purchased)."

(I have a hard enough time trying to understand liters per 100 kilometers.)

 

Imagine trying to understand miles per gallon! Can't average them; deal with improbable notions such as infinite MPG, better fuel economy means higher numbers, etc.

Here's a tool to convert back and forth between Earth and USA systems.

 

We just filled ours for the 2nd time. 56 mpg on the 2nd tank according to the computer. Manual calculation was nearly the same. Wife has a light foot and drives it in a mix of highway/city driving in her commute (the highway portion can sometimes be some stop-n-go).

56 mpg? Dang that's good...

 

On my 2004 and 2005 Priuses I've noticed in cold weather that the fuel bladders are less compliant/pliable in the cold than in hot weather, so have felt that the greatest variable in calculated MPG is the unknown amount of temperature dependent fuel accomodation when "full". But when temperatures have been similar between fill-ups, the calculated and displayed MPG have been within 0.2 MPG. With this in mind I haven't even calculated MPG with my 2010 Prius.

 

I almost appreciate the sarcasm!

I'm quite able to do the conversion, but a difference of 0.2 liters per 100km doesn't tell me that much. (A conversion to MPGs doesn't have value here.)

One of the things we are after is a measure of how much higher the MID readout is than the measured calculation based on gasoline purchases. While I'm assuming he was referring to 0.2 liters less consumption according to the MID than according to his measurement, it doesn't give the full report. It appears the discrepancy varies as FE improves, so a percentage reading seems to be a more valid comparison.

What I don't know is whether this percentage change metric is similar in the metric world.

In other words, if the difference is 3 to 10%, then we can use the data in a joint fashion, because it would match up fairly well to the data collected to date. If the calculation based on gasoline purchases showed fuel economy as 4.2L/100km, while the MID readout was 4.0L/100km, then I would say the MID gave a 5% "overstatement" of the fuel economy. This would match up with data collected on my car, and I'd say that we might be able to fairly easily incorporate data from Canada.

 

SAme problem here.. when my tank goes over 60 especially notice the big discrepency..

 

I'm getting 55 on the road and 46 to 48 City. My best is 56 on a 200 Mi country trip. Mileage improved after 1000 mi. Consumer Reports says it go 44 overall with about 32 City and 55 country) in this month's issue- saying its the same as the '09 edition. I keep mine in the ECO mode, but I do 2 steep hills daily, and the mileage dies on the hills in the city .

 

If the measurement is to be the percentage overstatement, then the math would be 4.4/4.62 = 0.952 ==> 0.95

so. . . that's 5%.

Yes, the % should be the same if you convert!

(Another way to do the math: (4.62-4.4)/4.4 = 0.05 or 5%)

 

My calculator says 4.62/4.4 = 1.05
Is your calculator made in China?

Ken@Japan

 

Hi F8L
The only way to be accurate when doing MPG measurements manually is to always fill up your thank until you actually see the gasoline in the filling hole. Fill-up twice that way and take your mileage in between.
But TMC say that it'is not good to fill the thank that way, they say you should stop filling when the gun pops up in your hand plus to or tree more shots.
I did it four times (the way it is not advised to) with my 2010 P-I and i can assure you that the computer is extraordinarely accurate.
Accurate to the point that i had no error since the computer display stops 1 digit past the point.

 

I have been keeping careful records of my fuel consumption. My calculations have been about 3 mpg less that what the car's computer says. It may say 50.3 mpg but when I do the math I am only getting about 47 mpg. I have nearly 7000 miles on the car.

 

Hi Footloose

Could it be the human factor ? 7000 miles is a lot of data for us to rite down on paper. Funny i had mine on last june 25th and it's exacly what my dash reads 7000 miles !

 

This is not correct. The only error introduced is the difference between the amount in the tank at the end of the first fill and the amount in the tank at the end of the last fill. The others matter not in the least, because those gallons are completely used.

I understand you see this as accuracy, but when looking at it from the perspective of seeing many, many Prius owners' data, your situation just happens to be at the fringe of the data. It also happens to be at the fringe which is closest to that which matches the mileage calculated from using gasoline purchases.

I can't speak for anyone else's human factor issues, but my miles add up just fine through about 8,300 miles, and I still have a consistent 5% overstatement of fuel economy on the MID.

 

I agree on your fill method. Only the first and last matter. If you fill a little less after tank #3, that will affect calculation for that tank, but will be made up for with tank #4. The cumulative calculation won't be affected.

Wayne Gerdes at Cleanmpg.com believes that early cars were 6mpg over, later cars 5% over and at least one recent car was accurate.

Read his comments after his report of hypermiling with the Gen III Prius: 2010 Prius-III P&G, SHM and WS FE techniques rehashed, defined and refined - CleanMPG Forums

 

MPG gauge is about 2 MPG higher than calculated. Build date 07/09

 

I propose a new hypothesis for the discrepancies between indicated MPG and calculated MPG. The indicated MPG doesn't account for gas used bt the ICE when the vehicle is stopped, such as when the vehicle is warming up. Drivers in stop and go traffic During warm up will experience the ICE running at stop lights (like a normal car). The ICE may fire up at other times without vehicle motion.

The calculated MPG captures both the gas used in motion AND not in motion. It therefore will always be lower than the indicated MPG.

A prediction: the discrepancy should get worse as temps drop.

 

My observation is not match your assumption.
I did 1043km round trip including two 500km non stop highway driving.
It was 95% highway driving where ICE was always running.
My indicated MPG was still 5% higher than calculated MPG.

Ken@Japan

 

I propose a new hypothesis for the discrepancies between indicated MPG and calculated MPG. The indicated MPG doesn't account for gas used bt the ICE when the vehicle is stopped, such as when the vehicle is warming up. Drivers in stop and go traffic During warm up will experience the ICE running at stop lights (like a normal car). The ICE may fire up at other times without vehicle motion.

The calculated MPG captures both the gas used in motion AND not in motion. It therefore will always be lower than the indicated MPG.

A prediction: the discrepancy should get worse as temps drop.

 

Displayed FE includes all the gas burnt, being in motion or stopped. Confirmed by using the remote starter: when I enter the car, the FE in L/100km as gone way up.