How accurate is your MPG computer vs. your calculator?

My last reliable one was 52.2 Screen versus 52.15 calculated. That's within the margin of error for gas pump accuracy I think.

My most recent tank apparently brought me the longed for bladder expansion (keeping fingers crossed). I put in a lot more fuel than expected.

Same Station, Same Pump, Same Fill procedure, so with any luck I can get my first 500 mile tank this time.

 

I'm now at 5500 miles. My display shows anywhere from 5 MPG below to 3 MPG above my actual calculated results. Sometimes they even match!

But I wonder if the difference is because the bladder doesn't fill to the same size every time. My thinking is if it's colder, it doesn't stretch as much. So if I actually used 8 gallons, I can only put 7 back in, making my calculated mileage look higher than it really is.

 

23 tank loads

mfd average mpg 51.1
calc. mpg 51.7

 

I have noticed mine was more accurate when I was doing more country driving..
Now that I am doing mostly highway.. It is off.. from 2-5 mpg.

I will be updating my lifetime milage spreadsheet in the next week or so..

Scott
04 silver #5 36k miles

 

I note that each time I emphazise driving in neutral my Ampg remains somewhat unchanged and the calculated Mpg decreases. So instead of getting the anticipated 50-60 mpg for my concientious efforts I actually get less than 50mpg. Tanking up consist of maximum topping off w/o spillage. Best Mpg seems to result from Cruise control set at about 40mph ( City/Suburban) and early braking preperation.

 

At first the MFD and the pump calculated MPG were WAY off. I guess the bladder made it very hard to consistently fill the car.

Starting this Spring, I started to let the tank go to 1/2 or even down to 1/3 a few times. Then I would top off until the filler neck was full. I was doing a lot of driving to my Hobby farm this summer so had plenty of times to gas up, the farm is 2.5 hours from Winnipeg.

After 6 fillups, I noticed it became much easier to consistently match the pump calculated fuel economy to the MFD fuel economy. For example, the last 8 tanks, if the MFD claimed something like 4.8 l/100km, from the pump I would calculate 4.7-4.9 l/100km.

Maybe you have to "stretch" the bladder so it becomes consistent?

 

I've kept detailed calculations of all 29000 miles on my car. The display reads 1.5 mpg too high. This seems to be about an average offset compared to other people who have kept detailed records over long times.
The basic conclusion is that your display minus 1 or 2 mpg is the most accurate way of knowing the actual MPG. Calculating it based a tank or two is way more inaccurate because of inconsistent fillups due to the bladder, temperature, gas pump auto shut-off differences, etc.

 

OK, you made me curious. Ive been tracking my "at the pump" MFD display refills since Oct '05 when I received my car. (That's 73 refills) I just added a column to my spreadsheet and recomputed ACTUAL MPG (Miles/Gals).

I have a/o today 27,388 miles on the car and the variance between DISPLAYED MPG and COMPUTED MPG is.... (Drum Roll Please) .025 MPG. ACTUAL MPG is .025 HIGHER than Displayed. Probably due to rounding, in XL I computed thousanths while the MFD displays tenths.

AVERAGE Tank MPG todate for the car is: 46.7, MIN is 41.6, and MAX is 51.6.

I'm satisfied...