calculated vs. computer MPG - Please post your results

Good deal! Although I prefer a site with a little more detail that can be shared with anyone, fueleconomy.gov is another good site. Well done!

Bob Wilson

 

Model III with Sunroof and Nav:
First tank 491 miles, Car estimate: 53.8; my calculation: 52.6
Second tank 418 miles, car estimate: 54.9; my calculation: 52.3

 

Did my first fill up yesterday.

483 Miles.
Per Computer: 56.9 MPG
My calculated: 56.7 MPG.

Pretty close I'd say.

 

It is interesting to me how many people have such close numbers on the first tank. I can understand this if the dealer did not reset the Trip Odo when filling the tank, but those few miles hardly seem enough to close the gap.

 

I suspect the dealer does not use the fill to first click; pause; and fill to second click method:

1. 611.7 miles - dealer tank to fuel exhaustion, Shell 89, E10
2. 651.3 miles - second tank to fuel exhaustion, Shell 87, E10
3. 660.9 miles - second tank to fuel exhaustion, Shell 87, E10

Note, I don't know many other folks who drove their first three tanks dry. <grins>

Bob Wilson

 

It seems from recent post that newly manufactured Prius's are more accurate in the MPG dash display than earlier ones, like mine, which is inthe 2,000 range of manufacture.

So, maybe Toyota did make a secret correction on the MPG dash guage on recently manufctured Prius's.

Just my thoughts;

alfon

 

I have never seen any display standard for the fuel economy numbers. Anyone?
My experience is the VW Golf GT displays approx 10% better than actual.
The Gen2 Prius displays 2-3 % better, but no one complained about that.

Ken@Japan

 

Sorry, but I have to disagree. I don't think you are giving the Toyota software engineers enough credit. The obvious way to do the accumulated mpg calculations is to keep track of distance traveled (using some small distance counter like a wheel rotation) and fuel consumed (using some small counter like fuel drops injected). These two integer values are incremented indefintely per trip meter reset. The displayed value is just the scaled ratio of the two values. It is always as accurate as the measurement of fuel and wheel rotation counter can be regardless of any displayed values that you might think are cheating it.

3PriusMike

 

The results for my last three fills:502 miles 9.02gal 55.6mpg(car Comp 57.4) 493 miles 8.7gal 56.7mpg(computer59.9) 490miles 8.8gals 55.7mpg(computer 57.1). Last trip I took my TomTom with me and noticed my car speedometer is off realitive to my TomTom gps reading. The car is from 1-2 faster than the gps measurement. Over 55mph it is usually 2 difference. My driving is a mixture of about 50% highway and city driving.

 

total odo gal gal total tank mpg running mpg date
376.00 8.274 8.274 45.44356 45.44356
803.00 8.975 17.249 47.5766 46.55342
1,175.00 8.246 25.495 45.11278 46.08747 8/5/2009
1,571.00 8.751 34.246 45.25197 45.87397 8/7/2009
1,961.00 8.19 42.436 47.61905 46.21076 12-Aug
2,329.00 7.482 49.918 49.18471 46.65652 17-Aug
2,801.00 9.693 59.611 48.69 493 46.98797 20-Aug
3,222.00 8.717 68.328 48.29643 47.1549 26-Aug
3,644.00 8.207 76.535 51.41952 47.6122 29-Aug
4,017.00 7.819 84.354 47.70431 47.62074 3-Sep


I noticed it, too. The trip display usually reads about 50. Incidentally, the speedometer reads a mph slow at 60 mph. This is as compared to gps, which is quite exact. So Toyota is getting a few more percent there, too. Hmmm. This is hard to read the bottom line is 47mpg, and the display says 50 or 51.

 

My question is a bit different. For my Gen II 2006 Prius, the owner's manual cautioned that fuel economy would drop if the gas tank was allowed to go below 1/4 full. In personal experience over 3 1/2 years, it seemed like it really did! I may have missed it, but I did not see this same caution in the Gen III owner's manual. Perhaps this is because the bladder from the Gen II is gone. Does anyone know if the Gen III maintains better fuel economy if the gas tank is not allowed to get below 1/4 tank? Thanks! :typing:

 

I have never heard of it.
Would you please let us know which page of the owner's manual is written about it?

Ken@Japan

 

I just had my N=1 refill with a grand total of 415 miles, well I got a little anxious, at 7.791 gallons. So the first 7.791 gallons have given my Gen III 53.3 MPG as opposed to the calculated 55.7 MPG.

 

After 2 tanks of gas, I find the computer calculated data to be 2-3 MPG overstated than actual calculated ... computer states 52 and actual is 49 ... what are you doing to get into the mid-50's range ... I must not be driving it correctly ... thanks/Mark

 

43.6 on the trip meter
41.3 calculated
exactly 5% difference for the 2nd tank in a row

mostly short trip driving, with a few longer interstate runs. The
good news is this is actually an improvement. I've been working harder at watching the instant mpg and HSI, and not regenerating or running down the battery. Weather has also been a bit better.

 

First fill-up this weekend. I refueled sooner than I needed as there was still a quarter of a tank left, but here are the stats:
428 driven miles. Computed 56.7, calculated 54.0.

 

At N=9 refills (3311 miles), my lifetime MPG is 54.79 MPG at the pump, 57.39 imputed from the per-tank MID display.

The display error has expanded to 4.75% or 2.6 mpg, but changed little since the 4th fill.

 

My indicated average is 59.06 and actual is 56.09, making a consistent 5% difference over the first 2377 miles. Considering this along with the well-reported 2 mph optimism, I'm beginning to wonder if the odometer is also reading high by 3-5%.

 

In the past, I've trimmed the upper and lower MPG elements, a total of 10% of all samples, to try and minimize outliers. The hypothesis being that the end points have a greater impact on the tend. But in this chart, I changed the outlier definition.

I calculated the % of error between actual and calculated and then dropped the 10% having the greatest error. These larger, relative errors can occur from a fill-up change and are less likely to reflect a constant, vehicle trend. With 138 samples, 14 would be ~10%. Then assuming this is a linear error, so both the indicated and real values intercept at 0, we get:


If I take out the zero intercept, the trend line formula changes. So folks have two fomulas to choose from:

1. actual_MPG = (.9545 * indicated_MPG) (R**2 = .952)
2. actual_MPG = (.9084 * indicated_MPG) + 2.5071 (R**2 = .9545) (NOTE 1)

So off hand, the calibration error appears to be between 5-10%. Ken@Japan reported a 10% effort from a conversation with a Toyota engineer. Which ever calibration formula is used, the adjusted MPG, the likely error will be fairly close to the actual ... with the exception of the higher MPG numbers. If the indicated MPG is above 55 MPG, these formula are not going to be terribly accurate.

Once again, thanks to everyone who contributed data. With a sample set of 138; improved outlier removal; and with or without zero intercept; we have fairly usable correction formulas.

Bob Wilson

NOTE 1 - notice that the fixed offset, 2.5 MPG ~= 5% * 50 MPG.

 

Just got back from Reno. Had a great time.

Here are are trip stats:

Fill up: 412.0 miles/ 8.364 gals/ 49.25 MPG/ guage 51.8
(Note above was filled at sea level a few days before and my wife went back and forth to her business.)

Fill up: 295.7 miles / 5.947 gals/ 49.72 / guage 52.1
Oregon fillup about mp # 70 on Interstate 5

Fill up: 366.9 / 7.413 / 49.45 / 52.5
Nevada / Carson City

Fill up: 164.0 / 3.325 / 49.32 / 52.3
At Reno / driving around up to Lake Tahoe, Virginia City

Fill up: 390.2 / 7.364 / 52.98 / 55.3
Oregon/ Interstate 5 / about MP 130

Fill up: 252.7 / 4.816 / 52.47 / 53.8
Oregon home at Seaside

Miles / 1,881.5 / 37.499 gals / 50.174 MPG

Speeds / at or slightly above Speed Limit to keep up with traffic
Weight / me my wife and about 200 lbs of luggage

MPG was better last time but car was new and I believe I was a little easier with my driving. This time I drove at higher speeds climbing several mountain passes.

This is not flat road driving.

I am impressed we got this kind of mileage for driving the speeds we did.

Alfon