calculated vs. computer MPG - Please post your results

The station for the first 2 fill-ups might have miscalibrated pump(s).

 

2007.7 miles / fuel used 38.915 gal's/ reg gas 10% Ethanol / acutal mpg calculated 51.591 mpg / indicated 54.0 mpg. (Real world driving)

Last trip, mostly highway 55-70 mph / ac on about 75 % of the time / my wife and myself in the car / filled up at Seaside ( sea level) / filled up again in Safeway, Longview WA (above sea level) / 144.1 miles / 2.494 gal's / calculated 57.778 mpg / indicated 58.9 mpg/ fuel used reg gas with 10% Ethanol blend

Our prius has Michelin Energy Saver tires / 195x65x15 / with 44 psi of nitrogen in all four tires / MAX Sidewall pressure / we now have 24,000 plus mile on those tires / they are fuel efficient, believe me

Oil is Mobil One 0w20 with four quarts in the crankcase so it is not overfilled for the best possible mpg's.

alfon

 

Is there a way to get the calculated and hand computed MPG around equal? The MPG im getting is about 3MPG off from what I hand compute vs what the Prius is telling me.

 

Averaging about 2.5 mpg (calculated) lower than computer readout.

 

This questions is most likely "asked and answered" somewhere else but I think its germaine to this thread -

What is the "usable" volume of the 11.9 gallons?

I too have filled up with less than 10 gallons and wondered just "how low can I go" without actually running out of fuel (and yes, there are many factors that can contribute to how quickly one runs out of fuel) but thinking about average driving conditions is it +50 miles past the "0" miles to empty indicator on the HSI? +100? +125?

 

515.7 miles / 9.956 gals / computer 55.0 mpg / calculated 51.797
10% Ethanol Reg Gas ( Safeway Brand, Longview, Washington)

mixed driving / 85% highway / speeds / rural highway 45-60 MPH/ and interstate 60-70 mph.

 

My approach is to do several careful full fill-ups right as the 1st bar starts blinking during steady driving (that happens to be 20 miles to empty on mine). I will use that data and the rated capacity of the tank to determine how much fuel is left at "0 miles to empty". So far it looks like about 2 gallons.

Once I am satisfied i know how much fuel is remaining, for everyday driving I will normally stop at the first station I like after I hit 0 miles to empty.

If I didn't feel like stopping then, I would be comfortable running it down to 1 gallon remaining, so about 50 miles past 0, depending on driving conditions. There is always the possibility of a planned gas stop not being open, or getting stuck in traffic or other emergency so 1 gallon is the limit for me.

Something I have observed is that miles to empty is more consistent than the flashing bar. A steep grade or sharp corner can cause the bar to start flashing early, then quit flashing for awhile. The miles to empty seems to be lot better damped so I will go by that once the flashing bar gets my attention. Both use the same sensor in the tank, the miles to empty seems to process the signal differently and apply more damping.

 

bump.

 

I would think that your best mpg would be driving on 1/3 or less of a full tank. Less fuel = less weight = a bit better mpg. Filling up the tank adds a load of weight that needs to be moved around town or up hills. I could care less because the mpg in my prius hovers around 50 anyway. It is interesting how my angst goes up a bit after a fillup and for the first few miles the readout is at the high 30's-low 40's. My how my perspective has changed. Amusing when 40mpg is a cause for concern.

 

Ive been wondering exactly how much gas you can put in . I ran mine 6 miles past 0 without running out. I'm on my 10th tank since new and have been filling up the tank to the brim, so i could get accurate calcs. I have found that if i fill to pump stop, then try to put more in once i get 9.9 gal. let the gas trickle in slowly with the nosle barely in i end up at 11.7. One of these days i will though a can of gas in the back and run it out, to see exactly how much gas you can put in it.

To date best miles on a tank has been 645

 

When you fill up when the pump first auto clicks off you can still put
in 2 gallons up to brim.

alfon

 

150.0 miles / 2.803 gal / indicated 54.4 MPG / calculated 53.514 MPG / 10% Ethanol regular gas ( safeway brand).

It is my theory that short trips, such as this, the MPG guage is more
accurate than on long trips.

Alfon

 

I thought you weren't supposed to top off this tank. I know its not a bladder like the gen II but doesn't it still make a problem with the fuel injectors or something like that....

you can tell I'm a real technical gear head.

 

I've driven 40 miles past the 0 miles remaining mark no problem. I still was only able to get ~10 gal in during the fillup on this 11.5 mile tank. The indicator is very conservative.

 

This is my first post, & if I am not following the recognized etiquette, my apologies. I have made only two fuel consumption checks since getting the car in mid-August. My Prius is a IV with moon/solar roof. (Made in 04/10). Both journeys were roughly 400 mile round trip, mostly highway. Both trips were made with two occupants (driver & passenger: combined weight 277lbs) and light luggage (two small suitcases).
The first trip was to Vermont and involved considerable mountain climbing. On the highway I had the cruise control on for much of the time. On the second trip (to Stamford, CT), the highway travel was much flatter, I hardly used the cruise control, and tried driving very economically. Both journeys involved 65+mph.
On the first trip my FCD read 46mpg and my actual MPG at fill-up was 43mpg.
On the second trip my FCD read 56.2 mpg and my actual MPG at fill-up was 53.7 mpg.
I realize information based on just two trips does not signify much, but this is to get the ball rolling and I'll add to this later on.
Tony

 

Another short fillup and it appears my theory that short fillups
are more accurate vs computer and true mpg.

229.5 / 4.608 gallons / 51.1 indicated / 49.804 calculated

10 % Ethanol Reg gas

alfon

 

It is my theory that cumulative number on multiple fill-ups is more
accurate than single fill-up.

Ken@Japan

 

computer: 52.6
actual: 54.4

 

The reason its bad to top off your tank is because excess oil can damage the carbon filter where only vapors are supposed to be, not oil. When oil enters the carbon filter it can drip on the ground, wasting your $$ and damaging your engine.

 

I believe the major concern is spilling gas on the ground more than
anything else. I could not find filling the tank to its max capacity,
which would be to the brim of the tank filler, in the owners manual.

I had several cars, and motorcycles that I would fill to the brim with no problem for the last 44 years of driving.

With our Prius I can't pump my own gas in Oregon so the attendant
stops the pump at the first auto shut off.

But in Washington self serve is legal and at times I have
filled to the brim,with no problems.
alfon