Is the computed MPG real?

<div class='quotetop'>QUOTE(jaspinwall @ Apr 9 2007, 03:46 PM) [snapback]420349[/snapback]</div>

Must be nice. I've had my 06 Prius since mid-September 2006 and the best I've gotten was 49 mpg during the first 600 miles. Since then, nothing better than 39. Filled up 4/9/07 and mpg read 42 during the first mile and then dropped to 21. However, only have driven 6 miles on this tank so far. Perhaps a 30 mile round-trip on Wednesday will get it back into the 30s. But from the posts on this site, off-the-scale mileage seems to be the rule rather than the exception, so I may have the only Prius that gets 30-40 mpg consistently. Life's too short to obsess over mpg.

 

<div class='quotetop'>QUOTE(jaspinwall @ Apr 9 2007, 12:46 PM) [snapback]420349[/snapback]</div>

After 35,000 miles and three years, my calculated MPG is little over one MPG less than what the computer says. Others here on PriusChat have similar results.

 

no, that's where mine is hanging out, too.

That would be fine. Toyota has given us a car that drives just like we expect, only it's getting 1.5x to 2x the gas mileage while we drive it like that. Can't really argue with that.

 

Mine started out at 42mpg when we bought it with 28 miles on the clock, and now it reads 43.1mpg. It doesn't fluctuate at all, it stays consistant. Never been below 42, never been above 43.1 and we've around 3500 miles now.

Dave

 

<div class='quotetop'>QUOTE(jaspinwall @ Apr 9 2007, 04:46 PM) [snapback]420349[/snapback]</div>

It's real, within a very small margin for error. Nice, huh.

Tom

 

The computed MPG is based on a sum of the pulses from the wheels with each rotation, for distance, divided by the time the injectors are open, for fuel volume. Standard tires are 855 revolutions per mile. These are both fairly tiny increments and the computed fuel efficiency is fairly accurate.

Tracking your fuel efficiency by taking miles driven (to the nearest tenth, or 528 feet) and dividing by gallons (to the thousandth, or 0.128 ounce) is complcated by the variability of the volume of the gasoline bladder and the fact that gasoline volume is not standardized for temperature or energy content. Over time, maybe a year, the computer MPG and hand-computed MPG will converge. Don't get upset if your hand-calculations are way off from the computer. It's still better fuel economy than you'll get in almost any other vehicle.

 

<div class='quotetop'>QUOTE(jaspinwall @ Apr 9 2007, 03:46 PM) [snapback]420349[/snapback]</div>

I find the MFD to be a bit optimistic compared to my calculations but a reasonably good estimate. Start shopping for gas a 2 pips, really consider buying gas at 1 pip. BUY GAS WHEN THE PIP FLASHES!

Bottom line--enjoy the car. Don't get hyper about the mpg. BTW, did you inflate the tires to 42/40?

 

<div class='quotetop'>QUOTE(JimN @ Apr 9 2007, 10:44 PM) [snapback]420596[/snapback]</div>

Why? I went around 30 miles with a flashing pip before getting gas. Then I could only get 10 gallons in the tank.

 

<div class='quotetop'>QUOTE(Rest @ Apr 12 2007, 09:55 PM) [snapback]422334[/snapback]</div>

It's been referred to as the "guess gauge" on this forum for good reason. Since I got my Prius two years ago and started hanging out here, I've read reports of triple digit mileage on the flashing final pip, and others who ran dry before they got to double digits... YMMV

Vince

 

enjoy your Prius. i strongly recommend backing up your mileage figures with the old paper and pen method. lifetime stats are easy to maintain and i think you will enjoy logging in 55 mpg+ tanks. also the speeds you mention are in the sweet spot for mileage. as long as you mellow out on the AC, i think you will find 60 mpg is only right around the corner because as the temperatures rise, so will your mileage!

 

<div class='quotetop'>QUOTE(jaspinwall @ Apr 9 2007, 01:46 PM) [snapback]420349[/snapback]</div>

Speaking from my extensive rookie experience with my 3-week old Prius... I got 53 mpg on my first tank. I'm 400 miles into my second tank (have 3 pips on gage left) now and have 55 mpg. I assume VA has been colder than Albuquerque.. so 52 is probably really good. The variables that will really affect mpg are temperature (higher = better), driving conditions (30-40 mph good, short trips <15 minutes bad, uphill bad, downhill good, AC/Heat/Defroster bad, etc). I recommend you read the postings about the different power modes in the Prius... stealth, stealth warp and all the rest, you will develop a symbiosis with your car like I have. Enjoy it!

 

<div class='quotetop'>QUOTE(JimN @ Apr 9 2007, 10:44 PM) [snapback]420596[/snapback]</div>

I"m finding the guess gauge quite aggrivating myself. I headed straight for the station after getting down to one pip, only to find the car would not take more then 8 gallons? I guess its all a learning process for the car...

 

"Computed MPG..." I suppose the OP meant the MFD's indicated MPG.

In 2½ years and two Prius I have found that the MFD's calculated MPG are less than 2% different from the hand-calculated MPG. The hand-calculated will vary somewhat from tank to tank, due in part to the vagaries of the fuel bladder, but works out over many re-fuelings.

For me, there's not enough difference to be significant.

 

Don't forget about rounding error. MFD calculates to one tenth, gallon reading is to one thousandths on the pump. Mileage I'm guessing increases when you hit the next mile with no rounding. The bladder and temperature filling issue would need at least one year to even out.

 

<div class='quotetop'>QUOTE(douglas001001 @ Apr 13 2007, 02:41 PM) [snapback]422899[/snapback]</div>

there are many many sources of error in calculating by hand, especially since the bladder is variable in its volume and variation in shut off devices on pumps (which you should listen to instead of getting covered in gas when your bladder deflates). i'm guessing, with all these errors, one has to assume .1 mpg accuracy is the absolute most you can truly get on paper. and that's pushing it. The MFD with it's internal measurements I believe is much more accurate than anybody ever can be.

 

<div class='quotetop'>QUOTE(jgills240 @ Apr 13 2007, 03:05 PM) [snapback]422920[/snapback]</div>

I was offering a why it takes a while for mfd and hand calculated to converge with the sources of rounding/error on both, as you state .1 is certainly good enough.

In the end what people should do is take their own personal averages under whichever method they use as a benchmark for future tanks and how they are currently driving.

 

first fill 196 miles 4.6 gal when pump stopped. around 42 MPG computer said 49 MPG.