Is the computed MPG real?

<div class='quotetop'>QUOTE(fairclge @ Apr 14 2007, 12:54 PM) [snapback]423331[/snapback]</div>

first fill will almost always be the worst simply because your "full tank of gas" will probably not be as full as you want it to be. my first fillup was 8 mpg off but i figured that the full tank i got from the dealer was a good gallon short which i pretty much affirmed when the first bar disappeared at 30 miles. having driven my 04 for over 2 years and having the first bar disappear normally in the 70-110 mile range, i knew the first fillup would not be good.

 

I always come out less with the pen & paper calculation that with the car's computation. Normally anywhere from 2-6 MPG less. I've noticed if I accelerate a tad quickly, then feather the gas pedal on level surfaces, I can really maintain some high averages on my typical 8 mile compute to work. I also try hard not to accelerate going up the hill and wait to give it some gas till I get going down the hill. I'm starting to think that most American's drive exactly the wrong way by accelerating up the hill and coasting down. People behind me probably think I'm putzing, but I do go the speed limit and as long as it's a two lane in both directions, I feel they can just pass me. I've got two questions. First, am I contributing to Prius owners being stereo typed as putzers, and two, am I just tricking the computer and not really improving my actual mileage that much? I'd like to hear what the experts have to say!

 

<div class='quotetop'>QUOTE(boutime @ Jul 16 2007, 05:19 PM) [snapback]479921[/snapback]</div>

i do the same thing and as long as its two lanes going your way, let em think whatever they want. around here, i am beginning to see all kinds of vehicles doing it as well. with each step up on the gas price ladder, i think you will see more and more following suit

 

I have my new car one week today.
I have 475 miles on the car. The computer kept showing 50+ MPG. I did not trust it so I put in gas 2 times.
Once 6 gallons at 300 miles, so it did compute to 50 MPG.
The second time 3 gallons 450 miles.
So the computer reading seems to be right on.
I drive mostly in the city. Did do one HWY trip of 150 miles at 65 mph.
EM

 

<div class='quotetop'>QUOTE(DaveLeePrius @ Apr 9 2007, 05:05 PM) [snapback]420403[/snapback]</div>

It sounds like you never reset the MPG calculations. The 2004/2005 resets it automatically on each fill, but I believe the newer Priuses require you to press the Reset button.

While that gives you an accurate lifetime average, the problem is that after thousands of miles, the accumulated weight of all the previous miles makes the MPG value so steady that you could go weeks with crappy gas milage and your display would not show it. At this point, if you were to drive an entire tank at 30 MPG, your average would only drop to 42.3.

 

<div class='quotetop'>QUOTE(Marlin @ Jul 17 2007, 11:38 AM) [snapback]480239[/snapback]</div>

I believe that to be inaccurate. No Prius will automatically reset the mileage calc if 'fill' is only a few gallons. All Prii (at least through 2006, we don't have a 2007) will automatically reset if you put enough gas in. I don't know the break point, but guess half a tank is enough to do it. It will reset shortly after you trun the car on.

If you want to know your lifetime mileage, you need to keep track of total miles and total gallons put in. You can use the MFDs cals, but you MUST weight it mathamatically with miles driven on that fill. Meaning - 5 data points at 45, 46, 47, 48, 49 MPG do not necessarily mean you have a lifetime average of 47 if the first 4 readings were 200 miles and the last was 600. You can only average them directly if you ALWAYS fill at the exact same 'miles on the tank' reading. The actual 'at the pump' numbers are most accurate, over time. The average for the first few tanks might bounce a lot even if the MFD readings do not.

There is variation in pumps, and cars. Some pumps seems sensitive to the back pressure caused by the bladder and kick off early. There are a couple in my area that will stop at LEAST 1 gallon less than what I know I have used (based on MFD being ~2% generous) and the miles I have driven. It also seems to happen earlier when it is cold (winter in VT cold, not winter in San Diego cold). Especially in winter, those like to kick off at 8 or 8.5 gallons, even though I have used, and eventually put in, ~10 gallons (with no spillage or gas in the fill pipe!!)

My last pip starts flashing when I have used about 10 gallons.

 

<div class='quotetop'>QUOTE(bruceha_2000 @ Jul 17 2007, 02:20 PM) [snapback]480490[/snapback]</div>

your belief is incorrect. on 2007's you must hit the reset if you want to track tank mileage

 

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

I've had my prius for two tank refills and both of them were within 2-3 mpg of what the MFD showed for mpg. However, Today I drove for 461 miles and the MFD showed that I was averaging 47.7 mpg and the gas gauge still showed two bars. I was really surprised when I filled it up that is took 11.1 gals and the math comes out to 41.5 mpg. It is a very good thing that I did not trust the fuel guage indicator!

 

I am on my 3rd tank of gas with my new prius.

At the end of the first tank, the MFD showed 45.6 mpg, but my calculations showed 41.5 mpg.

End of the second tank, MFD showed 47 mpg, but my calculations showed 41.5 mpg.

The difference seems to be off quite a bit, 5 mpg. My calculations are Distance traveled/Fuel gallons pumped. Am I doing something wrong? Is my MFD being too generous?

 

<div class='quotetop'>QUOTE(lagreca @ Aug 4 2007, 03:26 AM) [snapback]490425[/snapback]</div>

Situation normal. I now have over 15000 miles on the car. Usually the MFD is within a couple mpg of my calculations. It can also be off 10% or more. FWIW: I put the pump on the slowest setting and don't pump past the automatic shutoff. This is the most one can do to get a consistant "full" tank.

Your numbers appear low. What is your tp? Go to at least 42/40. Consider blocking your upper grill. Doing these things should give you a few more mpg at very little cost.

 

On my last two fills, the MFD was .1 mpg more than the calculated.(.1 and .03 respectively)

 

<div class='quotetop'>QUOTE(JimN @ Aug 4 2007, 09:17 PM) [snapback]490763[/snapback]</div>

I have my TP set to 42/40. I live in San Diego, so I don't think I need to block the grill. Especially right now with our high temperatures.

I'm just curious why the big difference between the MFD and calculated mpg.

I will certainly try filling the tank more slowly at next fill. I was just filling it like I have filled my other cars, at full speed. When it shuts off, I usually try one more, just to make sure it didn't prematurely shut off.

Thanks.