how do i tell my real MPG??

39.7 is your real mpg. As Evan points out, the computer does a pretty good job calculating mileage, and if anything, is usually just a tab optimistic. Manual calculations are accurate only after many, many tanks. I'd want a year's worth of data before I'd consider it close to good.

Tom

 

<div class='quotetop'>QUOTE(qbee42 @ Jul 1 2007, 10:53 AM) [snapback]471067[/snapback]</div>

How about the following model for a manual calculation:

1. Go to a service station that will serve as a start and end point for the drive cycle.

2. Fill up until the pump's auto shut-off kicks in. Note the pump that you filled up from for later reference.

3. Note your odometer's mileage as your starting mileage.

4. Drive a circuit that involves the exact return trip to neutralize variables such as hills or wind (head or tail) effects.

5. Return to the gas station and the same pump you filled from in 1. above.

6. Fill until the auto shut-off kicks in.

7. Note the ending mileage and the amount of gas that was pumped.

8. Divide miles driven by gallons filled and you are at a pretty accurate MPG. Check the variance between computed and calculated and you have a pretty reliable variance factor to apply to future checks.

 

You might be looking at two different averages. You didn't mention if you reset the average when you last refueled.. If you did not reset it then the display will be showing your lifetime average while your calculations are giving you your average for your last tank.

 

It automatically resets on the MFD. If you use the dash then you will have to reset it manually.

 

<div class='quotetop'>QUOTE(Tadashi @ Jul 1 2007, 07:56 PM) [snapback]471203[/snapback]</div>

Depends on what you mean by "It".

The number of miles on a tank resets automatically on the MFD (if you put in more than 3 gallons). The average on the MFD DOES NOT reset unless you press the reset button on that screen. The average is the total average since the last time you pressed reset on the MFD.

 

When I first got my Prius, I kept track of the mileage manually. When I filled up and checked the manual numbers to the MFD, it came out almost identical; off by a tenth of a mile or so. I did that for the first two or three tanks and after that, I started to rely only on the MFD. Also, I found that I only had to reset the ODO to get the Avg MPG on the MFD to reset too.

 

<div class='quotetop'>QUOTE(Trevor @ Jul 2 2007, 10:39 AM) [snapback]471586[/snapback]</div>

On the 04 and 05 models, both the average MPG and the number of miles automatically reset when the tank is filled up. There was a change for the 06 model year, where the average MPG no longer automatically reset when the car is refueled.

 

I need an opinion. I just filled up and posted the results in the "I beat the EPA..." thread. The FCD showed 77.0 and my hand calculations showed 74.1. I travelled 806.1 miles and put in 10.88 gallons. Which number is correct; the FCD or my hand calculation? Either way it's good but accuracy is important to me.

 

<div class='quotetop'>QUOTE(diamondlarry @ Jul 2 2007, 01:42 PM) [snapback]471750[/snapback]</div>

You could call that your best tank, but I'd still rely on an average hand calculation over several tankfuls, due to the variation in bladder capacity, and the slightly variable MFD readout.

 

Confused new owner,

Just got back from a 527 mile round trip to the mountains.
Filled up before I left, drove appoximatly 248 miles with cruise at 68-70 mph, including mountains. Upon arrival I gassed up taking 8.5 gals.
Average MPG showed 53.4 ??? OK figured iy was the cruise control.

 

Sorry about the mis-send before this one

Confused new owner,

Just got back from a 523 mile round trip to the mountains.
Filled up before I left, drove appoximatly 248 miles with cruise at 68-70 mph, including HIGH mountains. Upon arrival I gassed up taking 8.5 gals. Average MPG on display said 53.4 ??? OK figured it was the cruise control. which i left on to do it's thing.

On the return trip, with some small side trips, I only used the cruise control on the long flat runs, I gassed up when i got home with a display reading of 52.6 average mpg, actual gas put in was 4.85 gal.

No way did I get 53 mpg, after doing the math I actually got 39.22 mpg average from both directions

My question is, How does the computer actually come up with 53.+ number?

PS Car only has 1500 mile on clock, so I know it's still breaking in.

 

I believe it measures the flow of fuel in the engine. I have found that my MFD can sometimes be accurate and sometimes way off so I always do a hand calculation. I still track the MFD for kicks. Look at the link in my signature block for the file and you can see how much it varies.

 

<div class='quotetop'>QUOTE(IsrAmeriPrius @ Jul 2 2007, 02:54 PM) [snapback]471643[/snapback]</div>

Thanks for clearing that up. Glad to know that everyone is right? The OP seems to have an 07, so I'm guessing that the average does not reset like the 06.