how do i tell my real MPG??

iioeo · 07-01-2007, 11:31 AM

so i was totally bummed that my "average" mpg, according to the display, has only been about 39.7, but when i refueled yesterday i calculated my total miles since my last refueling it was 46.5! i made sure to drive it till it was blinking on E so that i could get an accurate gallon read.

anyone have info on this?

**efusco** · 07-01-2007, 11:37 AM

In your case I'd tend to believe the MFD read-out. Over time the calculated MPG will be a more accurate representation of reality as varibility of tank filling will level itself out when calculated over distance driven.

Don't get discouraged, FE tends to improve over the first several thousand miles.

Look at the stickies on the Fuel Economy forum here. There are some articles that can help make sure that you're getting the most out of your car. Things like looking at the tire pressure and, perhaps, adjusting it upward, having the dealer check the alignment (a notorious problem in the new Prii), and then just common sense driving things that we tend to not do in conventional cars like not speeding, slowing early and anticipating stops, increasing following distance a bit, moderating AC use, etc.

Even small changes can have pretty significant impacts on your FE. If you get really hungry for great MPG we can start teaching you about Pulse & Glide techniqes and other advanced stuff that is not for the average "get me to work and home again" driver.

qbee42 · 07-01-2007, 01:53 PM

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

apriusfan · 07-01-2007, 03:23 PM

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

Quote:

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
[/b]

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.

Trevor · 07-01-2007, 07:02 PM

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.

Tadashi · 07-01-2007, 07:56 PM

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

Trevor · 07-02-2007, 01:39 PM

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

Quote:

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

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.

JackDodge · 07-02-2007, 02:07 PM

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.

IsrAmeriPrius · 07-02-2007, 02:54 PM

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

Quote:

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. [/b]

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.

diamondlarry · 07-02-2007, 04:42 PM

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.

07-01-2007, 11:37 AM	#2
efusco Troll Slayer Join Date: Nov 2003 Location: Nixa, MO Posts: 14,708 My Car: 2004 Prius Package: #9 Nominated 1 Time in 1 Post TOTM Awards: 0 Friends: 26	In your case I'd tend to believe the MFD read-out. Over time the calculated MPG will be a more accurate representation of reality as varibility of tank filling will level itself out when calculated over distance driven. Don't get discouraged, FE tends to improve over the first several thousand miles. Look at the stickies on the Fuel Economy forum here. There are some articles that can help make sure that you're getting the most out of your car. Things like looking at the tire pressure and, perhaps, adjusting it upward, having the dealer check the alignment (a notorious problem in the new Prii), and then just common sense driving things that we tend to not do in conventional cars like not speeding, slowing early and anticipating stops, increasing following distance a bit, moderating AC use, etc. Even small changes can have pretty significant impacts on your FE. If you get really hungry for great MPG we can start teaching you about Pulse & Glide techniqes and other advanced stuff that is not for the average "get me to work and home again" driver. __________________ Evan E. Fusco, MD Seaside 04BC9/OEM EVb/NAV & auto-BT/XM Radio, CAN-View, Shark Fin, BT Stiff. Plate, Skid Plate & Strut Support

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07-01-2007, 11:31 AM	#1
iioeo Junior Member Join Date: Jun 2007 Posts: 12 My Car: 2007 Prius Package: #1 Touring Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	so i was totally bummed that my "average" mpg, according to the display, has only been about 39.7, but when i refueled yesterday i calculated my total miles since my last refueling it was 46.5! i made sure to drive it till it was blinking on E so that i could get an accurate gallon read. anyone have info on this?

07-01-2007, 01:53 PM	#3
qbee42 Senior Member Join Date: Mar 2006 Location: Northern Michigan Posts: 6,236 My Car: 2006 Prius Package: #7 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 6	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

07-01-2007, 07:02 PM	#5
Trevor Senior Member Join Date: Jul 2004 Location: Staten Island, NY Posts: 323 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 1	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.

07-01-2007, 07:56 PM	#6
Tadashi Senior Member Join Date: Feb 2005 Location: Fort Hood, TX Posts: 794 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	It automatically resets on the MFD. If you use the dash then you will have to reset it manually.

07-02-2007, 02:07 PM	#8
JackDodge Independent voter Join Date: Sep 2005 Location: Bloomfield Hills, MI Posts: 2,330 My Car: Other Hybrid Package: N/A Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	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.

07-02-2007, 04:42 PM	#10
diamondlarry EPA MPG #'s killer Join Date: May 2007 Location: Elkhart, IN Posts: 557 My Car: 2007 Prius Package: #5 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 3	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.