Large drop in MPG

blackprius · 06-06-2006, 02:27 AM

Hi:

I read the initial article about what to expect for mpg. I have owned my Prius for 10 months and it has run very well for most of the 10 months. My first month of ownership was in August. August in Phoenix is still very hot, 100 -110 F degrees. I used to drive 90 miles round trip to work, my fuel mileage was between 45-48 mpg with the A/C on. In the Winter, the fuel mileage was between 48-51 mpg (in Phoenix) Recently, the fuel milage has gone to pieces in a hand basket. My mpg has gone from 45 to 34.5 over a few weeks period. I drive my 2005 Prius until the blinker blinks, add fuel, do the math and bingo caluclate my mpg.

What I have notice is that if it is a cool day in Phoenix, 75-85 degrees, I get much better mpg even with the A/C on. Once it gets hot, the engine is continously running. Even today, at the stop lights with the battery more than half full but not green, the engine was still running to charge it up. Basically, in the winter I can expect to get battery power assistance from 10 to at least 40 mph. Now in the heat, the battery power stops after 12 to 15 mph and all I see is the engine running to drive the front wheels and charging the battery on the display. FYI, I use Mobile One Synthetic 5-30, what the Dealer puts in for Synthetics. Furthermore, I have even had the dealer install the BioPro QX3 which they promoted as adding some mpg. I even added Chevron Full Fuel System Cleaner to see if it helps.

I am at a lost on the poor fuel mileage. If this continues and the Toyota dealer cannot help, I may need to sell it and get a Yari or Scion.

seasalsa · 06-06-2006, 08:53 AM

Are you running A/C Auto? If so what temperature do you have it set at? 78 will seem cool when outside temps are hovering around 100 degrees. What tire pressure do you use?

brandon · 06-06-2006, 10:16 AM

So you're saying these are your MPG calculations and not those displayed on the MFD?

blackprius · 06-06-2006, 11:46 AM

<div class='quotetop'>QUOTE(brandon @ Jun 6 2006, 06:16 AM) [snapback]266610[/snapback]</div>

Quote:

So you're saying these are your MPG calculations and not those displayed on the MFD?
[/b]

Hi Brandon:

I don' trust the computer (MFD) on mpg. Several months back the computer displayed an average of 51 mpg. When I fuel it up and did the math (how many miles driven divided by how many gallons put in) I got a mpg of 41. Yesterday, when I filled up the computer displayed an average of 38.5 mpg.

<div class='quotetop'>QUOTE(seasalsa @ Jun 6 2006, 04:53 AM) [snapback]266572[/snapback]</div>

Quote:

Are you running A/C Auto? If so what temperature do you have it set at? 78 will seem cool when outside temps are hovering around 100 degrees. What tire pressure do you use?
[/b]

The car is running with A/C Auto and recirculation on. The temperature is set to 75. The tire pressure is whatever the Toyota Dealer mechanic set it at for the Goodyears.

brandon · 06-06-2006, 11:57 AM

But how can you be so sure your method for determining the MPG is correct? Yes, the MFD calcualtions are usually 2-3 MPG higher than actual, but you assume you are calculating correctly. How do you know how much gas has actually been used?

**Tideland Prius** · 06-06-2006, 12:04 PM

You have to take an average of sevaral tanks. If I force fill a tank, I'll get super bad mileage for that tank and it's easy for me to blame the car for a miscalculation. On the other hand, if the pump was sensitive and cuts off early and I don't round up or what not, then it's easy for me to get better mileage than what the computer said.

Also, A/C usage will drag mpg down esp. in MAX COLD.

seasalsa · 06-06-2006, 01:13 PM

<div class='quotetop'>QUOTE(blackprius @ Jun 6 2006, 08:46 AM) [snapback]266665[/snapback]</div>

Quote:

Hi Brandon:

I don' trust the computer (MFD) on mpg. Several months back the computer displayed an average of 51 mpg. When I fuel it up and did the math (how many miles driven divided by how many gallons put in) I got a mpg of 41. Yesterday, when I filled up the computer displayed an average of 38.5 mpg.
The car is running with A/C Auto and recirculation on. The temperature is set to 75. The tire pressure is whatever the Toyota Dealer mechanic set it at for the Goodyears.
[/b]

If you are really serious about MPG you need to get your own tire guage and check them at least weekly. 42PSI front and 40PSI rear seems to be the best for really good mileage. A single tire, under inflated by two pounds per-square-inch (PSI), increases fuel consumption by one percent.

blackprius · 06-07-2006, 05:16 PM

<div class='quotetop'>QUOTE(Tideland Prius @ Jun 6 2006, 08:04 AM) [snapback]266681[/snapback]</div>

Quote:

You have to take an average of sevaral tanks. If I force fill a tank, I'll get super bad mileage for that tank and it's easy for me to blame the car for a miscalculation. On the other hand, if the pump was sensitive and cuts off early and I don't round up or what not, then it's easy for me to get better mileage than what the computer said.

Also, A/C usage will drag mpg down esp. in MAX COLD.
[/b]

That is good to know. My calculations or the cars calculation (what ever is higher) still put the mpg at best at 41 mpg.

<div class='quotetop'>QUOTE(seasalsa @ Jun 6 2006, 09:13 AM) [snapback]266725[/snapback]</div>

Quote:

If you are really serious about MPG you need to get your own tire guage and check them at least weekly. 42PSI front and 40PSI rear seems to be the best for really good mileage. A single tire, under inflated by two pounds per-square-inch (PSI), increases fuel consumption by one percent.
[/b]

That is a good point your bring up about tire pressure. The dealer will not put more than 35 psi on the tires because with the ever increasing heat in Phoenix, the pressure supposed increases significantly. Therefore, if I put in 42psi with the Goodyear tires (which are rated at 44 psi) wouldn't I damage my tires? I believe the manual recommends 32 to 35 psi. I may be wrong. In my Honda Oddssey with Run flats and tire sensors, I typically see a jump of 6-7 psi as I drive and the temperature gets hotter.

Thanks for the information.

tumbleweed · 06-07-2006, 05:49 PM

Here is an interesting article on tire pressure vs temperature from Tire Rack:

http://www.tirerack.com/tires/tiretech/tec...e.jsp?techid=73

Looks like the "rule of thumb" is about 1 psi for 10 degree F.

Keep in mind the tires are rated for 44 psi cold. So the increased temperature/pressure from driving on them is taken into account. You could most likely email Goodyear and ask them what is safe for your area. Sounds to me like the dealer is being quite conservative.

I don't think tire pressure is your whole problem though, must be more than that to lose so much milage.

Jack Straw · 06-07-2006, 07:45 PM

06-07-2006, 05:49 PM	#9
tumbleweed Senior Member Join Date: Oct 2005 Location: Eastern Oregon Posts: 1,006 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Here is an interesting article on tire pressure vs temperature from Tire Rack: http://www.tirerack.com/tires/tiretech/tec...e.jsp?techid=73 Looks like the "rule of thumb" is about 1 psi for 10 degree F. Keep in mind the tires are rated for 44 psi cold. So the increased temperature/pressure from driving on them is taken into account. You could most likely email Goodyear and ask them what is safe for your area. Sounds to me like the dealer is being quite conservative. I don't think tire pressure is your whole problem though, must be more than that to lose so much milage. __________________ former Prius owner, waiting for a G III present car BMW 3 series

06-07-2006, 07:45 PM	#10
Jack Straw Senior Member Join Date: May 2004 Posts: 137 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	zzz

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06-06-2006, 02:27 AM	#1
blackprius Join Date: Jun 2006 Posts: 3 My Car: Package: Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Hi: I read the initial article about what to expect for mpg. I have owned my Prius for 10 months and it has run very well for most of the 10 months. My first month of ownership was in August. August in Phoenix is still very hot, 100 -110 F degrees. I used to drive 90 miles round trip to work, my fuel mileage was between 45-48 mpg with the A/C on. In the Winter, the fuel mileage was between 48-51 mpg (in Phoenix) Recently, the fuel milage has gone to pieces in a hand basket. My mpg has gone from 45 to 34.5 over a few weeks period. I drive my 2005 Prius until the blinker blinks, add fuel, do the math and bingo caluclate my mpg. What I have notice is that if it is a cool day in Phoenix, 75-85 degrees, I get much better mpg even with the A/C on. Once it gets hot, the engine is continously running. Even today, at the stop lights with the battery more than half full but not green, the engine was still running to charge it up. Basically, in the winter I can expect to get battery power assistance from 10 to at least 40 mph. Now in the heat, the battery power stops after 12 to 15 mph and all I see is the engine running to drive the front wheels and charging the battery on the display. FYI, I use Mobile One Synthetic 5-30, what the Dealer puts in for Synthetics. Furthermore, I have even had the dealer install the BioPro QX3 which they promoted as adding some mpg. I even added Chevron Full Fuel System Cleaner to see if it helps. I am at a lost on the poor fuel mileage. If this continues and the Toyota dealer cannot help, I may need to sell it and get a Yari or Scion.

06-06-2006, 08:53 AM	#2
seasalsa Senior Member Join Date: Feb 2004 Location: Kent, WA Posts: 1,199 My Car: 2004 Prius Package: #2 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	Are you running A/C Auto? If so what temperature do you have it set at? 78 will seem cool when outside temps are hovering around 100 degrees. What tire pressure do you use?

06-06-2006, 10:16 AM	#3
brandon Proud member of the "Lunatic Fringe" Join Date: Oct 2004 Location: Manhattan, KS Posts: 769 My Car: 2005 Prius Package: #6 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	So you're saying these are your MPG calculations and not those displayed on the MFD?

06-06-2006, 11:57 AM	#5
brandon Proud member of the "Lunatic Fringe" Join Date: Oct 2004 Location: Manhattan, KS Posts: 769 My Car: 2005 Prius Package: #6 Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 0	But how can you be so sure your method for determining the MPG is correct? Yes, the MFD calcualtions are usually 2-3 MPG higher than actual, but you assume you are calculating correctly. How do you know how much gas has actually been used?

06-06-2006, 12:04 PM	#6
Tideland Prius Super Moderator Join Date: Oct 2004 Location: Canada Posts: 14,183 My Car: 2005 Prius Package: B Nominated 0 Times in 0 Posts TOTM Awards: 0 Friends: 12	You have to take an average of sevaral tanks. If I force fill a tank, I'll get super bad mileage for that tank and it's easy for me to blame the car for a miscalculation. On the other hand, if the pump was sensitive and cuts off early and I don't round up or what not, then it's easy for me to get better mileage than what the computer said. Also, A/C usage will drag mpg down esp. in MAX COLD.