Significant MPG loss at 50,000 miles - HELP!

For those who have checked tire pressure, alignment, parking brake, grabbing brakes, engine oil level, I have to think a failing 12V is coming into play here... Just from what I have read over the years, no actual knowledge on my part.

 

New tires came when on the timeline of mpg decline? Even lrr tires can drag you down a bit during breakin, but not to 38. Also, I don't think nitrogen vs air is relevant, but would be curious what psi is "ok." As you look for your ultimate answer, consider upping to max sidewall to see if those new tires will do better. For my $, just add plain air.

What "services" did the dealer do for you? Check your oil level. You may also need to check the viscosity in case they put in 10W30 dino or something...

 

So you are driving around with an ailing 12v battery? If you know your battery is going bad then why are you so upset with your bad fuel economy? Replace the battery.

Since you did not provide any specifics on how long you've been tracking this drop in MPG I don't know how anyone can really help you. A drop in MPG over a single tank doesn't really say much. I assume this was over more than a few tankfuls? We need more info before we can really help you Anita.

 

Dealers often use a pass/fail tester that is not accurate for the Prius. Your 12V battery is dying. Replace it.

 

If the pre-cat Fuel/Air sensor (wide band lamda on Prius) is getting bad, it will result in the fuel-injection riching the mixture. Without getting technical i have read all i could find about the Prius enginemanagement for a few weeks now.
After that i have installed an EFIE (electronic fuel injection enhancer) that lets me adjust the signal from that sensor. This i do to lean the mixture.
Result: Summer small road drives at lower speeds gives 62mpg and fast highway(66mph) long trips ends up at around 54mpg. The only other mods on my 2005 Prius are plugged lower grill and 50psi in my 205 wide tyres.

 

brakes should not be a problem now.

 

I had similar reductions in mpg starting last June. Possible causes sited were new tires, tire psi, oil, etc, etc. Now it's suddenly back up to 50+ mpg again. I think we're at the mercy of what's delivered at the gas pump.

 

I know you mentioned in another thread that you changed tires. Per Tire Tech Information - Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires, changing from a set of worn tires to identical new ones can result in an mileage hit.

When you're talking about all these variances, how are you determining this? Are you just doing manual calculations for each tank (miles driven / gallons put in the tank)? If so, unfortunately, the small fuel tank coupled w/the bladder (which makes the tank capacity vary) makes things tricky. I sometimes see big swings via manual calculations from tank to tank but MFD values don't show as much of a delta.

Examples for a few fillups (my mileages are low since I have VERY short city drives now):

	Column 1	Column 2
0	MFD values	manual calculations
1	40.2	36.93
2	39.5	41.2
3	44.5	41.7
4	42.4	46.98
5	40.5	35.95

You're going to just need to track both values over many tanks to form any meaningful conclusion and try to run your tank down pretty low (2 pips or just into 1 pip terrain) to avoid short fill ups.

 

Does anyone know what the shortest and longest mileage range until they see dead/failing 12V battery? This seems to be an ongoing problem with a lot people and unfortunately i just accepted the bad mpg. For a year and a half I used to drive from Anaheim to San Diego - 180 mile daily commute and see 48 mpg, sometime w/ a lead foot. Once i hit 55k miles i took a major mpg hit to 37 and have only rebounded to about 40mpg with all suggestions. I now only have a 45 mile daily commute and sadly have accepted the poor 40mpg.

 

Have you started a new thread to answer the questions at http://priuschat.com/forums/gen-ii-...answer-these-questions-esp-if-youre-new.html?

We know nothing about your commute other than it being 45 miles and don't even know things like your oil level, tires, tire pressure, and 12 V battery voltage.

What is your definition of "bad mpg"?

 

Yes about 8-9 months ago i brought this up on a new thread. Checked everything suggested by other members as stated above and made the necessary changes. My definition of bad mpg is a decrease of 6-8mpg from what i originally had and what the advertised mpg should be. I've taken it as a wear and tear effect on my vehicle.

 

I skimmed some of your posts and it really appears you haven't given us sufficient information to help or tell you that your mileage is normal for your drives, conditions, equipment, etc. Please answer the questions in the questionnaire in a new thread.

For instance, we don't know what tires you're running on, your current 12 V battery condition, anything about your drives, other than you mentioning that you drive 75-80 mph. Those speeds will yield "poor" mileage. Also, one of your prior posts says "1) used to drive a truck and 2) lead-footed @ 80mph". We have no idea if you still are a lead foot.

As for "advertised mpg", please read http://priuschat.com/forums/other-c...uth-about-epa-city-highway-mpg-estimates.html about the EPA tests. Hopefully you realize the tests aren't done on a real road, the highway tests are at an average speed of ~48 mph and the test doesn't measure actual fuel consumption. Even Your Mileage Will Still Vary says

Also, temperatures has fallen, which will hurt mileage and your area might've switched over to winter fomrula gas.

To top it off, I believe the gasoline formulas used in CA cause a slight mileage penalty (http://www.arb.ca.gov/fuels/gasoline/gasoline.htm mentions Phase 3 Reformulated Gasoline (CaRFG3). I wish I could find some credible references about CaRFG3's mileage penality, if any.