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Haven't been here for a while but have a concern about deteriorating fuel mileage in my 2005 Prius. I am located in Vermont, rather mountainous driving. This car has 83,000 miles. The average summer MPG of this car (originally purchased new June '05) is: 2005: 53, 2006: 55, 2007: 54, 2008: 52, 2009: 49--a loss of 6 MPG from peak mileage attainted. It has been a cooler, wetter summer than normal which I estimate to be a 1.5 MPG negative. Some other factors: I am driving the same way as always: pulse and gliding sometimes; come up slow on red lights, etc. On third set of tires: Goodyear Integrity with 8,000 miles wear (psi: 40 lbs. front/38 lbs. rear), always make sure oil not overfilled at 5,000 oil change/servicing.

I talked to my local Toyota service dept. They come up with possibilities of inverter coolant needing changing (normally done at 90,000) and spark plugs (also 90,000). Wondering if anyone has other ideas, or have also had deteriorating mileage in an older Prius.
 

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My guess is the colder, wetter summer has contributed to most of your MPG loss. I doubt that the inverter coolant or spark plugs are contributing.
 

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Firepa63 - Thanks for the feedback. I have driven at least two tankfuls this summer where the weather has been mostly dry and warmer, but have never done better than 50.5 MPG. In '06 and '07 I would normally get 54-55, even using some AC and with some rain. It's definitely more than weather related.
 

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I usually get 53-55 mpg in the winter and 46-47 in the summer (due to air conditioner use). However, this past month we have been hitting 110+ degrees F pretty consistently, and the mileage has really gone in the tank. I had one fillup at 39 mpg, and most others have been around 41-42. The weather can really affect your mileage, unless you just leave the heater or AC off and just bear with the uncomfortable weather.

Our best mpgs are in the spring and fall when the daytime temps are between 65 and 85, and I drive with the windows down and everything off.
 

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The ethanol makes some sense. I noticed the 10% label on the tank at Sunoco yesterday. I'm wondering if the 10% has been a year round additive until this summer? And Phoenix, my seasonal mileage has been the direct opposite of yours in my northern region, except no longer this summer. I'll go for the new plugs and perhaps change the inverter fluid at 85,000.
 

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Enthanol is a political thing. Although the exhaust emmissions are slightly reduced, this benefit is cancelled because their are more of them because of reduced milage. They were once state required but I believe it became a federal mandate for all states slightly before the Iowa caucasus in the last election. (Your politicians "at work.")
 

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Oxygenated fuels are required in Arizona part of the year. This is not a new requirement to subsidize the ethanol industry, but has been a long-time requirement (at least in Arizona), for smog control.

My mileage has been creeping up a little this week, and our 2002 Prius is almost up to 44 mpg -- It's only been in low-100's! Can't wait until it gets in the mid-90s again, so I can roll my windows down.
 

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All points true. But the smog advantage is eliminated because you have to burn more of the 87 octane because of the drop in milage. Ditto on savings from the purchasing middle east oil.
Ten per-cent ethanol destroys lifespan of fuels in tanks for lawn mowers, boats and the government has forbid it's use in aircraft that have been certified for use with auto fuel. God onlyy knows what it does on "O" rings in injection systems. We already know it destroys carbuerators.
It's a political thingy. Just more government control of citizens.
 

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richard schumacher said:
E10 is not the cause of a 10% drop in MPGs. E10 contains 97% of the energy of gasoline per gallon, so it causes about a 3% reduction in MPGs.
3% ? Nope more like 6-10%. The 3% calculates out probably because of the reduction in energy in a gallon if E10 gasoline, but it is only part of the story. All modern cars operate and control the fuel mixture by feedback from the oxygen sensor/s as you probably well know. The detection of left over oxygen in the exhaust means a lean incomplete burn so the ECM counters by enrichening the mixture. This happens in all computor controlled engines. E10 fuel (oxygenated fuel) leaves a bit of oxygen in the exhaust so it "tricks" the computor so to speak.
This is easily proven by the fact that an older carburated vehicle has much less mileage drop with the E10 gasoline.
Probably more like the 3% energy loss you mentioned.
I've talked to Toyota engineering about this, with the fact that they need to find a way to detect the presence of alcohol in the raw fuel and adjust engine parameters accordingly (ignition timing etc.) and so far no response. I believe this could recover much of the lost performance. Oh well I guess I will have to figure a way myself.
 

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I found it on the internet (of course) and I'll see if I can find it again and post one of the links I used. Also did some backyard engineering myself with the several vehicles I own, one carburated and one ECM controlled. Experimented with regular fuel and also E10 because my state was using both types of fuel then, one for summer the other for winter. It is all year E10 now however.
I took oxygen sensor readings with both fuel types and noticed a leaner voltage reading with the E10 therefore causing it to enrichen the mixture a bit more just like it is supposed to do. I knew how 02 sensors work and could not find any other reason for it to read differently except the fuel.
So-- after some internet reading and my own testing I kinda came up with some of my own conclusions I guess.
Still have my 2001 Prius and have kept records since day one. Used to get 50-53mpg in the summer, 45-46mpg in winter same roads same routes. Since E10 45-47mpg in summer and 42-44mpg in winter. Has not changed for the last couple years. Anyhow thats my story and I guess my opinion only at this point.
Auto makers may not care about incremental engineering changes at this point hence no responce from Toyota. Our cars are probably interem units while they work toward all electric ones in the future I suspect.
 

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It might be a longshot, however people tend to leave their vehicles idling alot more, and tend to keep them on longer during the winter due to the cold. Is it possible that due to a colder winter, you start it and let it run for a bit when you get in? Doing so twice a day, daily could result in using more gas mileage. Or that you stay in it when going places and leave it running? Although this might not be the whole issue, it could be part of it.
 
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