I have a 2007 prius with around 125000 miles. my mileage dropped about a year ago. I had the fuel jets cleaned, but it didn't really make a difference. the dealer shop said it probably was the ethanol. I decided to experiment and see if the higher octane gas would help. I filled it up last friday and topped it off today. I had been getting around 39-42mpg. when I topped it off today, I got 60mpg. now I've been reading that the higher octane could damage the car. is this true? I've also read that the higher octane doesn't increase the mpg, but it certainly seems to have improved mine. should I or shouldn't I?
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What mixture are we talking about? It is true that the energy density of an ethanol mix is less than you'll see from a pure gasoline fuel. If you look carefully at the labels on the pump, it may be that your regular fuel is labelled as containing ethanol but the premium is not. In a purely isolated experimental setup, you could expect to see better mileage from higher energy fuel.
Caveat: a human driving on roads with other vehicles is a terrible experimental device.
We have plenty of discussion on this point but summary is probably not in the short run. Over a period of 100,000 miles, there might be a measurable difference.
This is well into the realm of: it's your car, you have to make the call. If you are truly seeing a 50% increase in mileage, that's pretty hard to argue with.
I'm not aware of any proof that higher octane fuel damages engines, although running a lower octane can, as it will cause detonation.
What octane rating do Toyota specify? I know a lot of Japanese performance cars are designed to run on the higher octane fuels (97-99 RON / 92-95 AKI) that are more common there than in a lot of other countries, but most normal cars are designed to run on regular 95 RON (90 AKI). If you've been running on a lower octane fuel then you will be getting less power and poorer economy than designed, as the engine will have adjusted itself to prevent detonation.
A lot of 'premium' fuels also have other additives and chemicals in them that can help clean the engine, and so some people recommend running a tankful of premium every so often for that reason - I've never tried it personally though.
The ethenol is most likely the culprit, it has a much lower BTU then gasoline, so the less of it you have, the better your milage. Octane rating is a measure of a fuels ability to resist self-ignition. And to clarify that that, when fuel is injected into the cylinder, valve closes and the piston moves up, the enviroment becomes presurized and the presurization lowers the ignition point. When the ambiant tempurature is high enough, that ignition point can occur on the upstroke prior to the piston completing its cycle(knock). When THAT occurs, it can be extremely damaging, very quickly(broken connecting rod if your lucky). Hence the knock sensors which automatically adjust your timing to prevent preignition
A 90 octane rating doesnt actually have anything to do with the ammount of octane in the fuel, it means that the fuel has the same self-ignition point as a 90% isooctane 10% heptane mixture. Which is why you can have 100+ octane ratings, because the fuel has a higher combustion point then pure isooctane.
Higher octane fuels are required in higher compression engines, in order to resist pre-ignation caused by the higher presures. Super chargers and turbo chargers raise the compression by packing air in at a higher presure, usually requiring higher octane fuels(unless the engine is designed otherwise)
Since it only refers to the self-ignition point, running a higher octane rating then is required is extremely unlikely to damage anything(unless your running race fuel), since the combustion still occurs via spark at the point it is supposed to.
"Most fuel filling stations have two storage tanks (even those offering 3 or 4 octane levels): those motorists who purchase intermediate grade fuels are given a mixture of higher and lower octane fuels. "Premium" grade is fuel of higher octane, and the minimum grade sold is fuel of lower octane. Purchasing 91 octane fuel (where offered) simply means that more fuel of higher octane is blended with commensurately less fuel of lower octane, than when purchasing a lower grade. The detergents and other additives in the fuel are often, but not always, identical." Wikipedia
(looks to me like the Ethanol is in the lower octane fuel, and getting premium means you have much less ethanol)
On a side note, up untill it was banned, lead was used to raise the octane rating on fuels.
Anyways, I'm going to do some experimenting... I drive about 100 mi a day, I'll test the different mixtures this week for fun.
When I first got my 2007 Prius in 2009 it was getting 37 MPG. After a month it dropped to 34 MPG and stayed at that level for four years. During that time I asked the dealership why would the mileage be so low. They asked where I was getting my gas. I told them I was buying it from supermarket stations. They said that’s why. They said to get gas elsewhere like Texaco, or Shell, or other brand names.
My vehicle had around 45,000 on it (today it has 60,100). So I experimented and there was no increase. I asked later on and they said the same thing, adding it could be tire wear and other things. Then they asked if I reset the MPG gauge. So the next time I filled up, I found out it didn’t reset, so I manually reset it. I still got the same miserable results.
Then one summer day in 2012 I put $15 worth of Chevron gas in, and when I got to my destination, I was getting 44 MPG. So I started using Chevron gas and the mileage (depending on time of year) would get between 40-46 MPG. Oddly enough, about three months ago, I briefly talked to a woman who also had a 2007 Prius and she was getting 50 MPG using supermarket gas.
I have been hesitant to use Premium gas because the dealership said the car was designed to run on the low grade gas, and a high grade gas could harm the engine. So maybe I will go to the high grade gas and see how much of an improvement it will make, if any, and calculate how cost effective it is.
I have a 2007 Prius with 67000 miles. When I fill my car with gas, the mileage is pretty good until i fill it with the same brand again, then it drops to 36/37. If I change brands every time, i get 39/41. this is in town. I was recently told by a mechanic to start using mid range fuel & see what it does.Does any one have other ideas? My tires are at 38 front & 36 rear.