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?

  • Does a prius really get such bad mileage? How did they dupe people into paying so much for a hybrid that's getting the same mileage a 1.5l civic was getting in '92? Feb 22, 2013 at 3:25
  • Revisiting this question much later: is this really a hybrid? If so, there's an excellent chance that the batteries are going: they're right at their warranty point.
    – Bob Cross
    Feb 22, 2013 at 13:14
  • @BobCross All Priuss are hybrid, could be the battery but most last much longer without issues. I think it could be more related to the type of driving, as the mpg can very wildly on hybrids. Mar 14, 2013 at 3:02
  • @Larry, I'll bet it's not most batteries that last much longer considering how warranty duration calculations are done. Regardless, these batteries are older. They won't have the efficiency that they did when new. On the third hand (!), it's the driver's car: she has to make the call what works best.
    – Bob Cross
    Mar 15, 2013 at 22:57
  • @BobCross Because of the high cost ($2000-$4000) of replacement batteries the manufacturers have designed the batteries to last the life (they consider life 10 years) of the car. Consumer reports has an article Nine-year-old Prius retains its youth. We have a local taxi (Ford Escape) with 400,000 plus miles on the original battery. I saw a special on TV about several NYC taxi's with similar results. That's not to say a battery can't fail, but that usually the exception. Mar 16, 2013 at 20:18

5 Answers 5


the dealer shop said it probably was the ethanol

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.

now I've been reading that the higher octane could damage the car. is this true?

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.

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?

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.

  • and btw Prius guys... My daily driver is a 1987 diesel mercedes with about half a million miles on it now. I paid $1000 for it a few years ago. It gets close to 40 MPG. All I've ever had to do to it was replace a couple fuses. Conservativly, my cost per mile is .13 Cents if I only expect 300k on the car(my last did a million) Yours is .26 cents at 300k(unless you start throwing in battery packs).
    – Matt Bear
    Feb 22, 2013 at 0:35
  • Now when you hit the million mile mark, is where your extra 5 mpg and the higher cost of diesel catches you up, .16 vs. .13, of course only that mercedes is going to hit that mark... Just saying, if your concerned about MPG, and $$ diesel mercedes is the way to go. Which is very surprising.
    – Matt Bear
    Feb 22, 2013 at 0:41
  • 1
    While I'll agree with you that a Diesel will be cheaper to run, I'd have to point out that a newer Diesel ought to do much better than 40mpg - Many sold in Europe these days are quoted as 60-70+ mpg... Of course, your Merc is probably a lot more comfortable!
    – Nick C
    Feb 22, 2013 at 13:33
  • The new mercedes C Class gets 45+ stock... The diesel VW jetta can do as high as 90, Germans :p
    – Matt Bear
    Feb 22, 2013 at 20:59

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.


Use 91 or 93 octane trust me. It will not damage we have 2007 prius they are designed to run on higher octane to make sure there is no pets ignition going on lower Octain ignites at a lower compression and exploding before it supposed to will damage the rods and slowly wear the motor to run bad. As the prius will try to correct it self use the 91 or 93.

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