Regular, Mid-grade, Premium. Does it make any difference?
 

So, for my last tank in the Geo, I put on over 400 miles. The gas gauge was inching towards 'E' just in time for a 26.6cent/gallon gas price increase. Yippee! :rolleyes:

To top it all off, I decided to fill my tank with Petro Canada's "Super Clean 91" gasoline for $31.44. I'll see if it makes any difference in my fuel economy.

Then I wondered, has anyone tried using a 'premium' fuel to see if there is a FE difference? I would definitely need to see a 10% improvement in FE to offset the extra cost, but has anyone tried?

I have tried premium at various times and have never seen improvement. I am sure your car does not have high compression to require it. There may be additives in winter blends that would make a difference. Let us know.

Cars that have knock sensors advance the timing if the fuel octane is high. This usually produces FE gains and more power. Cars that don't use knock sensors won't have FE improve much if at all.

Higher octane usually only benefits cars with high compression, typically above 10:1. Otherwise, not worth it IMHO.

When the knock sensor "sees' any knock then it pulls back timing. it won't advance timing any.

Read your owners manual, it will tell you the grade of gasoline to use, I was suprized to read that some toyota mini vans ask for premeum, and state that you will not harm anything, but that you will notice reduced mileage, and proformance if you use a lower grade, so unless you did something to increase your compression, or have an engine that is running hot (hotter thermostate?), or are running an older car like my carburated '84 civic dx hatchback ran better, and got 42 mpg if I ran premeum gasoline.

The old low-tech answer is: "if it doesn't knock on regular, you'd probably waste money on the higher octane gas."

From my own experience, I drive too many cars that I can hear "transition knock" - i.e. when you hit the gas quickly or when it shifts gears, etc. I hear a bit of knocking, and of course the knock sensor responds quickly and pulls back some timing.

The problem, in my mind, is that there is some knocking in the first place. It might not be the end of the world on just one tank of gas, but I believe these occasional knocks over 100K miles can hurt your headgasket.

Remember that your knock sensor is a reactive mechanism, and it doesn't fix the problem until after it hears knock.

Now mind you, I have driven with some other people who I would have thought had well-trained ears, but they can't hear the knocking in their own vehicles. It is quiet, and only lasts briefly, so for most people it is hard to tell when there is knocking unless you use a scan tool. I wish that new cars with "gauge packages" had a knock-retard gauge, showing how much timing the KR system was pulling out.

I'm not going to say that there is a vast headgasket-killing conspiracy by the major auto makers, but I will say that I hear knocking in cars more often than there should be.

As you have guessed I run 93 octane in my car- not because of better mileage, but because it takes more than 89 to stop the knocking. (There is a noticable improvement when I go from 87 to 89, but to eliminate 100% of knocking 93 is the only thing that seems to work). My mileage did not change notciably when I changed octane.

It is true, on the other hand, that the lower octane fuel you use, the better your gas mileage (as long as there is no knocking or KR).

[Soapbox] I wish the OEMs would take some more care to program their timing curves accurately...[/Soapbox]

-Bob C.

bobc455: have you tried mixing 89 and 93 together?? you might just need a bit more than 89 octane to stop the knock. I know it would be a hassle to have to mix but might save you a few cents.

I haven't, but it's not a bad idea. I have run 89 when the "hot" weather was done but before we switched to winter blend, and I'll probably run 89 again once the weather gets very very cold.

I have run 91 once (there was a Sunoco that offered 4 grades of gas) and it worked fine, but it was only $0.02 cents/gal cheaper than 93 and the station was out of the way.

Since 89 is only a mix of 87 and 93 anyhow, I could also do the same thing by putting 3 gallons of 87 in before my 93. That would probably save me $0.60 per tank. Too bad the gas pump has to reset before switching grades...

-Bob C.