just a thought...
 

Anyone tried using 91 octane instead of the recommended 87? I have noticed a 2-4 mpg jump with the a/c on.

Unless your car engine is designed for the higher octane, it won't help you. Just stick with the 87 octane regular. Higher octane won't gives more better mpg, only burn the fuel more efficient which resulted in more hp output. And if your car is only recommended for 87 and you try 91, in the long run it will most likely damage your engine instead because they are not design to burn 91 octane. Not to mention a waste of gas money that can adds up over time.

Yes I used to pump a lot of 91 on my old 99 Protege, that was a stupid mistake. I really did not notice the increase in mpg, only more power.

A/C turn on/off will change your mpg regardless of 87 or 91. Probably not so much on the newer cars these days, but I still think it does. I highly doubt is the result of 91 octane gas.

i also thought that it would not make a difference, however, i checked the mileage on a tank of 87 (miles driven divided by gallons used) came up with about 22.5 mpg. checked again with 91 and came up with 25mpg. that's with the a/c on both times. And if the cat burns out i guess I will put a high flow cat on it. :D

Something else changed as it isn't scientifically possible for a higher octane fuel to yield improved fuel economy in your car. Period, full stop, the end.

Even if the ECU was reprogrammed to advance the timing for higher octane fuel, there is absolutely no way a Mazda5 will get even a half of a mile per gallon improvement much less the two to four mpg improvement claimed by the original poster.

lnwlf, like it or not, believe it or not, higher octane fuel has exactly zero extra power per unit of measure than does good old fashioned "Regular".

I never said, or even meant to imply, that a modfied engine (where said modifications require higher octane fuel) cannot make more power or get better economy.

Back to the claim of the OP; he states that his stock Mazda5 is returning better fuel economy simply by running a fuel with a higher AKI. To that I say, "Ain't never going to happen."

So, what part of my post is "wrong"? :)

How many time is this topic gonna come up?

Octane is a rating at what temp, and pressure, the fuel will ignite.
You put a higher octane in than what your car is designed for and you will only burn what your engine can, and like Virgin said, throw the rest out your exhuast.

They do research on this $hit, just do what the book tells you and you'll be alright.

so I guess the extra 30-35 miles per tank is just my imagination. oh well... as long as my imagination saves me money

Yup, absolute imagination. The fact remains that by spending the extra money on a higher AKI rated fuel you are WASTING money, not saving it.

correct me if I'm wrong, the higher the octane rating the more compression it can handle before detonation? I believe what your saying, but numbers never lie

Yes, the higher the AKI rating (we don't use a direct "octane" rating to grade fuel here in North America), the more compression the higher AKI rating necessary to prevent detonation. That said, once fuel that contains the proper detonation resistance is identified, using fuel with a higher AKI rating will do nothing but waste money. In your case, your car is designed and built to use fuel with an AKI rating of 89, and using anything beyond that is simply a waste of money unless you somehow modify your engine so that it requires a higher AKI fuel to prevent detonation (something you have apparently not done).

So, while you claim that the numbers don't lie, I submit that they do. Why? Because there is no way on this earth that you (or anybody else for that matter) are a good enough driver to determine from one tank to the next that you drove under the exact same conditions and demanded the exact same amount of power from your engine in those conditions. Said another way, your anecdotal numbers of mileage improvement are statistically and scientifically irrelevant.

FWIW, I drive a Mazda3 and use Regular fuel 100% of the time. I've tracked my mileage religiously for the 31,000 miles that I've owned the car and have seen tanks of gasoline run low after as few as 250 miles and after as many as 475 miles. Given that I've seen a 225 mile swing in a single tank of gas, your "30-35" extra miles is completely within expected variance for any given tank of fuel. My bet is that if you were to put your car on a dynamometer and have the Brake Specific Fuel Consumption (BFSC) calculated, it wouldn't vary even 1% between tanks of fuel, regardless of which fuel you put in your car.

Long story short, save yourself some money and go back to Regular fuel. Not only will your wallet thank you, your engine will too.

will do, will let you know how it goes in a week or two. If all is as you say, then I stand corrected. One more question... the wheelbase on the M5 is a little over 4 inches longer than the M3... do you know if an aftermarket cat-back for the three will fit underneath the 5?

While I don't know for sure, I rather doubt a Mazda3 exhaust will fit on a Mazda5. Of course you can probably guess my next question... Why would you want to put a cat-back on your Mazda5?

that's the usual response, lol, trying to gain a little more power, not much, especially with the family in the "vagon". tossed the resonator for the intake last week and noticed a much needed improvement, 3 inch pvc slips right on the factory filter box. now that air has little restriction getting in, a little less restriction going out will help as well

the 3 and the 5 are identical as far as drivetrain goes, the exhaust is run the same and exits from the driver side rear, it's the wheelbase that's got me wondering. of course i can always have it lengthened.

I'd be willing to bet the variance is more reliant on another variable than your fuel's octane: your right foot's pressure. And where you're driving. And what the environmental conditions are.

I've seen up to 38MPG highway on low-tread tires along the flat Jersey shore in the middle of July without my AC. And I've seen as low as 14MPG in the dead of February with the heater on 3/4 'cause it was colder than a witch's tit. You'd be surprised how often the many variables that go into driving are forgotten, and someone automatically assumes it's their octane. And even then...not all fuels of the same "grade" are created equal. I'd take Sunoco 87 over some no-name's "regular" any day.

i think what we are all trying to say is that you need to keep things nominal. if you are supposed to use regular gas, use it. the engine is designed to spark early and detonate late. if you are going to a higher octane, you are causing the engine to have to work harder to ignite.

even if there is no difference here, you are paying more money for gas. that may not be much more, but it will be in the long run. even old gearheads will tell you that too much octane will cost you power.

well, as yall stated... very little difference in milage going from 91 to 87. and as i said... i stand corrected and thanks for your input and advice.