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

 

I have run straight 89 in the summer months "to help compensate for the 100*+ weather we have here then" w/no noticable improvement in either performance or mileage.
I have also tried a mix of 87/93 with the same results.
91 should not be necessary in your car. It doesn't have the fuel map to adjust for it and is probably sending more hydrocarbons down to the catylitic converter making that work harder and get hotter (= early burn-out.)

 

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.

 

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.

 

The only way I can think of in a modern car that extra octane would help, is if somehow the initial factory ignition timing is set way too high or to go advanced way too early, which would likely be a function of the ECU's software. Using 87 the anti-knock sensor would come in to override the timing advance to keep ping down to a minimum.
Using a higher octane, the engine would not get to that point, or get to it at a much higher load level, and the anti-knock would not come into play meaning more power from the engine.

But that's a pretty far fetched scenario even imo. Everything is controlled by carefully written software these days and to have one ECU sneak through w/advanced performance maps is not very likely.

 

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".

 

Now Dale, I have to say here that you are quite right, and quite wrong too.
Higher octane, by itself will not afford better power or mpg. But add a few other things and it might be a necessary evil in order to achieve that extra power... or mpg.

Case in point, my 1980 Civic (Inga.) 1500 CVCC engine. 5-speed. Carburetted. Basic computer control (VERY basic by today's stds.)
Before the mods, +/-28mpg.
After the mods, AND 91-93 oct fuel, 32mpg... @ 70+ mph EVERYWHERE!!!

Here's what I did to her:
1) A mild "street" cam, purchased from AT Engineering, Ct. Not far from Lime Rock Raceway.
2) Distributor modifications based on what I had learned. MUCH quicker advance (lighter springs, full advance @ 1750rpm) and limited advance (to an overall max of 17*,) and advanced static timing +12* over OE.
3) Drilling the STOCK carb jets in the otherwise STOCK carb, both air and fuel, out by one numbered drill bit.
4) Running higher oct fuel, because anything else and she would ping, ping, ping.

I LOVED that car and wish to this day that I still had her. Blame that on Pennsylvania winters and road salt.

 

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.