I don't condone the use of lower octane gas on our cars because I have found out the hard way just how sensitive our engines can be to mistiming/misfiring (apparently I'm one of the few who experienced bad engine knocking/pinging). For the first few months, I used 93-octane. I now compromised and use 89 (no 91 where I live, otherwise I would have used that) with minimal pinging. I'll switch back to 93 for my next few tanks, though, and see how my gas mileage is affected.

 

Can anyone confirm that the fuel capacity is 15.9 gallons.

 

Always about 260 with 89. 93 has consistently given me worse results.

Oops - had to edit this. Meant to say 89 instead of 91. We don't have 91 'round these parts...

 

I usually got 240-250miles per a tank on 87, but now I get 220-230miles on 91.

For performance, I don't feel much about difference b/w 87 and 91.

 

I've tried the whole range of standard octanes and haven't gotten a difference in mileage. Getting more highway time and staying out of stop and go are the only things that bump it up.

 

I bought my 8 two months ago with 19,000 miles on it and the first tank was 14 mpg, the second was 15 mpg, third was 16 mpg and now I'm up to 18 mpg. I always use 93 octane from Shell or Chevron since my first fill-up with the car. I'm actually driving more agressively now that I am getting the "feel" for the car and it's limits so the only thing I can figure out is the last owner must have been using a low octane fuel and now the engine is liking the higher octane better?!?!

Beats me, but I ain't complaining and I'm sticking to 93. It's only 10 cents more a gallon which translates to $1.25 a tank. That's a small price to pay for my own ease-of-mind.

 

I've been burning nothing but '87 in my car since November with noticable change in performance or presence of knock.

that said, no noticable change in mileage either.

 

Best mileage ever for me was a long highway jaunt at 75mph on Shell V-Power. 26.5 mpg (360 miles @ ~13.6gal fillup). A similar trip on I think it was Exxon 93, was 24 mpg. Similar temps/weather and same route. Usually get 22-23 unless I use Vpower gas (I also normally drive 80mph).

Normal day-to-day routine (70% city) I get 17-19.5 mpg on 93 octane (redlining 2-3 times per trip, which is approx. 22 miles).

All very interesting, however terribly unscientific. I don't keep track of every tank so I have no mean data.

L flashed AT btw.

Edit: the one time i tried 87 octane, the car hated it. Felt very laggy. Luckily I only put in about a 1/2 tank and burned through it quick...maybe 13 mpg. Could've been bad gas, but I'll play it safe.

 

Thanks for posting this. A few quotes from the article:

"And today's engines designed for premium run fine on regular, too, their makers say, though power declines slightly."

"Prejudice and preference aside, engineers, scientists and the federal government say there's little need for premium."

" "I personally use regular even though my owner's manual says you'll get better performance with premium," says Lewis Gibbs, consulting engineer and 45-year veteran at Chevron oil company. He's chairman of Technical Committee 7 on Fuels, part of the Society of Automotive Engineers (SAE) Fuels & Lubricants Council. Gibbs knows gas."

"The main advantage of premium-grade gas is that it allows automakers to advertise a few more horsepower by designing and tuning engines to take advantage of premium's anti-knock properties. But auto engineers generally agree that if you use regular in a premium engine, the power loss is so slight, most drivers can't tell."

""I go back and forth, and I'm hard-pressed to notice" whether there's regular or premium in the tank, says Jeff Jetter, principal chemist at Honda Research and Development Americas. He drives an Acura designed for premium. "

Okay, I better stop now .

 

I do have story about this. I used to put in 87 Octane in my car until a little mishap on the highway. I was going about 30 in the acceleration lane i decided to floor it and hit 9k. Afker i hit 9k my engine light cam on and i could not rev over 7k. This has happened to me once before as well. I called a tech at mazda and they told me it could have been something with the gas. Sure enough when i switched to premium the problem went away.

 

I get 170 mpg in the city, and 265 on the freeway using 87. with rotories everyone who knows something about the engines, say they need to run rich, and on higher octane, should i start listening, because i dont want problems right when my warranty ends.

 

After having my catalytic converter replaced due to clogging, and being reflashed to R flash I think my 8 is faster, and more powerful than it ever was. I fear that those that may not have a clogged cat, or CEL's referring to it, many 04's may very well be somewhat clogged, just not hugely affecting engine performance. Mine started to stumble at 6000rpm, and would not rev past 7000rpm...sounded bad, and made no power above 5000. After the new cat, power is way up...and for the first time I can feel the power building all the way to rev limiter. There is a jump at 6350-6500, then another at 7250-7500. With DSC off, I have a great deal of tire spin from an aggressive launch, a spin shifting to 2nd, and a chirp to 3rd.

Sorry, I got off track....my mileage has improved. I was getting as much at 220 per tank before with mixed driving, and as high as 26 on long trips at 65-70 mph. Now on my last per tank with mixed driving I managed 267 at the fuel light. No long trip numbers yet.

 

I forgot the main point. I use premium to cool the burn in summer. The heat in summer combined with the tremendous heat from the engine I think would promote pinging and detonation...so in the summer I wont use regular. I do use regular in the winter to aid in cold starting....and it seems to run better on regular when cold.

 

damn, i fyou're getting 170mpg you rock!

 

I wouldn't put in lower octane rating for your car especially for those in the US. The "lower horsepower rating" problem Mazda encountered when introducing the RX-8 to the US market might have been a result of their negligence towards the octane rating in the US. These cars are tuned for premium fuel in the US therefore they should only be fed so, just because pinging can't be heard doesn't mean its not happening and the rotary engine apex seal fails usually because of these detonation. Back here in my country, there were lots of cases of engine failure in Japanese imports, all due to difference octane rating. Higher RON allows an engine to be tuned more aggressively thus yielding more horsepower, that could also be the reason why ECU Flash yields more power after certain recalls were done. Just make sure you know what RON your engine is tuned for, there's no point in putting more expensive fuel if the engine is tuned to use lower RON.

 

I have 265 miles since my last fill-up and still have a bit over a 1/4 tank left :p

 

The last time I was at the dealer they gave me a service bulletin stating Mazda recomends only 89 and above. It also stated using 87 in hot weather has and can cause vapor locking. It also stated the lower octane causes more carbon build up and contributes to engine misfire when idleing

 

That (RON 89) accounts for the lower hp RX-8 the US is getting, my side here uses 97 so I'm only getting 241 crank hp, the Japan uses higher MON so more agressive tuning possible thus the higher 250 hp.

 

10 years later the affect is more drastic - using 93 Octane I'm about 80 miles for half-tank, vs generally 120 miles on 87 octane. Anyone figure out 'why'?

Sorry for necroposting, but a new thread would have been met with 'Search, n00b!"

 

Try calculating how many miles per gallon you get instead of how many miles you get per tank or half a tank.


I get 10mpg to and from work. My last job which was a bit further, i got 14mpg. On road trips i get 20, and on the track i get 6. So where and how you drive make a big difference.

 

Thanks - I got the 'how' part down pat. I'm looking to figure out the 'why' as in - does anyone yet know why RX8 fuel economy IMPROVES by a large margin on lower octane fuel?

 

I've never been convinced it does, and I've tried both. When actually measured 100% highway, I got the same between them. If you aren't doing 100% highway, you have so many possibilities of load variance that trying to compare them on even a single tank is faulty, much less an "about half a tank".

And 80 miles on half a tank is ... rather poor. I tend to get about 70 miles by the time my gauge gets to the 3/4 mark. I got 313 miles in 13.8 gallons a week ago. On track, a healthy RX-8 will get 9mpg for near constant full throttle.

Something on your car is busted or worn out, or you are driving with your foot to the floor all the time. In neither case is your gas mileage measurement really indicative of anything having to do with the octane.

 

I have used only premium fuel in mine for the two months I have owned my 8. I have a 2010 with AT and have averaged about 16 mpg with mostly city driving, trying to red line once a day and usually keeping the RPM above 3000. I have gotten as low as 13 mpg, but I treated the streets like a race track and the other cars as traffic cones to go around. I spent a lot of time above 4000 rpm.The one time I had to do a cross state trip I was getting about 25mpg going between 75-80 mph. On mixed driving with mostly highway miles I see low 20's mpg. I don't really see the need for lower octane gas (I'm happy with my mpg, because I don't want a Prius) and don't want to risk engine problems with using it.

 

Week prior, 87 octane from the same station, with no change in commute = 115 per half - which is what my car's done since new +/-, except I got 24mpg average on a 3500 mile road trip As you can see with the OP - this has been the case with my car since new. This isn't a single tank - this is 10 years of ownership, and each time I switched octane the MPG adjusted drastically.

 

Well, reverse engineer it with some OBD2 data.

If you are confident that there is no change in driver behavior, then you WILL be able to see the difference in OBD2.

If your fuel trims and AFRs remain the same, then your MAF and Fuel Injector Duty Cycle will drop.

If your MAF and fuel injector duty cycle remains the same, then your fuel trims will drop and your AFRs will climb (functionally impossible with the ECU trying to hit a target AFR)

If you can't find any difference in the OBD2 data, then the change is consistently behavior related, even if it is imposed behavior.

By "imposed behavior", I mean something like if I wire open my wastegate on my MSM's turbo, I will immediately get better gas mileage simply because I will no longer be automatically running in a higher load cell on the fuel map (even out of booth, the turbo has intake vacuum level implications). In the RX-8's case, if the ECU is pulling timing to save the engine, cutting power, then your mileage could increase simply because when you are accelerating, you aren't hitting nearly the load on the engine, thus burning less gas.

Just some theories.