I've recorded my gas, mileage and vendor for every tank since I bought the car. I found that Shell gives me the best mileage too (by 1-2 mpg). I tried the lower octane, got 1 mpg better mileage, but got some pinging out of it. So it's Shell 93 all the way now.

 

I use Shell 93 99% of the time. Just recorded my best hwy at 24 mpg. cruise set to 73mph.

 

In my experience, Shell (93) > everything else as far as my mpg goes. 2-3 mpg better on the highway (eg 23mpg with BP and 25-26mpg with Vpower, 100% highway, same route, same speed(80), etc). Somewhat better on my commute, though it's hard to judge accurately, as the variables change so much.

 

Been using Chevron for 17 years exclusively, and recently found out they are one of the only gas distributors to have their own lines that run next to the big lines, their own additives (could be a bad thing, but most folks say they are good additives)
and a nice clean modern stations (at least here in AZ).... Tried all three grades in my '05 RX-8 (30K on the OD when I got it) and I use mostly the mid grade (which is I think 89 here in AZ). I keep close tabs on mileage, and I average 18.5mpg. 17.5 if I am a bad boy, and 19.4 for a full highway tank at 85 mph. All the other cars I have owned stay pegged full for 50 or miles...not the RX-8 though! One 15-20 mile trip, and I am down a notch or two! I usually fill up around 245 miles...that is still hard for me to swallow, but I do love the other aspects of the car so much....

-C

 

It's really weird. I was headed down to LA and I was thinking about this same thing. I fueled the car up in San Jose and headed down the 5. Hit about 260 miles before I had to refuel, and I came pretty close to using it all up. So... I usually use Shell, but I had to use Mobil since there wasn't a Shell in sight (until after I got back on the highway) so I went to Mobil. Drove the rest of the way to Banning and then had to refuel again... but this time I was able to pull out 275 miles and it still had at least a gallon left!? So... I don't know what's going on.

 

I have put bad gas in my car before that certainly hurt my mpg. I have been going to Fred Meyer pretty consistantly since I've been here in Oregon and now I'm wondering if I have been using gas that is affecting my mpg. I used to consistantly use Shell and averaged 15-17 mpg mixed city/hwy miles. Now I'm mostly city and getting very low 12s to 13s. Last tank was 12.138 mpg. I have been freakin' out about it and will do my next couple of fills at Shell just to see what happens. I'll let you know as I go what my results are becuase anything has to be better than 12 mpg. I haven't seen those low of numbers since I bought the car in October of 2003 when my first tank yielded in the 11s. After a few fills I was getting in the 14-16s for city and in the low 20s for hwy. Those were the days!

*edit*Out of a wild curiosity does anyone think that the economy stations are doing anything to their fuel to save money? Are they adding fillers that are affecting the gas that would lead to poor performance in our cars that say a Toyota Tercel or Honda Accord owner wouldn't notice? Just wondering?

 

Don't know if it is a fact 93rd, BUT, what you DO know is that switching from one brand of gas to the other DOES make a difference. Like I said previously, I tried one of these no-name gas station (Space Age or something), and drove no more than 10 miles and it sucked up a 1/4 of a tank!!! Keep in mind that this happen when I was renting a extended van, not the 8..that in itself makes me want to avoid any of these little companies. I rather make the extra 10 miles to get the better well known product (aka Shell, Chevron, 76, BP, etc...).

As for your gas mileage you are getting..when was the last time you had your spark plugs replaced? Or..how about how hard do you drive your car? 14-16 sounds really bad..even if you are used to those mileage....then again, I know Portland commute is a pain in the @#$#....lots of idling, unless you can find another route that will lower your chance of stop and go traffic...that in-itself will eat your mileage up with us 8 owner :p

 

I use Shell V-Power 93 Octane on my 2004 6MT and I get 17-19mpg on average.

 

I forgot to mention, how often do you clean out your MAF (mass air flow) sensor?? Maybe that is clogged up and your car is mixing more gas than it should? I've read it makes a difference..I plan on cleaning it out with a can of electrical cleaner every 10k....

Anybody have any feedback on this? Pros, cons? Also, do you need to disconnect the battery terminal before you do this? Yes, I should search..hehehe..

 

I always put 91 (thats all thats available in California...) in my car and have never gotten better than 18mpg... in fact, I get the WORST mileage with Chevron, on a road trip I filled up with Chevron, on the freeway, on cruise control, I got just over 15mpg with Chevron... oddly enough Arco has been my best bet so far, but I'm definitely going to try Shell, I've never gotten better than 220 miles out of a tank of gas, if I were to get 300 or better I'd be stoked!

Also, to add... Like mikeb, I drive like a jerk around town... and I average 11mpg in town... I've calmed down a bit lately and averaged overall 15mpg on my last fill up (last night)

 

dang... 15mpg? tats definately on the low side, boosted?

 

^^^^15 mpg is not uncommon for an RX 8, without boost. I use 87 octane Chevron gas most of the time and average 18-19 in town, 20-21 on the highway.

Not great, but compared to my other vehicle, Dodge Quad cab pickup with 360 v8.....

 

I used to use on-base gas and almost quarter tank was gone before I got home. I have used other gases but Shell is the best so far. I think it cleans out the carbon build up.

 

HCTR, i already downgraded to 89, ur tempting me with 87 lol... but texas is about to heat up, may be going back to 93 soon?