dsmdriver

iTrader: (3)

And there's also people that just post about 3rd grade math over and over and over as if everyone does MPT even when they very directly have miles and gallons in their posts.

I get 14 MPG done the 3rd grade math way (tm). I am suddenly getting better economy on the last two tanks and am trying to figure out why.

What I'd really like to see is two RX8 owners that live near one another and get opposite ends of the spectrum trade cars for a tank or two. We really need to figure out if it's the driver or the car.

Georgia8er

I got 24.1 mpg on the last tank. Of course it was 95% highway at 75-80 mph. And I only have 13,000 on the odometer.

New Yorker

This is a long shot, but… could there be more variability in reported mpg (compared to other cars) because it's harder to calculate exactly how much fuel you've used in the 8? Sometimes I think there must be something different about the 8's filler tube or something that causes the pump to first "click" off at a different level each time.

I fill up at different gas stations (like I have with other cars), and I stop pumping at the first "click" (like I have with other cars), but with the 8, I get the feeling I'm filling up to a different point each time—which would make my calculations off a little. (I'd love to make it easy and just "top it off" to know I'm really filling the tank, but that's not good for the emissions controls. So I use the first "click" as a reference to stop fueling. But what if that first "click" is, say 15.2 gal at one station and 13.3 at another??

What about you guys? If you calculate mpg traditionally (miles divided by gallons), how do you know how many gallons you've used?

dsmdriver

iTrader: (3)

I really doubt that one station clicks off at two gallons away from another one. That's 12% of your tank! Those fillers detect when the liquid comes up and touches them, and the filler is thin, so what it basically means is that the fuel has backed up into the tube. The tube doesn't hold gallons.

Even if you assume a 10% difference in fill ups, that doesn't fix the HUGE range of numbers reported here. We have people saying 12MPG and those saying 22 MPG. Well, adjust each one up and down 10% and you get 13.2 MPG and 19.8 MPG. That's still a 50% difference in cars or drivers.

Also, averaging over a few tanks is always more accurate. I know that I always get right around 14 gallons into the tank after the light comes on, so it's not like there is much variation, and if you keep track over 5 tanks it's not like you manufactured gas at some point. It's only the innaccuracy on the last tank that factors in.

New Yorker

I see—didn't realize the filler is so small and insignificant. Thanks for the info!

DrDiaboloco

I usually top off with another .3gals after the pump clicks off.

Remember that the error introduced by the variability of top-offs really only applies on single-tank fillups. If you keep track over several tanks, or ALL tanks in my case, that error is effectively zero if you're dividing the total amount of gasoline pumped into the car over the total mileage.

dreamfrog

I agree. This car is MADE TO BE PUSHED.
I don't know what your idea of "low rpm" is, but that could be what's killing your mileage.

Added, later:

Okay, okay, sorry in advance. This has already been addressed in this thread.
I didn't want to delete my post though. I just LOVE this dang car!! And, love to read & talk about it. It's a passion.

dreamfrog

If you love her, REDLINE HER. She needs it, wants it, craves it.
She is made for it.

once again, {{{{{hmmmmm}}}}}}
Thank you guys for not thinking I am a sicko.

hehe.

BTW, when I drive, I vary greatly from shifting at 1.5 rpms to 7K rpms, to doing 50+ in second gear.
But, I ALWAYS get 260 miles per tank. Unless I have had a really crappy errand week. At 260, that's when the light comes on.
I don't worry about the calculation, because I know 260 is good. I don't care how good or if it's a little off, I expect some variation.

dreamfrog

Okay, okay, okay, I give. Someone please explain "Pre-Detonation" PLEASE.

sosonic

Here are things that I come up with to improve your RX-8 gas mileage:

1. Verify your calculation point. Are you running the RX-8 all the way dry or are you re-fueling when the yellow light comes on??? If you are waiting until the light comes on, than you may still have 1 gallon to 2 gallons of fuel left (so you may have used 13 or 14 gallons as oppose to 15 gallons). Don’t make the mistake of calculating that you have run the tank dry from that point when you do the math.

2. Check your oil? Changed your oil filter? Are you in a hot climate? Think about getting the fan mod? How many oil coolers do you have?

These are important, because they seem to be factors. I changed my oil filter from the oem one to a Mazdaspeed oil filter and picked up 1-2mpg. I had been checking and adding oil, but remember to change the oil filter at the designated time. Actually, you may want a high performance oil filter while you are at.

If you are in a hot climate, there appears to be issues with temperature. A 2nd oil cooler (if you have one of the ones that don’t have it), fan mod (search for Looking for a cooler running RX-8? Or go to http://www.mazsport.net).

3. What is the octane level of the gas you are buying? Who are you getting your gas from?

The RX-8 likes 91 Octane or better gas. If you put in lower octane than you are supposed to than that may be the reason why you are getting lower than expected gas mileage. If you insist on using low octane gas to save money, than consider at least adding an octane booster. I’ve used octane boosters with Xylene, with great success. Xylene also burns well, without leaving a lot of deposits, like other octane boosters.

Note/Edit- There seems to be some debate about which is the better octane level. It seems higher Octane is better for performance, but maybe not for gas mileage. Lower Octane, but high enough not cause knocking, may be better for gas mileage. I think the best bet is to stick with Mazda's Octane rating when in doubt, but experimenting with different gasoline from different vendors and with different Octane ratings may help or show something.

I’m getting better gas mileage when I fill up at certain gas stations. I’m not clearly understanding this myself, but it appears the additive package from different gasoline companies are… different. Thus the RX-8 performs better with gas from certain gas companies. I think you need to test this on your own, as oppose to me telling you who to go to and this may vary from state to state or from country to country.

4. You need to rev your RX-8 to at least 6K every day/every other day (higher for the M/T). Have you been doing this?

Some people are babying their RX-8 way too much. This can be hurting your engine, as oppose to helping it. The RX-8 rotary engine is different. Consult this forum or your user manual (just good to read anyway) for more info.

I’ve also seen some more specific advice in this forum, but I’m having trouble locating it right now, but soon as I do than I will let you know. But the above would be a big start for people having trouble with their gas mileage.

More....

5. Check your MAF ( Mass Air Flow) sensor/meter. Here is a link to a part for cost estimate and a reference.... http://www.drivewire.com/mazdaparts/...massmeter.html

6. Check your Catalytic Converter. It appears that its possible for them to become clogged or damaged in various cases without the a CEL light going off. This can be the cause of an RX-8 eating too much gas or not accelarating "properly", so should be checked by an Mazda dealer.

7. Check your PCM (Powertrain Control Module) / ECU (Engine Control Unit) . It may need to be re-programmed/flashed. It may be injecting too much fuel when your engine is started an causing carbon build up and other issues. It appears that various RX-8 have problems with their ECU / PCM.

Note- results of having the ECU / PCM flashed appears to vary.

8. Check Air Filter or consider buying Cold Air Intake. May improve gas mileage.

9. Check O2 sensor. Could be responsible for overly rich mixture.

10. Another factor worth mentioning is customer expectations. The RX-8 is a sports car. Gas mileage in most sports cars SUCK. You should expect 17 to 24 MPG from the RX-8 and most other sports cars. I think RX-8 users complain a little more, because the price range and the “cool factor” means people buying the RX-8 might have bought an economy car, but choose the RX-8 after looking at it. These same people that are used to a 4 cylinder econo car then have a hard time understanding how to take care of a rotary engine or anything that will get less than 25 MPG. Well, the RX-8 is for FUN. If you don’t want a cool looking and fun to drive sports car, than go ahead and get an econo box and call it a day. Otherwise, I say, go over a list of thing that may be causing poor gas mileage in your particular RX-8. Also, see your Mazda dealer and make sure they check and confirm that there is nothing wrong with your car.

Tanaka826

thanks for the info sosonic! very helpful!!