Ethan Melamed

Hey Rx8Club!

Disclaimer begin
So just because you see bad gas mileage in the title does not mean I'm trying to troll anyone, also I have done a search so please don't tell me to do a search. If you try putting in "Gas Mileage" or "Low gas mileage, you'll see that nothing really helpful pops up. I have been reading posts on this forum since I started shopping for an 8 and I have noticed that people on here can be "Testy?", especially when it comes to gas mileage. Which is understandable because it really is the one thing that people say to bash our beloved 8s. And also keep in mind that I am not saying anything bad about the mileage of rx8s in general, I bought mine fully understanding that it is no smart car. I am only saying that my own rx8's mileage seems bad, and that is in comparison to other rx8s and I am concerned about this for multiple reasons. I read this: https://www.rx8club.com/new-member-f...t-here-202454/ already so please don't refer me there either. Anyways please keep in mind that if you have nothing nice to say just please don't say anything at all. I'm just looking for facts and help, not miscellaneous opinions. Thanks! Also I would have posted this in the troubleshoot forum but I don't have the user privileges or something. I'm guessing its because i don't have enough posts.
Disclaimer end.

So since I bought the car I have been getting what seems to be low mileage even for an rx8 in the city. About 300 kms for a tank which is about 186miles according to google's unit converter. That equates to about
18.3 Litress/100km or
12 miles per gallon (I may have calculated this wrong I'm useless with imperial)

Maybe this is what some other people are getting and my car is not a gas guzzling freak of nature, but some people are getting closer to 400kms (250miles) and I would like to be around there. I have a brand new catalytic converter, I haven't changed the air filter but it doesn't look too bad. Also I keep my tire pressures pretty high 35-40 psi, closer to 40. So what steps can I take to trouble shoot this bad mileage problem? And maybe if you can include costs that would save me some trouble

Thanks!
Ethan

Edit:
some extra info: By "I haven't changed the air filter" I am not saying it hasn't been changed EVER, just since I bought it a little over a month ago.
The oil change sticker is still about 5000kms in the future, I've already put a couple thousand clicks on it so this leads me to believe it is running synthetic oil of some kind. And the mileage on the car is a little under 94 000kms ~ 55 000 miles

RIWWP

iTrader: (2)

From the thread you linked: https://www.rx8club.com/new-member-f...4/#post4533702
So what in that list have you already addressed, and what is left to address?

Ethan Melamed

I'm not much of an expert on this stuff. My question isn't so much as what to trouble shoot but how.. Um so I realise that is asking for a lot of info and someone's time but if you don't have it that's cool no worries someone hopefully will

Also i don't have much money so I realize the easiest way to that stuff, is to just straight up replace it and see if that did the trick, but that's not really in the cards for me at this point.

alnielsen

iTrader: (4)

A lot of time was spent in creating that thread to help new owners like you. The information you seek is there. You just have to read it there instead of waiting for a reply, which likely will not come now. The members here don't like lazy people.
We can't help that you don't have the amount of money necessary to properly maintain a car.

RIWWP

iTrader: (2)

Alright, running down the list: (Unsure of replacement prices for Canada for everything)

coils - Factory coils tend to fail around 30,000 miles, possibly sooner, sometimes later. You can test for coil failure with a $5-10 coil tester from an auto-parts store (looks like a spark plug with a clamp).

wires - Factory wires tend to fail around 30,000 miles, possible sooner, sometimes later. You can test for wire failure with the same coil tester (swap wires around on coils to isolate problem components)

plugs - plugs can start dropping in performance from fouling after about 10,000 miles, but typically can last till the coils or wires start failing, then they start fouling REALLY fast.

O2 sensors - they typically last a long time, but they can fail, especially if you have ignition or cat failure as well. The front is mandatory for efficient running, the rear is mandatory for emissions, and helps cruising mileage slightly.

carbon free engine - not much you can do about carbon caking overall, but an engine that has been lugged about at low RPM is going to have compression problems from being carbon choked

good compression - a compression test for $150-$250ish can tell you if your engine is failing or not.

catalytic converter - anything that kills mileage is probably also killing your cat, which will kill mileage even more itself, in addition to killing O2 sensors and your engine.

good transmission/diff fluid - Fluid changes every 30k are highly recommended, Mazda oil, Redline MT-90/75w90, Eunos, or Amsoil. Avoid off-the-shelf gear oils from local auto parts stores, especially Royal Purple, it destroys our transmissions.

no vacuum leaks - get an OBD2 bluetooth adapter and an OBD2 app for your smart phone. Check for short term fuel trim and MAF data at idle on a warm engine. Too little air flow and positive STFTs mean a vacuum leak.

clean MAF - $5-8 can of MAF cleaner, remove the sensor and hose it off. Let it dry and put back

clean ESS - Just a magnet inside a plastic shield, wipe it off with a rag. Reset the ESS profile data with the 20-brake-pedal-stomp and give it a few drive cycles to re-learn the ESS profile


And should add:
- Validate your engine coolant temp sensor with OBD2, if it's stuck low it can kill mileage, though should also pop a CEL

Ethan Melamed

What I'm looking for is to narrow down (trouble shoot) what the problem might be without trucking out money I don't have and just replacing everything on that list. I personally couldn't find that information in that thread so maybe if you can quote it for me or something like that you would be helping me greatly... Otherwise, you are just telling other people not to help me, in which case I personally would have preferred that you had just not posted at all, like I said in the disclaimer that I spent time on to reduce the amount of unhelpful posts in this thread. I'm sure you are a good guy, you're just not being helpful right now though.
All the best to you

Edit:
didn't see RIWWP's post before posting this. Thanks alot RIWWP for putting time into that post I'm reading it now and there's lots of useful information there thank you so much! I've copy and pasted it into a document so unless you think people will have anything to add that's not just flame you can go ahead and close this if you want.

RIWWP

iTrader: (2)

Al's point is that the thread has the best and easiest method to search the forum, and the list of possible parts that could be impacting the mileage. Using that search method on each part to research how to test is the next step beyond just finding the list of parts. No one is suggesting that you should just randomly replace everything, but we are giving you the tools you need to figure it out.

"Give a man a fish and he will eat for a day. Teach him how to fish and he will eat for a lifetime."

Ethan Melamed

Not sure if that's what al was trying to say but whatever. I'm not sure I did wrong by the site in this thread, after all aren't forums like this about compiling information? Anyways thank you both for your help. I'll be trying all of this after exams and I will put up a post about my findings when I'm done.

RIWWP

iTrader: (2)

Note, I'm not bashing you, just trying to help you understand.


Just picture how your teachers would react if their students would ask them "what are the answers to this exam?", rather than studying and researching ahead of time. I expect that your teachers wouldn't take kindly to that. That doesn't mean that they don't want questions though, as if you bring them a specific question on a specific topic to discuss or debate over, chances are they would enjoy that. It's really the same type of reaction you saw from Al.

To put it another way, you are correct that the forum does have a pseudo-purpose of compiling information. Just think about how that is then used? If we have to re-compile it for every single person that "just wants answers", it's defeating that purpose. You won't suddenly find a solution for "bad gas mileage" any more than you suddenly find an answer to "advanced mathematics", but within a textbook labeled "advanced mathematics" you will probably find the tools that you need to figure out the answer to your particular question.

Ethan Melamed

As a new member to a site that has been around as long as this one has all you can really do is read the stickies and the first couple pages of a search. Sifting through threads even using the great and almighty search tool that vBulletin provides is overwhelming (Its a good search tool but isn't specific to websites so can often times be useless). For example it would have taken me hours to get all of the information that you put into your above post. So generally, as is my way, if I have a question and google, none of the stickies in the specialized forum nor the first couple pages of a search in that forum answer my question, I post a new thread. I then hope for kindness, expect some rudeness, take away as much information as I can and thank whoever put their time into helping me. If that info had been compiled somewhere else then I missed it and shame on me, but I don't think it has and I hope it helps people in the future with the same problem.

RIWWP

iTrader: (2)

Actually all of the information in the list I posted is in the new owner's thread

Baran

I get around 12 mpg also :/ I drive 90 percent city

dynamho

12-14mpg is about right for all city short trips with the requisite redlining interspersed.

Gerald Nunn

300 km for the city and 400 km for highway driving is what I've been getting since I got my RX-8 with 16,000 km on it. I'm up to 70,000 now and it hasn't varied.

Karack

city mileage varies widely and i wouldn't take the sticky post as a rule of thumb answer for what you should get.

heavily congested city mileage is going to be a lot different than rural city mileage. even at times my city mileage can vary a bit depending on what time of day i drive and where i need to go and how heavy the traffic is between lights/how many lights i have to hit from point a to point b.

i wish i got 17 with strictly city driving, but it's more like 12.

in all honesty city mileage doesn't tell you squat as to whether your car is running correctly or not, highway and mixed mileage does.

UrbanCowboy

iTrader: (1)

I once has a track day where I got 96km to a full tank of gas which is 4mpg. all depends on how you drive.

hoosier1104

iTrader: (2)

I am getting 13.9 but I do not drive mine enough to have a valid input. My driving in my 8 is primarily short trips, gf house and back and the occasional trip into Indy. I have not driven mine since about the middle of Dec when the first snow hit us though.

Take it for what is worth.

UrbanCowboy

iTrader: (1)

You'll really know something is wrong if your getting terrible gas mileage cruising on the highway.

RIWWP

iTrader: (2)

Yup, all gas mileage benchmarking should be done 100% highway on the tank, since it removes a lot of the driving style variables. City mileage can vary hugely just based on the driver alone.

hoosier1104

iTrader: (2)

Mine is a combination of the 2 but the problem I have is the speed limit. This car LOVES to be driven that is for damn sure.

04Green

iTrader: (1)

RIWWP called it..

I am a fan of changing coils before plugs. And, if you do not know the history, coils first, and clean the plugs. Bad coils cannot fire good plugs. The autozone or equivalent ones are fine. Stay away from the EBAY ones, several on here have reported problems.

Some more data would be good. Model year, transmission?

DMaverickRX8

iTrader: (1)

What kind of mph/rpm is the most efficient for the 8 on the highway?

RIWWP

iTrader: (2)

The engine's most efficient RPM is around 5,000-5,500rpm. Your point of peak torque for your engine is going to be the same point, so it will vary a bit from engine to engine.

However, in 6th gear, this is 100mph+ and the aero drag is considerable. Lower speed will improve this, but too low and the engine efficiency will fall off too as it gets closer and closer to lugging. 3,500rpm is about the lowest you can get without the efficiency penalty out pacing the drag reduction benefit. My experience is that 65mph returned better mileage than either 60 or 70 if on hundreds of miles of level ground. That being said, letting speed fall off on uphills that won't kill your speed too much and pick up on downhills as much as gravity and aero will let you without additional throttle input, means that the ideal mph is a moving target.

DMaverickRX8

iTrader: (1)

Sounds good. What about for an auto? IIRC it does 70mph at 3000 rpm and 2700rpm at 65mph

RIWWP

iTrader: (2)

Probably around 70mph then, though it might take some testing to figure out.

Sunroof up and windows down by themselves can chop off 2-3mpg on the highway, so keep them closed if you are going for a peak. The moonroof being the bigger impact I believe, since it generates a huge low pressure zone behind it and disrupts the airflow across the entire rear half of the car.