Swanny2k4

I get 28 mpg combined mainly motorway miles but some city driving and a good blast here and there is that good? It's a 04 60,000 miles 192 bhp?

Tamas

iTrader: (1)

That's imperial gallons I suppose (from the use of the word "motorway"). Still, that's good fuel mileage.

Swanny2k4

Uk gallons not US so guessing that's metric?

Swanny2k4

And what's up with the word motorway? Lol

Slidin8

Motorway = way for motor vehicles to travel

Highway = way to get high?

What's up with the word highway? Obviously motorway makes sense

The English language originated in England, not the US FYI

Swanny2k4

I thought motorway and highway meant the same thing lol obv motorway is English and highway is American? Why were debating this I don't know? Haha

Tamas

iTrader: (1)

Huh? Gallon can never be metric.
UK gallons are slightly bigger than US gallons. 4.545 liters vs. 3.785 liters. So if you get 28 mpg with UK gallons, that would be about 23.32 mpg with US gallons.

As far as I know, motorway in the UK is the same like freeway in the US. Highway can be just a two-lane road. I only mentioned the word motorway because it is not really used in the US so that gave me a hint you're from the UK, using imperial gallons instead of US gallons.

Slidin8

its ok bro, I'm English so this sort of stuff bugs me

like the spelling of the word colour, it has a U in it, it is not spelt color. I'm pissed off right now because me laptop is set to english US and is saying i spelt colour incorrectly.

Rant over

BTW in New Zealand we use the term Highway so i dont even know why i'm having a whinge

Christian_T

I calculated 14-15 mpg and I do majority highway driving.
Is the computer flash covered under warranty?
CT

Bladecutter

There's no way you're getting 14 to 15 mpg on the highway on a 2011 RX-8, unless all your highway driving is being done in 3rd gear, at 85 mph or higher.

On an open highway run in my 2011 Sport (manual), I averaged 21.5 mpg.
That was a trip between Denver, CO and Santa Fe, NM back in Sept, and another trip between Denver, CO and Las Vegas, NM.

How many miles are on your car, and when was your plugs and wires last changed?

BC.

Christian_T

I am indeed. I bought the car with 7800 miles; it has 12300 now. Do you think it is a computer flash issue or a plugs/wires/coils issue?

RIWWP

iTrader: (2)

Things to check for why your mileage is so bad:
- Front O2 sensor (can be diagnosed with OBD2 live data by someone that knows what they are doing, even if there is no CEL)
- Ignition failure
- vacuum leak
- brake pads dragging
- engine seals are having trouble sealing (which would be exceedingly odd for that low of mileage)
- MAF sensor is dirty or fouled, reporting incorrectly (can be diagnosed by OBD2 live data)
- Something is severely hurting your aerodynamics, like a splash pan dragging or cupping the air
- injectors firing far too much, like the secondaries firing with the primaries all the time (Highly unlikely for this low mileage)
- Cat is clogging (unlikely, in general)


...thinking for others....


Short, no, this won't be a PCM flash issue. And at that low of mileage, most part failure sources are very unlikely.


Can you describe your driving, in detail?

RIWWP

iTrader: (2)

Another way to put it, basically anything that will make the ECU dump more fuel than needed (like injectors stuck open or firing when they shouldn't), cause power loss (so you have the throttle open more than you should have to), or causing the car to require more power than it should be (more aero drag, fluid drag, or mechanical drag)

Power loss could be too lean (engine is getting more air than the ECU sees at the MAF), too rich (dumping way more fuel than it should be), or simply not igniting the correct mixture that's there (misfires, compression loss/blowby).


13-14mpg on the highway cruise is most definitely not normal, and not acceptable.

Christian_T

My habits are not abnormal. I shift at 5000 rpm until it warms up; between 5000-6000 rpm once it does; highway speeds are at 70-80 at around 4000 rpm. I red-line it once a week. I drive it the way it was meant to and mpg is always 14-15 mpg.

I did not buy the car for the mpg but sheesh! Would all of that be covered under warranty since the car is so new (at around 12300 miles?).
CT

RIWWP

iTrader: (2)

Yes, you are within the bumper to bumper warranty, so everything should be covered. The problem is going to be convincing the dealer that there is something wrong, and even then, you will probably have to point them to what is wrong. We don't have much faith in dealers here. You very well might get a dealer that just tells you: "it's a sports car?!" Or, one that agrees that it's too low and just does an OBD2 scan, sees no codes and says "i don't know!"

Dealers aren't really paid to diagnose any further than that.

If you can get yours hands on an OBD2 device with live data (a bluetooth OBD2 dongle and an OBD2 app on a smartphone or tablet works fantastic) it will pay for itself many times over for years, and could help you get an answer.

Some things to look for if you can get to that live data:
If you can hold cruise at 60mph on level ground and be getting about 40g/s on your MAF and a ~14.1 AFR, then it should be acting normally and we can point at your driving (). That doesn't sound like it's the case though. If it's pulling more g/s, same AFR then you probably have a drag problem. If it's pulling less at the same AFR then the engine is steadily getting more air than the ECU thinks or there is a MAF problem. If your AFRs are swinging around, then it's getting more air, probably through either an exhaust leak or a vacuum leak, and the ECU is having trouble dealing with it. If it's running solidly richer, you have a severe fuel trim problem or something is stuck with your injectors. If it's running solidly lean, then you might have an injector problem on the 'stuck' closed side of things. Short term fuel trims (STFT) should be sitting at +2% to -2%, long term fuel trim (LTFT) should be sitting at 0%. If those are off, then it will help point to one issue or another. It's more how it behaves rather than a single raw value.

Some other possibilities for many of those, and it isn't the whole story, but it will probably get you most of the way there.

Christian_T

Thank you RIWWP.
If a dealer's not really faithful, any advice?
Would anyone repairing it honor the warranty or would I have to take it to the dealer?
Thanks again,
CT

RIWWP

iTrader: (2)

You will pretty much have to deal with the dealer at some level. But you are almost certainly going to have to arm yourself with information ahead of time. Just trying to get them to "find it and fix it" is what's not really going to work. It's worth a shot, but don't be too surprised if they fail to get anywhere through ignorance or unwillingness. We will all be surprised if they actually do find it

Something you can try short term is to clean your MAF sensor, your e-shaft sensor, and then reset all the learned info on the ECU with the 20-brake-pedal-stomp (key to on, not start, tap the brakes ~20 times rapidly until you see the oil pressure gauge sweep, key to off). That clears the fuel trims and sensor profiles. Idle may be rough for a few drive cycles, but get a feel for the mileage then. If it's still bad then it's probably a hardware failure of some type, from the prior lists. If the dealer can't really help, then I'd highly recommend getting an OBD2 tool (I'd recommend it anyway ) and doing some self diagnosis.

Christian_T

I'll update later on...after a visit to the dealer...sigh...Paul Miller Mazda...

PeteInLongBeach

Christian,
One other question comes to mind: exactly how are you calculating your fuel consumption?

Also, a word of advice: you don't need to shift at 5000 RPM every time - this seems excessive. Shift points should be dependent on driving conditions and requirements.

lgilbert50

iTrader: (1)

Only 5k, my normal is 7k or above but my fuel consumption is -10mpg
A repressed F1 driver

Christian_T

I got coils, plugs, and wires replaced...and with yesterday's calculation, I'm sitting at 16.9 mpg.
Thoughts?
CT

RIWWP

iTrader: (2)

ASSUMING that is 100% highway mileage you still have many things to check:
- engine compression
- Front O2 sensor
- Rear O2 sensor
- clogged cat
- clogged air filter
- engine block grounds
- fuel trims
- vacuum leak
- MAF fouling
- ESS fouling
- brake pads dragging
- tire pressures far too low
- engine coolant temps at cruise
- Driving style

If that is 100% city mileage, then you are in pretty standard range.

Christian_T

Three things down...many more to go.
I drive close to 40 miles one way; 30-35 is straight highway.
Still getting lousy mpg (15-16.9ish)...the saga continues...I'm making a list and going to someone outside the dealer and see what happens...
CT

Cookie Monster

Fuel mileage has as much to do with driving conditions and driving habits, as it does maintenance and condition of the vehicle.

1) do you let the car idle; if so, how long?
2) How cold is it on these trips?
3) what is the terrain like on your trips/area?
4) how fast are you driving? Acceleration. Average speed held at cruise, what gear/rpm?
5) how do you approach lights/stop & go/etc?
6) do you have construction or traffic jams on your trip?
7) what fuel are you using? Does it contain ethanol?

I'll try to give some examples why each of these are important.

1) idling is 0 mpg. If you let your car idle for 5-10+ minutes a day, it can lower your mileage by more than you realize. This is why hybrids have 'auto-stop'; it really makes a huge difference.

2) extreme temps make a big difference; cold more so. Extreme cold (below 40 F) means the car is running rich longer to get to normal op temps; it also usually means more idle time to get the inside warm for the occupants. Conversely, extreme heat is an issue as well, including continuous a/c usage.

3) hills/mountains, rough roads add to fuel consumption, especially with spirited driving. Rough roads are purely resistance; even slight hills eat fuel, and if you don't coast back down, you don't make up for any of the loss. Fuel efficiency is like a game and you are the underdog. You have to work at it, or just not care and be happy with what you get.
4) heavy acceleration, high speeds = minimal mpg; top gear, racing a Prius, gets in the teens, while in real world normal driving it is the easiest to get 40-50 mpg with minimal effort. Every bit of speed over 55 mph drastically effects your mpg. So, if I get 30 mpg at 55, teens at 70+ and much much less every 5 mph, wind resistance is exponential, not linear.

5) stop sign/light drag racing burns up fuel. If you do this as well as never take it easy, you'd be lucky to get 10 mpg. Slow accel and coasting to lights with minimal breaking maximizes your efficiency.

6) every time you break, you turn energy into heat, so lots of slows and speed ups eat fuel. Passing people eats fuel, staying in low gear high rpm eats fuel, traffic jams, if like ones in Chicago (80-0-80mph) sucks your tank dry.

7) 10% ethanol is becoming standard, it is less efficient, has less energy, therefore you get less mpg with it. Gas is more dense when cold, filling up early morning after a long, freezing evening gives you more fuel for you dollar.

I've owned several rotaries, able to achieve 30+ on all of them with lots of highway miles, including 32 mpg in my 8 before I let it go.

I've also owned several hybrids. I could get 60+ in a Prius, but 30s during winter because of issues above. I've gotten close to 90 mpg in a civic hybrid on a pure highway trip and 70 on a town + rural back roads route. I've gotten 45 mpg on a trip from Texas to Kansas before with a car loaded with 3 people's stuff making the rear end squat.

I've also gotten 12 mpg out of an rx7 I could get 28 out of; purely from driving spirited vs driving passively.

I bet if you honestly evaluated your driving habits, you locale, your route; you'd find your mpg acceptable.

Drive fast, burn fuel; bad weather, burn fuel; non-flat roads, burn fuel;

There are resources out there on how to maximize fuel efficiency. From extreme hypermiling to common sense behavior and maintenance, such as proper or max sidewall tire pressures, slow accel, minimal breaks, higher (safe) speed turns, etc.

lightning76

Bought car used. Drove around for a couple of days and noticed fuel guage moving down rapidly. Checked oil and air filter, both were nasty. Changed air filter, oil, oil filter and spark plugs.

Before that MPG was around 18 now MPG is around 16.9....

What gives? Why would mileage get worse?

I need help big time................