Maximizing gas mileage for the CX-9

J

jackaree

Member
I have a 2008 CX-9 that I bought used last year. It has about 30K miles & I love the vehicle overall, but the gas mileage is HORRIBLE. I knew when I bought it that it would eat more gas than my old Grand Am, but holy crap - I didn't expect it to be quite this bad. The thing drinks gas like I drink water!

Any suggestions from other owners on how to get the most mileage per gallon? I've tried putting in 89 or 91 octane fuel, but there was no noticeable difference. Yes, I could probably just drive slower, but even being an SUV, the CX-9 is fun to drive, so who wants to always go 55 mph? :) I've heard other people talk about how they've had chips installed in their vehicles, but none have Mazdas. Thanks!

Jackie in MN
 
How bad is bad? We avg 17-18mpg city/highway with our 2009 AWD, and I'm quite happy with that. It gets 21-22 mpg on our 4 1/2 hour trips to Chicago, and that's cooking along at 70-75. What are you actually achieving?
 
For a vehicle like CX9 with 4500lbs, tall 1st gear, and probably too-heavy flywheel, stop and go traffic kills the MPG.
I see 10mpg busing my kid to school 2 miles away with lights and stops on the way. I also got 21-22mpg on 120 miles all-highway trip to Sacramento traveling at least 70mph.

On heavy flywheel, I have no spec data, but everytime the traffic slows down, my CX9 seems to catch on to the guy in front even though both have the gas pedals lifted. It seems like heavy flywheel to me. Anyone with similar experience?

On MPG, check air filter (engine) and tire pressure.
Other than that, nothing helps w/o mod.
 
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I know that SUVs are not known for good gas mileage but our 2008 GT's city mileage has been slowly deteriorating. When purchased new on our drive back from Albuquerque, we were averaging betwen 25 to 27 mpg on the highway with about 15 to 16 mpg in the city. Two years and 16,000 miles later, the highway mileage dropped to 20 mpg and the city dropped to 12.8 mpg. I already replaced the air filter and maintained the tire pressures at 40 psi. This is my wife's daily driver which is mostly city driving but she doesn't drive fast. The only things I can think of are: 1) fuel injector cleaning? or 2) oil change? to improve the gas mileage?
 
Try the fuel injector cleaner route. I just used Red Line in a tank and my highway MPG went up by about 2 MPG over the previous trip I took. Seems to have made about a 1 MPG increase in town.
 
Stop and go absolutely kills the mpg on this car. I'm no hypermiler but I will use some gas saving techniques that may help a little bit. For one I will start coasting when I see a red light (and apply the brakes slowly if needed). This way I save a little momentum for when the light turns green. Also, if there's no urgency or traffic to merge with, I'll get up to highway speeds pretty slowly. I'm consistently getting 18-19 mpg in mixed driving. On pure highway trips I'm around 21.
I have found the trip computer to be surprisingly accurate. I check my mileage at every fill up and the number is always within 0.5 mpg of the computer. I've never had a car that was this close. Trip computers in many other cars tend to significantly overestimate mileage.
 
10' GT
weight: 6046lbs

Gas mileage sucks. 17-18 is what I am getting. 17.9 is what the computer tells me. Which I think is accurate, the only thing I find inaccurate is the miles remaining, it always says roughly 400 miles when I fill up and I get 315-340 miles per tank. When the low fuel light comes on the computer says I have about 30 miles left and that usually happens at 300 miles.

I found that slow acceleration and going to neutral on downhills help the gas mileage. It's a very heavy vehicle so gas mileage is going to suffer, the upside is that the heavier a vehicle is the safer it is, everything else being equal.
 
>> weight: 6046lbs

How did you come up with that number? GVWR?
Curb weight is a more accurate number representing how much a vehicle typically weigh on street.
 
Yunus said:
I found that slow acceleration and going to neutral on downhills help the gas mileage. It's a very heavy vehicle so gas mileage is going to suffer, the upside is that the heavier a vehicle is the safer it is, everything else being equal.
Click to expand...

I don't think going into neutral on downhills helps. Rather, it hurts since your engine is now idling (powered only by gas) rather than being powered by your wheels (transmission is connected) using little to no gas as it coasts. Plus, better to keep your car in drive since you never know when you'll need to accelerate quickly to avoid something.
 
Putting your CX9 in neutral (or any vehicle) while coasting down a hill is plain dangerous plus it does not save MPG. An iddling engine is always considered "energy loss" plus what ever you think you are saving in gas you are wasting it on brake replacement or repair because you are constantly regulating your speed by braking rather then with down shifting or engine brake.
 
Davicho said:
Putting your CX9 in neutral (or any vehicle) while coasting down a hill is plain dangerous plus it does not save MPG. An iddling engine is always considered "energy loss" plus what ever you think you are saving in gas you are wasting it on brake replacement or repair because you are constantly regulating your speed by braking rather then with down shifting or engine brake.
Click to expand...

Brings up a good point though. Assuming you don't WANT to be slowing down, keeping the car in gear robs energy because it will give you some engine braking/transmission inertia that slows you down. This is obvious because you speed up on a downhill when you put your car in neutral.

So the question is, in what scenarios is the energy loss associated with engine braking/transmission inertia outweighed by the energy loss of neutral idling @ ~1.0k rpm. In certain instances, such as a long-slow downhill highway where you can maintain speed and don't have to stop at the bottom, I can see neutral (1k rpm gas use) + freewheeling being preferred to gear (no gas use) + engine drag.

Obviously this ignores the (slight) safety issue posed by leaving your car in neutral given the incremental 0.5 second to put it into gear before accelerating.
 
Just to clarify something. Our CX-9s when put in neutral but still maintaining a constant or increasing speed (as in traveling downhill) will NOT iddle at ~1000rpm. Mazda has programmed the ECM to increase iddle rpm to around @~ 2000-2500rpm when this scenario appears to prevent or minimize transmission shock when/if put back into gear at a higher speed then rolling. So the savings in MPG may not be as high as you expect. Just an FYI.
 
I was stating the GVWR, after reading your comment I realized what I was looking at, I just saw the number and thought that was the weight, not the max weight.

I stand by the neutral though. I'm not talking about coasting all the way down Pikes Peak. The roads I drive on are slight declines followed by slight inclines. If I remain in gear the vehicle slows to 50 without pressing the gas to maintain speed. However in neutral the vehicle accelerates to about 65-70 while idling. So while you might be correct on the downhill portion saving fuel remaining in gear, when you then have to go uphill again the fact that you are already at speed you don't have to accelerate up a hill. If you maintain speed in gear going downhill then you are in effect accelerating downhill which I have trouble believing to save more gas than idle. The road I am describing is I-70 between Frederick and Baltimore in MD.
 
Siimple to test - change your trip computer to instant consumption and note what the consumption is while coasting (even though it is not recommended by anyone) and then again down the same incline in Drive.
 
AusieCX9 said:
Siimple to test - change your trip computer to instant consumption and note what the consumption is while coasting (even though it is not recommended by anyone) and then again down the same incline in Drive.
Click to expand...

Would this comparison be possible?

In the two cases:
1) If you put into neutral: You will have some measurable MPG as you are using fuel while the car idles at some RPM (may be more than 1k as per previous comment)
2) If you leave in gear: Your MPG will be infinity since you are using no fuel. But you'll be going slower at the bottom of the hill due to engine drag.

How about this alternative:
1) Neutral: Enter top of hill at a pre-determined speed (eg, 50mph), reset your MPG at top of hill. Coast down the hill in neutral. At bottom of hill, record your AVERAGE MPG for the entire downhill and your final speed at the bottom.
vs.
2) Gear: Enter top of hill at a pre-determined speed (eg, 50mph), reset your MPG at top of hill. Coast down the hill in gear, but ACCELERATE very slightly (to overcome the engine drag) such that your speed at the bottom of the hill exactly matches your speed in case 1. At bottom of hill, record your AVERAGE MPG.

This would seem more apples-to-apples to me.
 
Although I haven't been impressed with the city mpg of our CX9 (15.5-17), highway mpg on long trips driving 80+ mph with the AC, loaded up with luggage, at high altitude no less, has been 22-24 mpg.

I always remind myself, the window sticker said 15/21, and I'm right there, what do you expect? It's just that our 04 Pilot got 19-20 city all the time, so I figured the CX9 with better aerodynamics and a 6 speed would do at least that if not better. But it doesn't. Oh well. In the whole scheme of things, we drive ~11k miles a year, it's only a few extra $ in the end...the only thing that would make a huge difference money wise is if you could get 24+ mpg, and that ain't gonna happen...
 
My car is new and on my first road trip of about 380 miles, with some city, I got 31 MPG average.
 
AusieCX9 said:
Siimple to test - change your trip computer to instant consumption and note what the consumption is while coasting (even though it is not recommended by anyone) and then again down the same incline in Drive.
Click to expand...

Tried it yesterday on the way home. When you put it in neutral and are moving the MPG on the instant read goes to 99.9 (max reading) on the downhill in gear it read varying between to 48.5 and 75 or so. However I don't believe the readings on the instant consumption, they might be accurate but I don't trust them especially on the extreme ends.
 
You should believe the MPG meter.
It is based on fuel injection "count" devided by distance traveled.
That is average MPG since prior reset.
Instantanous MPG however is based on periodical computation.
i.e. it measures "average MPG" every few seconds.
I have ScanGuage-II on my Mazda. My wife's Prius also have on-board MPG meter.
When rolling downhill, Prius always show 99.9 since it shuts off engine (unless A/C is blasting).

The fuel injected on each count remains more or less constant. It injects more per second during acceleration. If you have a MFD on your CX9, inside diagnostic mode, you can see injection count on the fly, gyro steering angle, etc.
 
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