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Old 03-02-2005, 12:21 PM   #1
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Gas Mileage Ruminations

Our RX-8 is a wonderful car, with the notable exception of its dreadful gas mileage. The car is my wife's daily driver, and she consistently gets around 15 mpg. Her previous daily driver was our Honda Odyssey minivan. The same person, with the same driving pattern, averaged 19mpg with that truck/car.

I drove the RX-8 today, and (as always when I drive it) I found myself consistently one or two gears lower than I "needed" to be, not because of the fun of being in a lower gear, but because it's so smooth and so quiet, I don't really notice (my normal ride is my Honda S2000, which give one very audible feedback as the revs move up). I drive my S2000 at high rpm occasionally when not on the track or autocross, but I always do so on purpose (and still get 23mpg in horrible commuter traffic :D ); I found myself well over 4,000rpm frequently with the RX-8.

I wonder--if the driver were to try to shift at 3500 rpm when in normal commuting and not trying to accelerate hard, or go up a hill, might that have a signficant effect on gas mileage?
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Old 03-02-2005, 12:28 PM   #2
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The rumour is - keep the rpms under 3750, and fuel economy will be at its best. don't downshif unless neccessary, as 'more throttle and fewer RPMS' consumes less fuel than 'more rpms and light throttle'
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Old 03-02-2005, 01:02 PM   #3
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I think the longest I've been able to do anything near that was during the break in period. Even that was above the magical 3,750 mark (varying RPMs and speeds etc...). On the highway (long distance) I see between 21-23 MPG at speeds over 80 MPH (4,000RPM in 6th gear) that pretty much dissolved any thoughts in my head about 3,750 being some economical mojo.

My car gets between 14-10 MPG in the city and that number is directly related to how much fun I have driving it. I have accepted it. I do still run the numbers at the pump but I no longer worry about it while I'm driving the car...I buy the gas and then I burn it.
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Old 03-02-2005, 01:18 PM   #4
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Quote:
Originally Posted by MTCD01
I think the longest I've been able to do anything near that was during the break in period. Even that was above the magical 3,750 mark (varying RPMs and speeds etc...). On the highway (long distance) I see between 21-23 MPG at speeds over 80 MPH (4,000RPM in 6th gear) that pretty much dissolved any thoughts in my head about 3,750 being some economical mojo.

My car gets between 14-10 MPG in the city and that number is directly related to how much fun I have driving it. I have accepted it. I do still run the numbers at the pump but I no longer worry about it while I'm driving the car...I buy the gas and then I burn it.

How would driving FASTER than 3750 dissolve thoughts about driving slower than 3750 showing improvements?

Drive around 70mph for a long freeway run.
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Old 03-02-2005, 01:23 PM   #5
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The rumor is that if you worry about gas mileage and own an RX-8 then you really are in the wrong car.
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Old 03-02-2005, 01:27 PM   #6
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sometime my gas station gives me a free air freshner..
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Old 03-02-2005, 01:32 PM   #7
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Quote:
Originally Posted by StewC625
The rumor is that if you worry about gas mileage and own an RX-8 then you really are in the wrong car.
I understand the reasoning. The car is meant to be enjoyed, and we need to just accept the sucky gas mileage.

My real question, however, is whether gas mileage in this car can be improved from simply dreadful (and embarrasing for us environmentalists) to merely bad, with a little care? If one wants to rev the engine, one can, obviously. I am curious if, for the normal humdrum driving when it really makes no difference whether you're in fourth at 2400rpm or third at 3500rpm, gas mlieage is improved significantly by choosing fourth at 2400rpm.
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Old 03-02-2005, 01:37 PM   #8
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Not an expert, but I am reasonably sure that higher RPM driving will significatly decrease the gas mileage. Vice versa, driving at moderate RPMs should improve the mileage.
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Old 03-02-2005, 01:38 PM   #9
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Quote:
How would driving FASTER than 3750 dissolve thoughts about driving slower than 3750 showing improvements?

Drive around 70mph for a long freeway run.
If it were true I would have expected to see much less than 20 MPH while freeway cruising at 90MPH. How much better than 21-23 MPG do I need? I was very happy to see numbers above 20 MPG. Driving at 70 MPH may improve the MPG by 1 or even 2 but I doubt it has anything to do with the specific 3,750 number and much more to do with reduced RPM and reduced drag/load in general. Driving any car at 90 MPH vs 70 MPH will always reduce efficiency. The same car at 55 MPH or 65 MPH may be even more efficient.
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Old 03-02-2005, 01:41 PM   #10
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wouldn't momentum play a role in this? Im sure driving too slow will reduce efficiency...
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Old 03-02-2005, 01:55 PM   #11
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I'm sure based on this gearing, drag of this body, air temp, and elevation there is a magic number for speed that has the greatest efficiency. Thanks to my car I don't even worry about things like this anymore. I just drive.

I had a Honda CRX that got 40 MPG with the AC, headlights, and at least 100 lbs of stereo equipment sucking power all the way from Daytona Beach FL to Dulles VA (just over 500 miles). If I wanted better mileage I'd buy another 4 banger, instead, I bought a better car.
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Old 03-02-2005, 02:48 PM   #12
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I just bought an 04 8 recently. On the first tank I got 20 mpg, and that was keeping rpms under 4000. I am a couple of tanks in now, just filled up this morning and got 19.5 mpg, mostly highway driving in the 80-90 mph range. Not bad. I've read some posts on here that say mileage begins to improve over a certain mileage. Maybe because the computer is programmed to use more fuel during the inital period (just a guess)
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Old 03-02-2005, 02:52 PM   #13
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3750rpm

There are 3 sets of fuel ports on the rotor housings. The first set obviously stays open or no fuel would reach the rotors. The second set opens at 3750rpm, and the third set is way up there, like 7000 or 7500 I think, don't remember. So at >3750rpm you have increased the potential amount of fuel going to the rotors by 100% (obviously this is the *potential amount* of fuel, you don't send twice as much fuel at 3751 than you do at 3749. You have 6 openings from the gas tank to the engine instead of 3). I'm not a fluid physics person, but I'm sure it may be sending a measureable amount more gas into the engine from the pressure equalizing with the additional fuel port openings. Somebody who knows something (read: Rotarygod) instead of somebody who pretends to know stuff like me may be able to address this better.

But either way, that's where the magical 3750rpm number comes from. I'm sure it does play a somewhat measurable role in gas mileage, but how much I don't know.
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Old 03-02-2005, 02:55 PM   #14
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plus driving on an incline will help suck some more....nomatter what rpm
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Old 03-02-2005, 03:04 PM   #15
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I'm aware of the 2nd injectors duty cycle but I just don't believe it has a huge impact on mileage in a highway setting (constant RPM and speed for long duration). If it did you wouldn't be able to drive the car over 70-75 MPH on a long trip and still see numbers above 20 MPG.

In city driving sure running the tach up alot results in lower mileage (real no brainer here) and a bigger smile on your face (my biggest smile has been 10 MPG - 14 MPG is my average smile - on the highway I grin at 20+ MPG -- I hear the track smile weighs in at 8 MPG).
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Old 03-02-2005, 03:25 PM   #16
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You have an AT? With a MT in 6th gear, you get 20MPH for each 1000rpm. 70MPH is 3500rpm and 75MPH is right at 3750rpm, all ideal cases subject to environmental conditions, blah blah blah. So that'd be about right I suppose, you'd be doing 70-75 with just the primary ports open. Assuming you shift before 3750rpm of course which is doable but not fun.

I'm not sure of the gear ratios and other such numbers with the AT
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Old 03-02-2005, 03:45 PM   #17
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I drive mostly highway (about 80%) and have tried on several tanks to do all shifting at or below 3750rpm. 22mpg is the highest I've ever gotten and that was during the summer. Now during the winter, I'm lucky if I can break 19mpg. I guess I've learned to live with the poor gas mileage, but it just irks me that you can't get what the window sticker says. I think if the window sticker said 13 city and 19 highway there would be virtually no complaining about it, myself included, because you could plainly see what to expect when you bought the car. Of course we would then have another $1000 gas guzzler tax added to the price of the car.

I'm no expert on the workings of the rotary engine but I have to believe there is a way to improve the mpg. HJopefully over the next few years, Mazda can make improvments
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Old 03-02-2005, 03:50 PM   #18
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I think that most people ignore one of the major factors and that is the start/stop cycles. Most people, especially suburbanites are accustomed to jumping in the car whenever they need to get 1 item at the store or run the kids somewhere. The more times that you start the car warm and take it through warmup and shut-down again the more gas that you will drink.

The proof point for me was getting stuck in a 100 mile traffic jam. Filled up just before getting on the road and then traveled in 1st and 2nd between extended period of sitting completely still. Out of curiosity, I filled up as soon as I got off the road. Almost 23 mpg. (highest ever)

Other times, when I accumulate 100 miles with short runs and stop and go driving it will never be higher than 17. The rotary is not thermally efficient and goes from normal-rich to extra rich for warmup.
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Old 03-02-2005, 04:15 PM   #19
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Quote:
Originally Posted by mjd
I'm no expert on the workings of the rotary engine but I have to believe there is a way to improve the mpg. HJopefully over the next few years, Mazda can make improvments
What really baffles me is that we owned a '73 RX2 and a '74RX3, both of which got around 17/24 city/hwy. We _never_ had a tank with 15mpg. While reciprocating engines have gotten much more powerful and efficient, the rotary has only gotten more powerful, at quite a cost in fuel efficiency.
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Old 03-02-2005, 04:28 PM   #20
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The rumor is that if you worry about gas mileage and own an RX-8 then you really are in the wrong car.
Haha--Dammit, if I'd only known! :p
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Old 03-02-2005, 04:41 PM   #21
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Mine has steadily improved over it's life. Right now I average 20 MPG combined driving at 22,000 miles, so I'm happy. The problem is that gas prices keep rising and that's eating into my beer money!
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Old 03-02-2005, 06:04 PM   #22
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I think that some of our RX8s just behave better than others. I don't believe that people are getting poor mileage because of how they drive. If this were the case, wouldn't we all get about the same highway MPG?
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Old 03-02-2005, 06:11 PM   #23
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No one has mentioned that the secondary air intake opens at 3200 as posted in a previous thread. I have tried and still try to drive in that 3200 to 3750 rpm range. I have not seen a significant change in mpg but it sure is fun.
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Old 03-02-2005, 06:15 PM   #24
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Shifting at 3200 rpms is no fun! But I have followed that shifting pattern and have found my gas mileage to have improved (about 40-80 extra miles on the tank) sometimes. Sometimes, however, when I shift earlier (i.e., around 3000 rpm) I find my gas mileage to decrease. Not sure why that is. Just my luck, though.
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Old 03-02-2005, 06:35 PM   #25
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This is mumbo jumbo about shifting below 3750. Assuming you stay at the same throttle position before and after 3750, the amount of fuel going to the engine is not going to be significant enough to affect gas mileage to the point of 12-15 mpg. The AFR stays constant through the rpm range (relatively). If it did begin to dump a lot more fuel in at 3750, (enough to put mpg at 12-15) you would feel it bog down since it is already running pig rich.
I think poor gas mileage is coming from poor tuning and Mazda playing it safe for those that aren't running 91 or above, which keeps them from losing their butts on waranty claims.
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Old 03-02-2005, 06:35 PM
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