JasinC19

Hi everyone. I'm new to the board, although I've been following the RX-8 for some time now. I'm graduating in December and will be buying a new car then, so I'm seriously considering the RX-8. Everything on paper looks good, but I'm worried about the gas mileage.

The RX-7 was pretty poor on gas, and although Mazda claims to have cleaned things up a bit, what do you guys estimate the mileage to be?

Thanks

Jason Convry

FritzMan

Apparently they've improved efficiency with the new port locations.

2007RX-8

The last NA RX-7 (1991 model) sold in the U.S. was EPA Rated at 17 city and 25 highway MPG.

In the RX-8's favor, the Renesis engine is reported to use 40% less fuel than the last 13B at idle. I'm assuming that this percentage inprovement will be much lower at higher throttle settings.

In the last NA RX-7's favor, it was perhaps 100 pounds lighter than the RX-8 will be.

My best guess in that the RX-8 will be EPA Rated at 19 city and 26 highway MPG. Your right foot will dictate how much fuel you use in the real world!

wakeech

well, only 19 mpg for city driving??? that ~40% fuel savings at idle would save more than that, don't you think?? i mean, what're average ~2.6L 6 cyl. (for accurate comparison) engines running at these days?? +20mpg i'm sure... it's not a truck, and i'm not sure how many people would put up with pick-up-esque fuel economy at $0.80 per litre (which is, uh, jeez... times four, times 0.65 for American $$, you figure it out!!) for gasoline, in a compact car ('cause it's rated by mass right?? and the 8 is gonna be light, so i figure it'll be a compact...), no one would stand for it, or the magazines at least would tear it apart for that...

no, i think that Mazda knows they need it to be competitive, just as it was for it's time (17/26 or whatever was fine for a sports car in the 80's through early ninties...), and now with the rotary entering a new application, i'm sure that they've worked everything out...

hey, on the "world outside HP" topic for engines, how about the emissions?? i read somewhere that it was going to be a "Low Emissions" vehicle!! how about that, eh?? anyone else heard the same??

PatrickB

The Infiniti G35 has a 3.5L 6cyl and gets 19/24 MPG, and some people have called the 19 a generous rating, getting more like 17 around town. This is about 1 liter bigger than the effective displacement of the renesis. Piston engines are somewhat more efficient in general than rotaries, though. Personally I'm hoping for about, say, 20/27 from the RX-8.

-Patrick

Edit: fixed size of infinti engine from 3.0 to 3.5L and adjusted text appropriately.

BlueAdept

Hey, did you know that the US Gallon is only 83% as big as it should be.... LOL? Fuel might be more expensive per gallon in the UK, but we get a full 20% more.

So your 20/27 estimate will translate into 24/32.5 in the UK!... Cool, that's actually pretty good... better than my accord at least!

Grimace

I read, I think in a Mazda press release, that fuel mileage would be ~20% better than the 3rd gen RX-7 in all around driving.

Rotaries are not the most fuel-efficient engines, to be honest. But the RX-8 promises a lot of performance.

zoom44

83%= a full 20% more?

BlueAdept

Yes....

1 US Gallon = 0.8326747 Gallon

1 Gallon = 1.20095 US Gallons

LOL... See 1.2 times, or 20% more.

rpm_pwr

Oh wow! :D

Seriously, that's a worry. That would make it WORSE than a NA FC which doesn't make sense since the rx-8 is lighter and has much less overlap. I'd expect about 40% improvement which I have read somewhere.

-pete

wakeech

i'd thought that the FD and the NA FC's weren't too far off in consupmtion... but i've never really checked!!

uh, this thought just came to mind: JasonC19, if you're going to buy a car at US $30 000, and want to pay the insurance on that mother, why're you worrying about the gas?? i knwo that there was a thread on insurance before, but i forget everything that was on it... you're only a little older than i am, and well, i KNOW that my insurance for such a car would break my back. You're really worried about gas milage as a major cost?? (well, maybe that'd be a good thing, as the next one after insurance!! :D who'd want it in the shop at all, right??)

dw1784

I'm not as optimistic as you guys; I'm happy if it gets 15/23. I imaging its going to be 2nd, 3rd gear around town due to low torque. I have to agree with previouse poster regarding cost of ownership vs. fuel cost; if you love your car, what does it matter?

BlueAdept

We're not getting into this again, but low torque is not an issue, the car is simply lower geared and higher revving so for any given speed, the torque at the wheels will be fine.

dw1784

excuse the newbie here, I won't use the "T" word again in the future. I apologize for my inability to convey my opinion more clearly.

I was merely suggesting due to gearing and my guesstimate for the proper gear selection to suit my driving style (which is tame in SoCal), my fuel milege prediction is less than what is offered by the other board members.

Perhaps at Sevenstock in Sep 28/29, Mazada reps will have more info for us.

RX7 Guy

JasinC19,

I think that 2007RX-8’s post was very well written. FWIW, I’ve completed a graduate program in engineering, have been a pilot for twenty years (we hate running out of fuel) & have fifteen years of experience with rotary engines. I’d like to quickly apply my personal knowledge to elaborate on 2007RX-8’s good basic points:

First, automotive engineers use a measure known as BSFC to compare the relative efficiency of engines. BSFC is an acronym for Break Specific Fuel Consumption and denotes the number of pounds of fuel burnt per hour per horse power. Highly efficient piston engines achieve BSFC’s of around 0.45. A US gallon of fuel weighs 6 Lbs. so for example a 150 hp engine running at 70% power with a BSFC of 0.45 would burn 7.875 gallons per hour. 150 X 0.70 = 105 hp. 105 X 0.45 = 47.25 Lbs. 47.25 / 6 = 7.875 Gallons. As I recall, the normally aspirated 13B engine in my RX-7 has a BSFC of 0.52 so all things being equal, a my rotary engine will burn 15% more fuel than a highly efficient piston engine. Fortunately, all things aren’t equal…rotary engines are lighter & smaller than piston engines so they don’t need to be as powerful in order to achieve the same performance. How much the size / weight advantage off sets the efficiency disadvantage depends on the situation but as a rule of thumb, cut the 15% in half & say “rotary engines burn 7.5% more gas than efficient piston engines”. Keep in mind that “efficient” piston engines typically have high compression ratios that require premium fuel. Premium fuel sells for about 12% more than regular fuel so assuming the new Renesis engine runs on regular (likely given that it’s normally aspirated) the RX-8’s fuel costs should actually be a selling point rather than a negative attribute.

Now lets move from the known to the speculative, how will the Renesis compare to the normally aspirated, 13B engines??? 2007RX-8 pointed out the reported 40% reduction in fuel consumption at idle. This improvement will benefit mileage through out the power range as idle can be though of as a constant. The new zero gap configuration will reduce the amount of energy that’s allowed to escape into the exhaust ports without being put to work. My back of the napkin, don’t quote me, wild *** guess is that the Renesis will achieve a BSFC of something like 0.48.

Within the limits of conventional engineering, rotary engines aren’t capable of being as efficient as piston engines for two reasons. One…The combustion chamber more poorly approximates a spherical shape which has a negative effect on the combustion flame form. Two…The surface area to volume ratio is greater which allows more energy to be lost to radiation.

Bottom line, I’m guessing the average driver under average conditions will get about 22.25 mpg in the RX-8.

Grimace

I just re-read the R&T article on the RX-8 from May. It says the Renesis rotary in the RX-8 is "more economical than the RX-7's 13B, getting up to 30 percent better fuel economy in certain driving situations."

I'm assuming they mean the FD RX-7.

rpm_pwr

Think of it this way:

Say the rx-8 uses 10% more fuel than a normal car. It's no different than living 10% further away from work. That's how I justify an FD daily driver anyway

DreamWarrior

Alright, I'm going to preform some major thread bumpage here...but this is something I'm very curious about still. Now that a lot of us board members have our cars, if you all could report in with what you're seeing and how you're driving I'd love it! I won't be able to get a near accurate representation during my test drive, but before I sign the papers I'd love to insure from you all that own it already that I'll be seeing reasonable gas milage.

Realize, I'm coming from getting 17 MPG in a Camaro Z28, so as long as it is similar to that I'll not care...I'm used to that expendature, and use high octane fuel already. If it is worse (like I've been seeing from some of our Japanese brethren - or maybe miscalculating) than that it is going to shun my purchase a bit. I mean I'm buying the car TO DRIVE and gas is the one expense that never goes away as long as you drive the car. So it's the one expense that I'm almost unwilling to compromise further on.

So, help me out guys, I'm nearing purchase time (hopefully Friday) and I'd love to have the same glorious reaction to the car in my parking spot that you all are getting!!!

Chuck Clifford

My first tank of gas in my new Winning Blue RX8, I got 16.1 miles to the gallon. That was with a lot of idleing and ogleing. I expect that that will be my worst case scenario.

RomanoM

Most car magazines that have bothered to measure, have achieved 19-20 MPG in mixed driving.

That sounds about right, you'll get less in pure city driving and better in pure highway.

I do remember that in one case of hard track use the RX returned 14 MPG.

Vaillant

On one particular track day, my 1.8L Miata got 10.5 mpg. Usually I'm around 11-15 mpg on track days...and around 25 in normal driving.

So, if the RX-8 is getting a little less than 20 mpg when being thrashed by automotive magazines, I'd guess it'd get in the low 20's in real driving conditions.

- Matt

RotorMotor

Did I miss somebody post this???

The window sticker says 18/24

DreamWarrior

EPA...if you actually drive the car you probably won't see that...and you may easily see below it. I wanted more "accurate" figures based on real driving maybe with a little aggression put into it :D.

Quick_lude

Haha.. You actually go by those figures? Estimates at best. Do some research on how EPA arrives at those figures.

RotorMotor

Haha, settle down there.... No, I would never go by those figures, especially not when we'll all be driving "spiritedly" :D . Just with all of the guessing and quick glance over the thread, it didn't look like anybody had posted the "official" numbers, so I did.