Fuel economy and driving habits
08-03-2014, 07:13 PM
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Fuel economy and driving habits
As a disclaimer, I will say I was aware that the RX-8 is known for poor fuel economy. And I accepted this and bought one regardless. So let's just get that out of the way 
However, this car is my daily driver and there are plenty of times I can't push it and just need to get from A to B in a line of traffic. So I'm trying to figure out how to get the most of fuel economy during the dull times, while still enjoying stretching it when I can.
I read in the 'new and potential owners start here' thread this comment:
When it says 'happy' I'm assuming it means having power quickly available, not fuel economy. This would equate to driving around town at 50 km/hr in second gear?! I would have thought it would be most economic to be in the tallest gear practical when cruising? Given that rotarys are so fundamentally different, I can't really assume anything though.
Please enlighten me.
However, this car is my daily driver and there are plenty of times I can't push it and just need to get from A to B in a line of traffic. So I'm trying to figure out how to get the most of fuel economy during the dull times, while still enjoying stretching it when I can.
I read in the 'new and potential owners start here' thread this comment:
Now, all that being said, high RPM DOES generate a lot of heat. So don't just sit up in high RPM cruising there all the time, you are just cooking the engine and transmission. It loves going full throttle to redline, but then shift and let the engine cruise around 4-5k when you are just driving around. It's happy there.
Please enlighten me.
08-03-2014, 07:24 PM
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That quote is referring to highway cruise, not around town cruise.
Don't think about it as a single curve on a graph of speed vs RPM. It's several curves of several factors. The engine itself is actually most efficient at about 5,500rpm or so, and i mean this in terms of amount of fuel used over time, not over distance. Gearing is what turns that into distance. If you could drive in a complete vacuum with zero wind resistance, then 6th gear ~5,500rpm would be optimal cruising RPM. However, you can't drive in a vacuum, so aero drag starts making this suffer quite a bit, making the actual optimal point much lower. The lower speeds get, the less wind resistance you get, but also the more and more the engine will leave it's optimal efficiency range. 50kph (30mph), 6th gear is ~1,500rpm, and the engine is lugging badly, even if you are keep the engine stable there and not trying to accelerate. At a specific moment in time, 6th gear will probably return the best mileage at 50kph, if you can maintain a perfectly stable cruise without gaining or losing speed. If you have to accelerate/decelerate plenty, then 3rd or 4th gear might end up being more efficient
There is another factor too, doing this for extended periods of time. If you spend all your time driving at 50kph in 6th gear, you will so carbon choke your engine that your compression will start suffering badly, and further kill your efficiency. Same if you keep it under 4,000rpm all the time.
So basically, if you drive it to get the best mileage at those speeds, you can get something moderately acceptable for a time, at the cost of engine life and performance in the long run.
At those speeds, I always used 2nd/3rd, and ensured I was smooth on/off the throttle. Smooth matters more than RPM or gear at those speeds.
Don't think about it as a single curve on a graph of speed vs RPM. It's several curves of several factors. The engine itself is actually most efficient at about 5,500rpm or so, and i mean this in terms of amount of fuel used over time, not over distance. Gearing is what turns that into distance. If you could drive in a complete vacuum with zero wind resistance, then 6th gear ~5,500rpm would be optimal cruising RPM. However, you can't drive in a vacuum, so aero drag starts making this suffer quite a bit, making the actual optimal point much lower. The lower speeds get, the less wind resistance you get, but also the more and more the engine will leave it's optimal efficiency range. 50kph (30mph), 6th gear is ~1,500rpm, and the engine is lugging badly, even if you are keep the engine stable there and not trying to accelerate. At a specific moment in time, 6th gear will probably return the best mileage at 50kph, if you can maintain a perfectly stable cruise without gaining or losing speed. If you have to accelerate/decelerate plenty, then 3rd or 4th gear might end up being more efficient
There is another factor too, doing this for extended periods of time. If you spend all your time driving at 50kph in 6th gear, you will so carbon choke your engine that your compression will start suffering badly, and further kill your efficiency. Same if you keep it under 4,000rpm all the time.
So basically, if you drive it to get the best mileage at those speeds, you can get something moderately acceptable for a time, at the cost of engine life and performance in the long run.
At those speeds, I always used 2nd/3rd, and ensured I was smooth on/off the throttle. Smooth matters more than RPM or gear at those speeds.
08-03-2014, 07:38 PM
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That quote is referring to highway cruise, not around town cruise.
Don't think about it as a single curve on a graph of speed vs RPM. It's several curves of several factors. The engine itself is actually most efficient at about 5,500rpm or so, and i mean this in terms of amount of fuel used over time, not over distance. Gearing is what turns that into distance. If you could drive in a complete vacuum with zero wind resistance, then 6th gear ~5,500rpm would be optimal cruising RPM. However, you can't drive in a vacuum, so aero drag starts making this suffer quite a bit, making the actual optimal point much lower. The lower speeds get, the less wind resistance you get, but also the more and more the engine will leave it's optimal efficiency range. 50kph (30mph), 6th gear is ~1,500rpm, and the engine is lugging badly, even if you are keep the engine stable there and not trying to accelerate. At a specific moment in time, 6th gear will probably return the best mileage at 50kph, if you can maintain a perfectly stable cruise without gaining or losing speed. If you have to accelerate/decelerate plenty, then 3rd or 4th gear might end up being more efficient
There is another factor too, doing this for extended periods of time. If you spend all your time driving at 50kph in 6th gear, you will so carbon choke your engine that your compression will start suffering badly, and further kill your efficiency. Same if you keep it under 4,000rpm all the time.
So basically, if you drive it to get the best mileage at those speeds, you can get something moderately acceptable for a time, at the cost of engine life and performance in the long run.
At those speeds, I always used 2nd/3rd, and ensured I was smooth on/off the throttle. Smooth matters more than RPM or gear at those speeds.
Don't think about it as a single curve on a graph of speed vs RPM. It's several curves of several factors. The engine itself is actually most efficient at about 5,500rpm or so, and i mean this in terms of amount of fuel used over time, not over distance. Gearing is what turns that into distance. If you could drive in a complete vacuum with zero wind resistance, then 6th gear ~5,500rpm would be optimal cruising RPM. However, you can't drive in a vacuum, so aero drag starts making this suffer quite a bit, making the actual optimal point much lower. The lower speeds get, the less wind resistance you get, but also the more and more the engine will leave it's optimal efficiency range. 50kph (30mph), 6th gear is ~1,500rpm, and the engine is lugging badly, even if you are keep the engine stable there and not trying to accelerate. At a specific moment in time, 6th gear will probably return the best mileage at 50kph, if you can maintain a perfectly stable cruise without gaining or losing speed. If you have to accelerate/decelerate plenty, then 3rd or 4th gear might end up being more efficient
There is another factor too, doing this for extended periods of time. If you spend all your time driving at 50kph in 6th gear, you will so carbon choke your engine that your compression will start suffering badly, and further kill your efficiency. Same if you keep it under 4,000rpm all the time.
So basically, if you drive it to get the best mileage at those speeds, you can get something moderately acceptable for a time, at the cost of engine life and performance in the long run.
At those speeds, I always used 2nd/3rd, and ensured I was smooth on/off the throttle. Smooth matters more than RPM or gear at those speeds.
So when at 50-60kph you stay in third around town? I've been using fourth.
08-03-2014, 07:43 PM
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at those speeds, I usually was in 2nd more than 3rd, but maybe 50% 2nd, 40% 3rd, 10% 4th. But then I wasn't all that concerned about mileage, and could get 22mph + on the highway cruising.
The car is simply too enjoyable to drive with the revs up.
The car is simply too enjoyable to drive with the revs up.
08-03-2014, 08:25 PM
#5
Best bet, keep stuff clean, keep it tuned, and if you want, lower the front an inch. Makes a heck of difference in the aero dept.
I find 50 to 55 mph to be the most efficient speed. My daily drive has a lot of that.
And, get an ultra-gauge. It will tell you what is going on.
I find 50 to 55 mph to be the most efficient speed. My daily drive has a lot of that.
And, get an ultra-gauge. It will tell you what is going on.
08-03-2014, 08:31 PM
#6
Driving my unreliable rx8
50 k thats about 30mph right? Id probably drive that in third, anything lower and go to second. 4th would be okay but any gear higher will lug the engine. Once you start luging it you will loose economy quickley.
08-03-2014, 08:44 PM
#7
I will check this one with my u-gauge...
My 50 mph drive is at about 2,500. On a flat road, with ac off, the u-gauge says about 27 mpg. AC on, about 25.
If I am in 5th, I know it goes down a few mpg, but it sounds really nice. I find myself skipping the last shift, listening to the engine, with a stupid grin on my face.
My 50 mph drive is at about 2,500. On a flat road, with ac off, the u-gauge says about 27 mpg. AC on, about 25.
If I am in 5th, I know it goes down a few mpg, but it sounds really nice. I find myself skipping the last shift, listening to the engine, with a stupid grin on my face.
08-03-2014, 09:03 PM
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If someone is looking to maximize highway mileage...
- don't open the windows or moonroof, they each have a gas mileage penalty bigger than using the AC
- Remove the moonroof wind deflector if you have it
- install the OEM rear spoiler (believe it or not, it does lower the RX-8's coefficient of drag)
- Add a small splitter with a completely flat tray to the front axle, and make the rear of the underbody as smooth as possible from there back. Don't trap the exhaust heat though, mileage isn't worth burning down the car or blowing the transmission or diff from overheating.
- Add a proper rear diffuser of the right angle and with proper fencing
- lower the car
- get narrower and less grippy tires
- install Mazmart's water pump (less drivetrain drag)
etc...
Basically, there isn't much we can do for mileage when it comes to engine efficiency other than making sure all the hardware is healthy. To get beyond that baseline is all about drag reduction, absolutely minimize how much power is being wasted to wind resistance and rolling friction. Then it's all about driving smoothly and conserving kinetic energy as much as possible.
I hypermilled my 8 for a couple tanks at 27mpg (doesn't take long at 450 miles a week commuting) without any mods added, and I suspect that I could get 30mpg out of one if I started modding one for that purpose.
However it was boring as hell and not that healthy for the engine in the long run.
- don't open the windows or moonroof, they each have a gas mileage penalty bigger than using the AC
- Remove the moonroof wind deflector if you have it
- install the OEM rear spoiler (believe it or not, it does lower the RX-8's coefficient of drag)
- Add a small splitter with a completely flat tray to the front axle, and make the rear of the underbody as smooth as possible from there back. Don't trap the exhaust heat though, mileage isn't worth burning down the car or blowing the transmission or diff from overheating.
- Add a proper rear diffuser of the right angle and with proper fencing
- lower the car
- get narrower and less grippy tires
- install Mazmart's water pump (less drivetrain drag)
etc...
Basically, there isn't much we can do for mileage when it comes to engine efficiency other than making sure all the hardware is healthy. To get beyond that baseline is all about drag reduction, absolutely minimize how much power is being wasted to wind resistance and rolling friction. Then it's all about driving smoothly and conserving kinetic energy as much as possible.
I hypermilled my 8 for a couple tanks at 27mpg (doesn't take long at 450 miles a week commuting) without any mods added, and I suspect that I could get 30mpg out of one if I started modding one for that purpose.
However it was boring as hell and not that healthy for the engine in the long run.
08-04-2014, 08:42 AM
#12
Maybe you can find some benefit in hypermiling techniques. There's a whole community of people who basically try to get the most fuel economy out of their cars.. some go to extremes like radiator louvres and home-made start-stop solutions.
Summary here (skip the first dozen or so)
100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com
Applicable examples might be:
- Pulse and Glide: when you see that you have sufficient space in front of you, give a strong hit of throttle to get above your intended travel speed, and let the car coast in gear until it drops significantly below intended travel speed. The idea is your engine is most fuel efficient at high load, and the injectors are off entirely while you coast. Average travel speed remains the same as if you had driven normally, but in theory less fuel is used. Works well in a light-to-light scenario.
- draft behind heavy vehicles at speed
- find the lane of least resistance, the one that flows the smoothest (not fastest) with fewest interruptions for people turning, parking, etc.
- drive as if you have no brakes, don't build up speed you will need to scrub later with brakes and let the car coast to stops or slower sections. Any time you touch the brakes, you're scrubbing kinetic energy created by burning fuel. If you don't use the brakes, you don't need to burn that fuel.
That's the best I got man. Not sure how much effect these will have in our cars :/
In my experience, 16.6L/100 is way too much. I do 12-13 if I stay in the city. Depends how hilly it is though.
Summary here (skip the first dozen or so)
100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com
Applicable examples might be:
- Pulse and Glide: when you see that you have sufficient space in front of you, give a strong hit of throttle to get above your intended travel speed, and let the car coast in gear until it drops significantly below intended travel speed. The idea is your engine is most fuel efficient at high load, and the injectors are off entirely while you coast. Average travel speed remains the same as if you had driven normally, but in theory less fuel is used. Works well in a light-to-light scenario.
- draft behind heavy vehicles at speed
- find the lane of least resistance, the one that flows the smoothest (not fastest) with fewest interruptions for people turning, parking, etc.
- drive as if you have no brakes, don't build up speed you will need to scrub later with brakes and let the car coast to stops or slower sections. Any time you touch the brakes, you're scrubbing kinetic energy created by burning fuel. If you don't use the brakes, you don't need to burn that fuel.
That's the best I got man. Not sure how much effect these will have in our cars :/
In my experience, 16.6L/100 is way too much. I do 12-13 if I stay in the city. Depends how hilly it is though.
08-04-2014, 08:47 AM
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^ They do. Like I mentioned in the prior post, I hypermilled my 8 up to 27mpg over a couple of tanks with just driving technique, though again, it's really boring and the minimalistic usage of rpm/throttle is detrimental to long term Renesis engine health.
Frankly, the conclusion was: "If I wanted better gas mileage, I'd probably get the biggest impact by not deliberately igniting ***** of gasoline inside the tailpipe for fun." But it's too much fun, so I ignored mileage outside of just knowing what it was on long trips.
Frankly, the conclusion was: "If I wanted better gas mileage, I'd probably get the biggest impact by not deliberately igniting ***** of gasoline inside the tailpipe for fun." But it's too much fun, so I ignored mileage outside of just knowing what it was on long trips.
08-04-2014, 11:16 AM
#14
I checked..
50 in 6th, 27
50 in 5th, 24
50 in 4th, 21
50 in 3rd, 19
50 in 2nd, 12.
It seems the lower the MPG, the better the engine sounds.
Best I can thing of is make sure everythign is happy. That will keep all the fuel getting burned with none out the tail pipe. Outside the ECU tune, lowering the front an inch was the biggest single mod for MPG. I did it for looks, but it instantly raised MPG 5% or so.
I am looking for the european spoiler, not the bread basket, to see how that works. Cannot find one. Supposedly it was standard on those models simply for the MPG.
50 in 6th, 27
50 in 5th, 24
50 in 4th, 21
50 in 3rd, 19
50 in 2nd, 12.
It seems the lower the MPG, the better the engine sounds.
Best I can thing of is make sure everythign is happy. That will keep all the fuel getting burned with none out the tail pipe. Outside the ECU tune, lowering the front an inch was the biggest single mod for MPG. I did it for looks, but it instantly raised MPG 5% or so.
I am looking for the european spoiler, not the bread basket, to see how that works. Cannot find one. Supposedly it was standard on those models simply for the MPG.
08-05-2014, 07:24 AM
#16
More testing,
65 mph. AC off, windows up, sunroof closed. about 21.5.
65 mph. AC on, down around 19.5 to 20, when compressor cycles on. back up when cycles off. Cruse control seems to not have the hang of adjusting for the compressor smoothly, the mpg swings a lot. my light flywheel is not helping here.
65 mph. Windows down, sun roof open. 21 average, the extra drag seems to cause similar oscillations to the AC compressor cycling.
65 mph, drivers window half down, no difference than windows up. This is my normal driving mode until it gets hot.
65 mph. AC off, windows up, sunroof closed. about 21.5.
65 mph. AC on, down around 19.5 to 20, when compressor cycles on. back up when cycles off. Cruse control seems to not have the hang of adjusting for the compressor smoothly, the mpg swings a lot. my light flywheel is not helping here.
65 mph. Windows down, sun roof open. 21 average, the extra drag seems to cause similar oscillations to the AC compressor cycling.
65 mph, drivers window half down, no difference than windows up. This is my normal driving mode until it gets hot.
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