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5x Racing Mazda RX-8 Project MPG

Project Details:

Project Completed May 2010!

Overview and Goals:
When the time came to replace Brittany's daily driver 2000 Mustang GT, as Mazda racers we were naturally drawn to the Mazda RX-8 for it's looks, handling, usefulness, and super cool rotary engine. While we knew the rotary powered RX-8 did not get Mazda 3 gas mileage, we were still surprised to find out that the rotary got around the same miles out of a tank as a the V-8 Mustang GT it replaced. This is quite acceptable being a sportscar, as you sacrifice fuel mileage for power, ride quality for superior handling, creature comforts for weight savings, etc... In other words, we knew what we were getting into when we bought the car.

After the first tank of gas came and went, and the next, and the next, with about 275 miles squeezed out each tank (20.5 mpg), it left me thinking that there had to be some way to get better gas mileage and at the same time not sacrificing power, but GAIN power. There are several ways to save mpg's, but some will reduce performance, such as reducing gear ratios and unspirited driving. We want to preserve the spirit of the Mazda RX-8, and at the same time see if we can increase power output while getting better gas mileage.

What we will be doing with Project MPG is performing simple modifications at every oil change for one year and logging the gas mileage to see if what we did had any effect on mpg's. The modifications will be simple at first, such as: using different engine oils, installing a Green High performance air filter, using different spark plugs, upgrading the spark plug wires, replacing the transmission and rear end oil with lighter weight equivalents, etc...

Follow along with our project and hopefully it will help you get better gas mileage out of your RX-8, or any car for that matter. I hate to think that many RX-8's get traded in shortly after being purchased because people expect the car to get 30 mpg, perhaps because it is "Japanese", "has a 1.3L engine", or is a "lightweight sportscar" and forget the fact that it has a rotary engine that consumes more fuel than a piston engine. We will be adding information to this page continuously, so check back ofter for updates and planned modifications.

Initial Data

Subject: 2007 Mazda RX-8 GT 1.3L Rotary/Manual Transmission/Factory Fuel Economy: 16 mpg city - 22 mpg hwy

  • Calculating Data
    To begin, we calculate our gas mileage by resetting our Trip Odometer at every complete fuel tank fillup. After driving until our low fuel light comes on, we then do another complete fuel fillup and calculate the mileage of the tank by dividing our total tank mileage from the Trip Odometer by the amount of gallons used in the fillup. For example: 300 miles to a tank on the trip odometer, 14 gallons used in the fillup would be: 300/14= 21.42 mpg

  • With the addition of a ScanGaugeII Ultra Compact 3-In-1 Automotive Computer from Project Sponsor Linear Logic (manufacturer of the ScanGauge II), we were able to monitor our fuel milage in real time as we drove. In addition to being able to monitor our fuel mileage in real time, the ScanGauge II also provides a ton of useful information about what your car is doing, as it reads info directly from the vehicles ECU. So, now that we have a ScanGauge II we can accurately monitor and double check our fuel mileage efficiency to back up our data and provide us with some insight on what is helping, or hindering our fuel mileage.

    Our current findings:

    The type of driving we do is a good estimate of average driving conditions. The car is daily driven by Brittany 4 out of the 5 day work week, and driven by John (who is a racecar driver) at least 2 or 3 days out of the week. Brittany drives conservatively, John drives more aggressive, so a good performance balance is achieved by sharing driving duties. The car is shifted normally during long highway stints, and more aggressively during stop and go city driving.

    During our first 3000 miles of the project, we saw:
  • Average MPG: 20.48
  • Highest MPG from one tank: 21.75
  • Lowest MPG from one tank: 19.9
  • Mods Performed: Castrol GTX 5w20 oil

  • ScanGaugeII Installed
  • Average MPG: 21.05
  • Highest MPG from one tank: 21.9
  • Lowest MPG from one tank: 20.5
  • Mods Performed: ScanGaugeII Fuel Mileage Gauge
  • Previous Mods: Castrol GTX 5x20 oil
  • Gains/Benefits: Gained 0.57 MPG
  • Thoughts: While it did not yield huge MPG gains, the ScanGaugeII really shows you what your car is doing as far as MPG consumption goes. It really does make you easier on the throttle as you watch your MPG go all the way down to 6-7 mpg when the throttle is pushed even the slightest amount. I can see the ScanGaugeII working well in the long run as it trains you how to drive, there are a lot of habits that hinder your fuel economy. I did try the "in gear coasting" method discussed on rx8 forums as a method to save fuel and I can say that it does not work. Coasting in neutral saves more gas than keeping engine rpm's high while coasting in gear. After getting the ScanGaugeII dialed in, we found that you must disable the open loop/closed loop function to get accurate readings on the RX-8. Any speed over 77 mph or coasting puts the injectors in open loop mode and the ScanGauge does not measure mpg's accurately in open loop operation. Overall the ScanGaugeII is sweet, and I would recommend it to anyone that is looking for increased fuel economy or an all in one tool to keep an eye on your engine vitals and to use as a scan tool.

Modifications

Modification: The strict useage of Castrol GTX 5w20 non-synthetic oil

  • Result: None. We contribute this to the fact that there is no parasitic drag from oil in a rotary engine like there is in a piston engine where the crankshaft dips into the oil.

Modification: Addition of a ScanGauge II 3-in-1 Automotive Computer (special thanks to project sponsor Linear Logics) added to monitor fuel efficiency during the project duration    

  • Result: Marginal mpg increases of about a mile or two a gallon due to the fact that you are more aware of what your driving will do to your fuel economy. If you drive conservatively you will of course notice that you use less gas. This tool is great for anyone who wants to know what is going on in their cars system. It is like real time diagnostics and can help you find problems before they start.

Modification: Addition of a Green High Performance Air Filter

  • Result: None to marginal. This mod did give the car more useable torque throughout the rpm range and increase air flow to the engine because of the non-restrictive filter element.

Modification: Installed Magnecor KV-85 Racing Spark Plug Wires and NGK Laser Iridium Spark Plugs

  • Result: Increased fuel mileage by about 1 mpg. This is most likely due to allowing the ignition system to perform to it's maximum potential and produce the maximum amount of spark. The engine felt more powerful and felt like it had more useable torque as well.

Modification: The use of ethanol free gasoline for several tanks in comparison to normal E10 fuel

  • Result: Increased fuel mileage by about 1 mpg. The RX-8 reacts poorly to fuel with a high ethanol content, such as E10 with the full 10% added. The car just ran better as well, there's nothing like good old fashioned 93 octane!

 

Findings

  1. By using all the features on the ScanGauge, we did find that the ECU will go into open loop mode around 68 mph in 6th gear, and most likely above 4000 RPM's. Once the ECU is in open loop mode, it runs a default "performance" program that eliminates the oxygen sensor feedback and consequently richens the mixture, which is bad for gas mileage.
  2. Greater loads on the engine will reduce mileage ten fold. With the use of the ScanGauge, we were able to see that if the car was in too high of a gear, operating at too low of an RPM, it would load the engine more so than if you were operating at a higher RPM. So higher RPM's doesn't always mean worse gas mileage!
  3. The RX-8's rotary will be most efficient operating in 6th gear around 50-55 MPH, or between 2000-3000 RPM's. Driving consistently in this range should yield fuel mileage in the 25 mpg range!
  4. Air filters do not increase mileage as much as they make your engine breath better. The increases in mileage on a car that already has poor mpg ratings will be null to marginal. If a Green Air Filter is used on a car that gets good mileage to begin with it would probably result in a greater increase in mileage.
  5. Properly inflated tires doesn't do anything for fuel economy, but of course it is always necessary to have your tires properly inflated.
  6. E10 ethanol gas KILLS your fuel economy in an RX-8! There is no worse mpg killer than E10 with the full 10% of ethanol added, stay away from it if at all possible. You can smell when the fuel has a high concentration of ethanol, it doesn't smell like gas. If you can find a station that offers ethanol free fuel, use it and you will see more consistent mpg ratings. After all the stations went to the "10% or less ethanol added" fuel, our gas mileage was all over the place. Sometimes it would be the same (21 mpg average) and sometimes it would be really bad (19 mpg average). We believe that we lost about 30 miles or so to a tank when using E10 fuel, even our 2001 Mustang GT lost about 30 miles to a tank when ethanol was used, so it is universal that this stuff kills fuel mileage. Don't believe me? Read this article 
  7. Having your vehicle tuned up with new spark plugs and maybe even a set of Magnecor KV-85 spark plug wires will ensure you ignition system is putting out the most spark that it can, which is essential to efficiency. While we didn't see a noticable increase in mpg, we knew the car was running smoother and had more torque, when used conservatively will yield higher mpg's. It's hard not to rev this car, especially when you have added parts that help with the down low torque.
  8. Keeping the engine in between 2000-3000 RPM's will be the most efficient range for mpg as the engine is not being loaded excessively. If you keep the car at these RPM's for the whole tank, we would estimate 25 mpg is possible.
  9. The mpg's averaged out to be about 20-21 mpg for the length of our test, which is not too bad when compared to the other vehicles on the road. For instance, a Mazda Miata is only rated at 29 mpg highway and a Mustang GT is rated at 25 mpg highway. Throw the RX-8 in there with an average of 21, and highway rating of about 23 and you have a completely average fuel mileage rating!

Advice

  1. Get a ScanGauge II! They are wonderful tools to have in your car, they will tell you all about your vehicles systems and what they are doing as well as monitor your fuel mileage. The SGII was the single greatest thing that helped us acheive better mileage.
  2. Use ethanol free fuel if possible, or find a station that does not have the full 10% added. You can tell the difference when you smell the fuel while pumping. Sometimes no-name gas stations will have non-ethanol fuel or fuel without the full 10% of ethanol added.
  3. Change your spark plugs in between 30,000 - 40,000 miles to keep your ignition system efficient. We also recommend upgrading to a set of Magnecor KV-85 spark plug wires.
  4. Don't mash the throttle everywhere and you will get better mileage (duh).
  5. If you drive above 3000 RPM's all the time your ECU will go into open loop mode, which is a default "performance" tune that richens the mixture. This means worse mpg. 
  6. Buy the car regardless of the fuel mileage, it is worth it and the actual average mpg are really not that bad. 21 mpg is not a bad average for a sportscar.

Conclusion

Our goal was to monitor and evaluate the fuel mileage performance of our RX-8 and determine if there was anything that could help us acheive better fuel mileage. After a year of monitoring and logging fuel mileage, and adding certain subtle modifications, we concluded that the rotary engine is just not meant to be as efficient as a small piston engine. The engine is meant to be a fun, high revving, small liter powerplant meant strictly for purists. The racecar drivers, tuners, corner carvers, and enthusiasts are who this car is made for, not the soccer moms, gas misers, or "a to b" type driver. Although it is a shame it doesn't get better fuel mileage, it doesn't matter because the car is just that much fun! Hope you found this educational, and I hope it didn't sway you from purchasing an RX-8. The car will average out to around 20-21 mpg, which is better than any truck out there and about the same as a Mustang GT. Not too bad when you look at it in that light!

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