Seeing two other near-lookalikes of my car in your "reader rides" prompts me to offer a little engineer-geek experimental data I was able to conduct with my 2010 Mazda3s GT 5dr 6MT (not a Mazdaspeed though).
I ran careful checks of fuel economy with the trip computer, averaging each over the same 15-mile loop of Washington State I-90 with very light (essentially zero interference) traffic. I used cruise control on each loop, starting and ending at a standstill in the same location for zero altitude change and near-zero wind effect. The weather was dry and cool on all of the 2-3 days of the experiment.
My car now has 36,000 miles on it but this experiment was done between 10,000-20,000 miles.
I have taken the car on two long road trips to San Diego and Medora North Dakota, both from Seattle, and from milepost timing and speedo and odo-reading I have verified that true speed is actually slightly faster than the speedometer indicates and the odometer under reads the true miles covered. This is accounted for in the MPG plot pictured here:
Despite all we've heard about speed and economy, I was surprised how clear cut the effect of low speed and low revs are on gas mileage. It is plain as day. And it's obvious that a taller 6th gear could give better mileage. The only saving grace for a rational speed-demon is the hourly rate/cost of driving fast is pretty cheap when thought of as the cost for your time. If you slow down to save gas and money, you're paying yourself no more than about the minimum wage.
The above photo is on the top of Chief Joseph Pass Montana on a 19-degree F. dawn in September 2011. -- Mark
If you want to participate in the Readers Rides blog, click here to read the submission guidelines and email your text and photos to submissions@edmunds.com -- Donna
cynic783 says:
04:54 AM, 02/ 2/12
Geeky awesomeness! +1
powell_jr says:
05:16 AM, 02/ 2/12
Great info. Just goes to prove real world and EPA don't line up. Great engine, the 2.5. My wife has a CX-7 (na) and I never feel it lacks power. Mazda has had great success with the MZR, it is smooth and bullet proof. The CX-7 is my second car with some sort of MZR (my first was an 07 3 with the 2.3). Mazda makes a quality product.
deagle13 says:
10:06 AM, 02/ 2/12
Great car - I had the same car for a couple of years (just traded it in for a Subaru Legacy GT). Mine was a black GT 5 door with no other options (not even a sunroof)...
emajor says:
04:43 PM, 02/ 2/12
Sweet, actual data instead of anecdotes. Well done, Mark. Now I want to go do the same thing with our cars. I am surprised at how steep the decline is with velocity. This explains why our Yaris recently only pulled 31mpg while doing 80mph into a stiff headwind.
But 30 mpg at a typical 70mph? That's low for a compact, and very close to what I get in my VW 2.5. I can see why SkyActiv is now on the scene. Hey, VW, you listening? Not everyone wants to ante up another $5000 for a diesel to get good mileage.
roadburner says:
09:00 AM, 02/ 6/12
I get @30 mpg at 70 in my 2007 MS3. It's not exactly a rocket ship, so I sure wouldn't want to give up any bhp in exchange for lower fuel consumption.
mmmodem says:
03:10 PM, 02/ 6/12
Hate to burst your bubble but there is a fatal flaw in your experiement. Your mpg relies completely on the trip meter. All you've proved is that Mazda programmed the trip meter to adjust the mpg linearly proportional to the speed you are traveling. A true test would be to measure the amount of gas you put into the tank. Of which a 15 mile test loop would not be sufficient to overcome errors in the measurement.
roadburner says:
07:05 PM, 02/ 6/12
In my experience the OBC mileage is @3% higher than the number obtained by dividing the miles traveled by the gallons used.
blackadder5639 says:
11:52 AM, 02/ 7/12
The drop in fuel economy from 70 to 75 mph in 6th is shocking! 2.5 mpg drop for a mere 5 mph increase at around the speed limit?
Is the new (since 2006 or so) EPA hwy rating not supposed to reflect economy at 80 mph with A/C on? This car can't even make its 28 hwy figure at 75 mph! Interesting.....
whobodym says:
07:30 PM, 02/21/12
the author here, replying to mmmodem. I guess I can't absolutely prove it, but my impression is that my MZ3 GT's trip computer is calculating MPG based on the fuel flow signal from the engine control computer. When set to "current MPG" (instantaneous, a new value every 2 seconds) it certainly bounces all over the place, responding to even a 1/32" change in depression of the gas pedal. So I assume that "average MPG" is using the same signal but doing the calculation for a longer period of time -- as long as you want, until you reset it. As to its accuracy, I've corrected for that -- not much explained in my first post -- by checking the "average MPG" when left alone not reset, on a trip of 3,000 miles total length. I've also checked the odometer against the mileposts, and I'm correcting for that too. So I'm making the assumption that I can correct for what the trip computer says for any length of trip long or short, into true miles driven divided by true gallons purchased. On the 3,000 mi trip, the computer said 29.8 mpg and my true miles/gallon for the total trip was 31 if I recall. (and when you say "trip meter" I assume you don't mean the trip odometer -- that had no role in my experiment at all)