Bone stock 2018 Mazda CX9 Grand Touring front wheel drive (the Grand Touring has 20" wheels vs 18" on the base) loaded with 500 lbs of people, pets & gear:
EPA rated 22/28 (28 mpg happens just below 70 mph actual suggesting an honest EPA rating)
I was surprised that the rate of mpg decline tapered off at higher speeds rather than plummeting downward given the exponential force of wind resistance. I made a spreadsheet & dug up the BSFC map for Mazda's 2.5L turbo and discovered that at 55-60mph the engine is operating well under that peak efficiency island but it moves closer as you approach autobahn speeds:
- RPM is the calculated rpm to reach that speed in top gear (0.6) through the final drive (4.411) and the tires (30.0")
- TE% is thermodynamic efficiency calculated from BSFC. The difference between 38% and 30.7% thermodynamic efficiency might not sound like much, but that 7.3% change works out to 19% higher fuel consumption.
- BSFC is brake specific fuel consumption - the amount of fuel burned for a given amount of work. The lower the number, the less fuel required. Those numbers are estimated from the BSFC chart...
- BMEP is brake mean effective pressure. It is a measure of the capacity to do work and was calculated from torque.
- Torque was calculated from horsepower & rpm
- Horsepower is the total power required to maintain steady speed
- Dt & Acc HP is the estimated horsepower lost to the drivetrain & accessories. It is estimated at 20% based on a stock 250hp CX9 dynoing 200 hp at the wheels.
- Aero HP is the horsepower required to overcome wind resistance at that speed
- RR HP is the horsepower required to overcome rolling resistance at that speed
Even with California gasoline at $5.50/gal, the price of fuel is still too low to encourage people to slow down:
With California minimum wage at $15/hour, even a burger flipper's time is more valuable than the gas savings driving 55...
Calibration notes:
- I set the cruise at indicated 55, 60, 65, 70, 75, & 80 mph, but based on timing mile markers at 60mph, the speedometer reads 1.5% high (so I adjusted all the speeds down 1.5% assuming the error is linear).
- The odometer was dead on (trip meter read 20.0 as I passed the 20th mile marker - it flipped a bit early so it was probably 20.03-20.05).
- Any error in the fuel consumption display is less than the 1%. I calculated 0.997, but that implies far more precision than available from the instruments at my disposal (gas station pumps are allowed to be off by 0.5% here)
- I took the average of 2 directions (4-7 mph headwind one way and 4-7mph tailwind the other). Road is flat (gains 50 feet over 100 miles)
- Air Conditioning off, windows up (fan on high venting fresh air)
- 65-80* F external temperatures
- No intentional drafting, but light freeway traffic provided some aerodynamic drafts/wakes
Also of note, I was unable to discern a difference in mpg filling up with E10 87 octane vs ethanol-free 91 octane from Costco. E10 has 3.3% less energy content and Mazda's computer on this 2.5L turbo will automatically increase power from 227 horsepower on regular to 250 horsepower on premium so it should be able to maximize timing. But any difference in mpg was overshadowed by noise from various random environmental factors.
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