4
Testing the Car's Mileage -- and Our Patience
Measuring things like 0-60-mph times and panic stops is old hat for us. But discerning fuel mileage, that's another issue. At a cruise-controlled 60 mph, one of our cars, the Chevrolet Cruze Eco, returned 49.4 mpg. That's 0.0202 gallon per mile, or, if you were to picture that gasoline as a series of teaspoons, one burned every 4 seconds.
A Nissan GT-R can accelerate from 0 to 238 feet in 4 seconds; 238 feet I can measure. But teaspoons of gasoline? Being combusted deep within a densely packed engine? Measure this?
We've all historically computed our mileage just by filling the tank and dividing the miles driven by the gallons added. Nothing wrong with that (particularly if you correct for odometer error). But it's a slow-motion process, and when you're dealing with high-mileage cars (the Jetta TDI, for instance, has a whopping 600-mile highway driving range), it's frame-by-frame slow.
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There are far more efficient data-collecting tactics for this, the best being to directly install fuel-flow meters into the fuel lines. However, for myriad reasons -- including our inside-baseball realities that test cars like ours arrive shortly before we test them and the manufacturers don't look kindly on their cars' fuel systems being tampered with -- we've avoided meters (for this round-up). Yet, there remain other good crowbars in our mileage-measuring toolbox.
To produce power, an engine obviously consumes two things: air and fuel. And at constant speeds, their ratio is predictable: 14.7 to 1 -- known as the stoichiometric ratio. It's their perfect proportion for efficient combustion, and when a car's at a steady speed under moderate load, its oxygen sensors are beavering away to keep it so.
We monitored the cars mass airflow sensors (excepting the Elantra's, which employs a different strategy), and, as long as the car is in this steady mode, knowing the air passing through the engine points directly to its fuel consumption. This is what was displayed in a filtered, averaged way on our cars' instrument panels. As a check of this information, we simultaneously logged the cars' slow decline in fuel tank level, and, after calibrating the fuel tanks (incrementally measuring their level versus gallons added), their constant-speed fuel consumption data could be backed up.
Divergence
Our driver's mileage did vary. You hear all the time that you shouldn't expect your driving to duplicate the EPA's super-scientifically derived mileage numbers. Well, before we started our Tehachapi lapping, our drivers were firmly directed to observe the posted speed limits, which inadvertently put our own mileage reproducibility to the test. And the results were actually more divergent than I expected. Below, we list numbers -- and name names.
Oddities? Our differing driving habits had nearly 10 times the influence on the Elantra's mileage than on that of the Focus. Perhaps the Hyundai's more susceptible to enthusiastic outbursts? Mr. Martinez' idea of sticking to the speed limit was taken as more of a suggestion.
Driver Cruze Focus Civic Elantra Mazda3 Jetta
Evans 43.1 38.6 39.9 38.0 39.4 42.1
Kiino 40.9 37.7 39.9 39.1 42.0 43.0
Lassa 41.7 38.1 41.0 36.7 37.9 41.2
Martinez 35.8 37.7 37.8 31.3 34.1 39.8
Average 40.4 38.0 39.7 36.2 38.4 41.6
Std. Dev 2.7 0.3 1.2 3.1 2.9 1.2
Under Pressure
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Every time gas prices spike, the AAA and the EPA nag us to check our tire pressures. After our 40-mpg test-o-rama was complete, I did a little experiment with our long-term Civic Si (the performance-opposite to our test HF). At a cruise-controlled 60 mph, with the tires set to the door sticker numbers, the Civic returned 38.42 mpg. After stopping and immediately lowering them by 5 psi, the mileage (deduced from the mass airflow sensor) declined to 38.18 mpg, or 0.6 percent. Car engineers would kill their dear old moms for 0.6 percent. And all from a little squirt of compressed air.
AutoEnginuity
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To raise our data-gathering game we've reached out to Jay Horak and his OBD2 scan tool company, AutoEnginuity, which has become a leader in its field. Don't know what OBD2 is? Feel around under your dash, and you'll discover a hexagonal-shaped receptacle that technicians use to diagnose troubles and check your car's emission system. We're employing it to log speed, rpm, mass airflow, and fuel-tank level -- and, in the case of the Jetta, its diesel consumption in gallons per hour. Cool.
An Anomaly
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To measure the cars' speeds accurately, we've turned to our longtime GPS pals, Vbox USA (aka Racelogic). Their data loggers have become the industry standard for GPS-data recording, and they've brought along additional units to supplement those we use for our regular performance testing. A curiosity we've noted is that all the test cars (save the Jetta) are actually traveling slightly faster than their speedometers indicate. Our best explanation is their new (unworn) tires.
Measuring things like 0-60-mph times and panic stops is old hat for us. But discerning fuel mileage, that's another issue. At a cruise-controlled 60 mph, one of our cars, the Chevrolet Cruze Eco, returned 49.4 mpg. That's 0.0202 gallon per mile, or, if you were to picture that gasoline as a series of teaspoons, one burned every 4 seconds.
A Nissan GT-R can accelerate from 0 to 238 feet in 4 seconds; 238 feet I can measure. But teaspoons of gasoline? Being combusted deep within a densely packed engine? Measure this?
We've all historically computed our mileage just by filling the tank and dividing the miles driven by the gallons added. Nothing wrong with that (particularly if you correct for odometer error). But it's a slow-motion process, and when you're dealing with high-mileage cars (the Jetta TDI, for instance, has a whopping 600-mile highway driving range), it's frame-by-frame slow.
There are far more efficient data-collecting tactics for this, the best being to directly install fuel-flow meters into the fuel lines. However, for myriad reasons -- including our inside-baseball realities that test cars like ours arrive shortly before we test them and the manufacturers don't look kindly on their cars' fuel systems being tampered with -- we've avoided meters (for this round-up). Yet, there remain other good crowbars in our mileage-measuring toolbox.
To produce power, an engine obviously consumes two things: air and fuel. And at constant speeds, their ratio is predictable: 14.7 to 1 -- known as the stoichiometric ratio. It's their perfect proportion for efficient combustion, and when a car's at a steady speed under moderate load, its oxygen sensors are beavering away to keep it so.
We monitored the cars mass airflow sensors (excepting the Elantra's, which employs a different strategy), and, as long as the car is in this steady mode, knowing the air passing through the engine points directly to its fuel consumption. This is what was displayed in a filtered, averaged way on our cars' instrument panels. As a check of this information, we simultaneously logged the cars' slow decline in fuel tank level, and, after calibrating the fuel tanks (incrementally measuring their level versus gallons added), their constant-speed fuel consumption data could be backed up.
Divergence
Our driver's mileage did vary. You hear all the time that you shouldn't expect your driving to duplicate the EPA's super-scientifically derived mileage numbers. Well, before we started our Tehachapi lapping, our drivers were firmly directed to observe the posted speed limits, which inadvertently put our own mileage reproducibility to the test. And the results were actually more divergent than I expected. Below, we list numbers -- and name names.
Oddities? Our differing driving habits had nearly 10 times the influence on the Elantra's mileage than on that of the Focus. Perhaps the Hyundai's more susceptible to enthusiastic outbursts? Mr. Martinez' idea of sticking to the speed limit was taken as more of a suggestion.
Driver Cruze Focus Civic Elantra Mazda3 Jetta
Evans 43.1 38.6 39.9 38.0 39.4 42.1
Kiino 40.9 37.7 39.9 39.1 42.0 43.0
Lassa 41.7 38.1 41.0 36.7 37.9 41.2
Martinez 35.8 37.7 37.8 31.3 34.1 39.8
Average 40.4 38.0 39.7 36.2 38.4 41.6
Std. Dev 2.7 0.3 1.2 3.1 2.9 1.2
Under Pressure
Every time gas prices spike, the AAA and the EPA nag us to check our tire pressures. After our 40-mpg test-o-rama was complete, I did a little experiment with our long-term Civic Si (the performance-opposite to our test HF). At a cruise-controlled 60 mph, with the tires set to the door sticker numbers, the Civic returned 38.42 mpg. After stopping and immediately lowering them by 5 psi, the mileage (deduced from the mass airflow sensor) declined to 38.18 mpg, or 0.6 percent. Car engineers would kill their dear old moms for 0.6 percent. And all from a little squirt of compressed air.
AutoEnginuity
To raise our data-gathering game we've reached out to Jay Horak and his OBD2 scan tool company, AutoEnginuity, which has become a leader in its field. Don't know what OBD2 is? Feel around under your dash, and you'll discover a hexagonal-shaped receptacle that technicians use to diagnose troubles and check your car's emission system. We're employing it to log speed, rpm, mass airflow, and fuel-tank level -- and, in the case of the Jetta, its diesel consumption in gallons per hour. Cool.
An Anomaly
To measure the cars' speeds accurately, we've turned to our longtime GPS pals, Vbox USA (aka Racelogic). Their data loggers have become the industry standard for GPS-data recording, and they've brought along additional units to supplement those we use for our regular performance testing. A curiosity we've noted is that all the test cars (save the Jetta) are actually traveling slightly faster than their speedometers indicate. Our best explanation is their new (unworn) tires.
