2014 BMW i3 REx vs Chevrolet Volt comparison [photos: David Noland, Tom Moloughney]Enlarge Photo
Either way you measure, it's an official EPA 20 percent efficiency advantage for the BMW--not surprising considering it weighs 900 pounds less than the Volt.
To add to the unit-confusion, both the Volt and i3 read out their efficiency in yet a third unit: miles/kWh, the electric analog to mpg. Since that's what our cars were telling us, that's the unit we used in our test.
We began our side-by-side efficiency run from the parking lot of Tom's restaurant, Nauna Bella Casa, in Montclair, New Jersey. After 13.1 miles of stop-and-go suburban driving (average speed 22 mph), the Volt, leading the way with Tom at the wheel, had used 3.1 kWh of juice, which worked out to an average of 4.5 miles/kWh.
I followed in the i3, which at the end of the city/suburban run displayed 5.0 miles/kWh. (Unlike the Volt, the i3 obligingly reads out the final average.)
2014 Chevrolet Volt 5dr HB Instrument ClusterEnlarge Photo
Advantage: i3--but by a smaller margin (11 percent) than I had expected. Especially considering that this had been a stop-and-go city route, where the i3's weight advantage is more telling.Would the sleek Volt close the gap on the boxy i3 on the highway, where air resistance rules?
Surprisingly, no. The Volt managed 3.5 miles per kWh on our 15.2 mile run along Interstate 80 and the Garden State Parkway, compared to 4.0 mi/kWh for the i3. The i3's advantage actually increased to 14 percent on the highway.
Overall numbers for both city/suburban and highway legs: 3.9 mi/kWh for the Volt, 4.5 for the i3.
Bottom line: in our little test, the range-extended BMW was 10-15 percent more efficient than the Volt.
And, yes, efficiency is a virtue in its own right. But how does it pay off on the bottom line in this case? In terms of dollars and cents, the i3's efficiency advantage is almost trivial.
Based on our own testing numbers, the i3's efficiency advantage amounts to about one-third of a penny per mile.
Savings over a year and 12,000 miles of driving: a cool $40.
2014 BMW i3 REx owned by Tom MoloughneyEnlarge Photo
With their instant electric torque, both cars are spritely performers. But the i3 has a clear advantage in acceleration.
We didn't do timed 0-60 tests, but the factory numbers reflect what the driver feels when he stomps the "gas" pedal: Zero-to sixty in 9.0 seconds for the Volt, 7.7 seconds for the Bimmer. Pedal to the metal, the i3 is the clear winner.
But in normal around-town driving, at less than full throttle, the Chevy feels just as peppy as the i3--as long as the Volt is in Sport mode. Its more aggressive throttle mapping makes the Volt jump off the line from stoplights. I always, always drive my Volt in Sport mode.
In terms of top speed, the Volt gets the nod. It's electronically limited to 100 mph, while the i3 REx cuts off at 93 mph.
I've never driven my Volt faster than about 85 mph, but it seems to have decent reserve power at that speed. Likewise, Tom reports that his i3 REx has "plenty of punch above 85 mph."
The Volt suffers essentially no performance penalty in REx mode. The 84-hp gas engine is powerful enough to maintain full performance under virtually all conditions.
The one exception is very long, steep, fast uphill stretches of road--say, Interstate 80 from Denver to Silverthorne, Colorado.
For this situation, the Volt offers a solution: Mountain mode, which kicks on the range extender when there's still about 40 percent of battery charge remaining. The battery is thus able to help out the range extender to maintain full power up long, fast grades.
The i3's tiny two-cylinder 35-hp REx engine, which kicks in when battery charge falls to 6 percent, cannot maintain full performance under all conditions.
In low-demand driving (moderate speeds, short and not-too-steep hills), the REx provides the battery with all the power it needs to maintain the 6-percent level, or even charge up the battery a bit.