We reviewed the BMW M3 Coupe with a manual transmission and the full load of cabin tech back in April, but we couldn't pass up the opportunity to test out another one, this time the 2008 BMW M3 Convertible with the new double-clutch transmission. Although our convertible M3 lacked the cabin tech options, not even having the programmable M button on the steering wheel, it still proved a remarkable car to drive with all the performance capabilities of the loaded version. And we didn't think it possible to improve on the M3's performance, but the double-clutch transmission makes the car even more exciting to drive.
Test the tech: DCT versus MPG
When we took a brief drive in a DCT-equipped M3 earlier this year, we noted that the trip computer was reporting more than 17 mpg fuel economy. While that number doesn't sound impressive, it is a notable improvement over the 15 mpg we saw from the manual transmission BMW M3 Coupe we tested earlier. Speculating that the increased fuel economy was because of the double-clutch transmission, we paid special attention to this number for our full test of the BMW M3 Convertible.
The DCT is a manual transmission with two gear shafts, each with its own computer-controlled clutch. When driving, one clutch will engage a gear on its shaft, while the other clutch sits ready to engage the next up or down gear, with the computer determining the most likely next gear you will want. In manual mode, you can choose to shift up or down using steering wheel paddles or the shifter. Because the computer controls the clutches, there is no clutch pedal. The car also has an automatic mode, where the computer determines when to shift up or down. The BMW M3 Coupe we tested had a standard six-speed manual. The double-clutch transmission has seven gears.
However, the DCT isn't the only difference between these two cars. The BMW M3 Coupe gets a carbon fiber roof, while the BMW M3 Convertible uses a retractable hardtop, which means an extra 441 pounds of curb weight for the convertible. That weight difference should give the M3 Coupe an advantage in fuel economy.
According to the EPA, the BMW M3 Convertible with the double-clutch transmission should get 16 mpg city and 20 mpg highway. The M3 Coupe with the manual transmission is rated at 14 mpg city and 20 mpg highway. In our testing in dense urban traffic, twisty mountain roads, and 65 mph freeways, we saw an average of 16.2 mpg for the M3 Convertible with the double-clutch transmission. In our earlier test, the BMW M3 Coupe with the manual transmission got 15 mpg. So the DCT clearly offers an advantage even when laden with an extra 441 pounds. For a bit more comparison, we saw 16.2 mpg with the seven-speed automatic-equipped Mercedes-Benz C63 AMG, and 14 mpg with the dual clutch manual-equipped Nissan GT-R.
Why does the double-clutch transmission do better in fuel economy than a manual transmission? When we had the M3 Convertible in boring driving conditions, such as in traffic or in the city, we let the computer handle the shifting. We noticed that the program tended to upshift earlier than we would have, for example running all the way up to seventh gear at 45 mph, keeping the tachometer about 1,500rpm. In the manual M3 Coupe, we would most likely have left it in third gear at 45 mph.
In the cabin
Our 2008 BMW M3 Convertible lacked a lot of the cabin tech we're used to seeing from BMW. Without the navigation option, there is no LCD on the instrument panel. Instead, you get a standard stack with the stereo and climate controls. Take a look at our review of the BMW M3 Coupe to see what a fully loaded cabin looks like. Standard features include power adjustable leather seats.
The base stereo is fairly limited for audio sources, having AM and FM radio, an auxiliary jack in the console, and a single-disc slot that can read MP3 CDs. But even though the radio display only has two lines, BMW manages a pretty good interface for MP3 CDs. You can scroll through lists of folders and songs on the disc, and see songs by their ID3 tags.
As in other BMWs we've tested, we noted that the audio system in the M3 Convertible is powerful and delivers good quality sound, but it is far from the best we've heard. Its highs lack clarity, although the midrange comes through with strength. Its bass isn't thumping, but it is solid. This base audio system uses eight speakers. You can get a premium audio system for the M3 Convertible that uses 12 speakers and an 825-watt amp.