The bad: Fuel economy is not great, although the double-clutch transmission helps a bit, and the car commands a hefty price. When the top is lowered, trunk space is minimal.

The bottom line: The 2008 BMW M3 Convertible delivers an exceptional driving experience. The M3's double-clutch transmission and cabin tech are first rate, but options quickly jack up the base price.

CNET editors' review

• Reviewed by: Wayne Cunningham
• Reviewed on: 09/11/2008

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 double-clutch transmission uses a different shifter than the base six-speed manual transmission.

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 little 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.

Driving along at 45 mph, the double-clutch transmission's automatic mode puts the car into seventh gear, which keeps the tach to less than 2,000rpm.

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 stereo interface, although restricted to a two-line display, lets you scroll through folders on an MP3 CD.

The base stereo is pretty 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 tag.

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.

We had Bluetooth cell phone integration in our car, which comes with the BMW Assist telematics feature. Even restricted to a two-line radio display, BMW does an excellent job with cell phone integration. We paired up our phone, and very quickly could access our phone's contact list using the stereo controls.

The Bluetooth cell phone system is impressive, even without a large LCD.

The retractable hardtop is also a nice feature, despite the extra weight. Weekend track drivers will want to opt for the carbon-fiber roofed Coupe to reduce weight, but the M3 Convertible works as a more multipurpose car, allowing open-top driving in good weather. We found the top was a little slow in moving up and down, and it seriously compromises trunk room, but having it down lets you hear the engine much better than you can with it up. The sound of the engine changing speeds with gearshifts was particularly enjoyable.

Under the hood
With the BMW M3 Coupe, we reveled in its power and handling, sticking to third gear, with its wide power band, as we slung the car around corners, feeling the rear come out just enough so that the front was pointed in the right direction. Although a convertible top compromises performance, BMW engineers kept it to a minimum in the M3 Convertible. We did most of our hard driving with the top up, to keep the weight balanced properly, and found little difference between the two cars on public roads.

BMW designs its engines for individual throttle control on each cylinder, allowing for minute performance adjustments.

We took the car on the same route we drove the M3 Coupe, a series of long, winding roads north of San Francisco, then back down along the coast on Highway 1. The double-clutch transmission definitely changed the driving experience--we kept it in manual mode and spent more time shifting between second gear and third gear than we did with the manual transmission M3 Coupe, which was content to run in third gear for all but the tightest turns. Shifting with the DCT is effortless, merely requiring a pull on the left paddle for downshift and the right for an upshift.

As an M car, the M3 Convertible has a power button on the console. The car moves forward sluggishly with it off, requiring you to practically floor the gas pedal. Given this behavior, we're not sure if leaving it off improves fuel economy. Leave it on, though, and the throttle response sharpens considerably, giving the feeling that you've suddenly gained 50 horsepower. The four-liter V-8 engine is a remarkable piece of engineering, using BMW's Double-VANOS for valve control, with individual throttle control for each cylinder. The result is 414 horsepower at 8,300rpm and 295 foot-pounds of torque at 3,900rpm.

With the Power and EDC lights all on, the car is ready for the track.

Our car also had the optional electronic damping control, which gives the suspension three modes, comfort, normal, and sport. EDC should be a standard feature on the M3, as it seems like a crucial bit of tech to improve the car's handling. Comfort loosens the suspension up a little, giving a slightly softer ride. We relied on normal mode for much of our driving, as it automatically adjusts the suspension damping to your current driving style. We could feel the difference in handling when we went from diving into corners to cruising down the highway. One option our car didn't come with was the programmable M button, which lets you set the car for sport driving with a single button touch.

BMW incorporates even more programmable technology into the double-clutch transmission with its DriveLogic settings. A rocker switch on the console lets you change the gearshift response through five settings, shown as one to five bars on the instrument cluster display. You can have different settings for automatic and manual gearshifts mode, and the car will save them, reverting to the proper setting when you put the car in manual mode, for example. At one bar, the shifts feel softer, as the car kicks in a limited amount of torque during the gearshift. With DriveLogic maximized at five bars, the shifts feel harder, as the engine adds the maximum amount of torque the program allows during the gearshift. The difference from one bar to five is a soft push versus a hard punch in the back during gearshifts. We kept it at five bars for manual mode, enjoying the visceral feeling of shifts from second to third, enhanced by amazing engine sounds with the car's top down.

With five bars for the DriveLogic program, the car adds extra torque during gearshifts.

Driving with the double-clutch transmission, we liked the fact that we could keep pressure on the gas pedal while shifting. In traffic, the DCT handled itself well, making smooth shifts at low speeds and pitching no fuss when it had to sit in first gear at a stop light. BMW incorporates hill hold tech into the system, too, so that you don't roll backward during a hill start.

In sum
The base price for the 2008 BMW M3 Convertible is a hefty $66,150, and that's missing much of the tech we've mentioned. Two features adding significantly to our car were the electronic damping control, for $1,000, and the double-clutch transmission, for $2,900. BMW Assist, which brings in telematics and Bluetooth, adds $750. Other nontech options and an $825 destination fee brought the total up to $72,925. Cabin tech options we would have chosen for the car would have been navigation, enhanced audio, and iPod integration, which would have added $4,400 to the price, putting our convertible sports car close to $80,000. Getting the convertible adds a pretty serious premium to the price of a standard M3, but few other companies make convertibles that perform this well.

The M3 Convertible earns our top rating for performance, with the superb engine tech, DCT, and EDC all contributing to a sublime driving experience. Although our test car didn't come with much cabin tech, we give it credit for the optional navigation and audio systems, for a strong score in this category. Design is also very good--the car looks as good as it drives.