The "M" moniker has been used on various vehicles, all being mid-size luxury cars from Nissan's Infiniti marque. This includes the M30 Coupe/Convertible, and the M45 and M35 sedans, both of which are unrelated to the M30. This article contains info translated from the Nissan products described in the Japanese Wikipedia.
The Infiniti M30 (chassis code F31) was a 2-door luxury GT coupe introduced with the Q45 as the start of the Infiniti line in 1990, as a stopgap until other models could be produced. The M30 was basically a Japanese Nissan Leopard built with left-hand drive and rebadged as an Infiniti, and had added luxury features. The other main changes were in the engine, transmission and suspension. The M30 was only sold in the United States.
The M30 had no factory options, but dealers offered the addition of a cellular phone and a CD player/changer. The sole engine was Nissan'sVG30E, a 3.0 L SOHC V6 that produced 162 hp (121 kW) and 180 ft·lbf (244 N·m) of torque, also used by many previous Nissan models such as the Maxima and 300ZX. Like the 300ZX, the M30 was a front-engined and rear-wheel drive vehicle. The transmission was an RE4R01A, four-speed automatic with overdrive, as enabled or disabled by a button on the shifter. There was no option of a manual transmission. The differential was a Nissan R200-type open differential. It is generally regarded among M30 enthusiasts that the change of powertrain, weak suspension and the lack of options in these categories was a serious mistake on Infiniti’s part, and perhaps to blame for the lackluster sales. The M30 (coupe)has a stock weight of 3333 lb (1540 kg), yet only 162hp (121kW), creating an inadequate power to weight ratio for the intended image of a sport coupe. Unsurprisingly, the M30 convertible weighed even more, due to massive amounts of body and chassis reinforcements, but this is regarded as a good thing to some, because the chassis is noticeably stiffer than it is on the coupes. In addition, sports cars normally come with a manual transmission standard and have the option of an automatic. Instead, Infiniti didn't even give potential buyers the choice, as all M30s had the four-speed automatic.
The M30 was equipped with the Nissan Sonar Suspension II system that was also installed on the Maxima at the time, which featured a sonar module mounted under the front bumper that scanned the road surface and adjusted the suspension accordingly via actuators mounted on the strut towers. There was also a switch on the center console that allowed the driver to change between "Sport" and "Comfort" settings—essentially "firm" and "soft", respectively.
The M30 convertible was the sole convertible of the Infiniti line, until the Infiniti G37 convertible's introduction in 2009, . Infiniti selected about half of the models produced from 1991 to 1992 and had them made into convertibles by the ASC (American Sunroof Corporation). While the power tops on these did not need extensive maintenance for quite a while, they could be operated manually if necessary. All M30 coupes had power tilting/sliding sunroofs standard, with "auto-cut" while sliding to the closed position to prevent anything from getting caught.
The M30’s production run lasted for only 3 years, ending in 1992 due to lagging sales. Nobody knows exactly how many models were produced, but it has been said that just over 12,000 were made, half coupes, half convertibles, making the M30 the rarest Infiniti ever put into production. Previously rumored by M30 enthusiasts was that only 5,000 were made in total, half being coupes and half convertibles. The M30 was replaced by the Infiniti J30, a mid-size 4-door sedan. While the J30 is a completely different car and has a different chassis code (Y32), it was called the "Leopard J. Ferie" in Japan and was still referred to as the Nissan Leopard.
At the time, the M30 rated as one of the top-5 most safe cars to drive of the 90s. The car came with a driver's airbag and ABS (anti-lock brakes) as standard equipment, though it lacked belt pretensioners and roll-over protection (convertible). All M30s came with 3-point self-locking seatbelts located at each outer seat, and the rear-center seat had an adjustable lapbelt.
The M30 received 4 in both frontal- and side-impact crash test ratings, making it one of the safest cars out there at the time. Nissan paid special attention to the placement of the M30's side reinforcing, which is located most notably in the doors and quarter-panels. Of course, all of this safety came at a price; the M30 weighs over 3,200 lbs., and the massive amounts of reinforcement in the doors are very heavy on their hinges. Over time, the driver's door begins to sag drastically, whereas the passenger door not so much.
The M30 has a small cult following, similar to that of the Nissan Leopard F31 built in Japan. The car shares many similarities with more successful and capable Nissan sports cars, such as the 240SX, 300ZX and early-generation Skylines. As such, more powerful engines, manual transmissions, suspensions and limited slip differentials from these cars can be swapped into the M30 to create a much more capable car. In both the U.S. and Japan, the M30/Leopard has been used for drifting, as it can take parts from other Nissans and is rear-wheel drive.
The 2003 Infiniti M45 was based on the Y34-series Nissan Gloria, with styling almost identical to the Japanese-spec Gloria, and replaced the I30/I35 a.k.a in Japan as the Nissan Cefiro as Infiniti's mid-size car.
In contrast to the Japanese models, which were available only with V6 engines, the M45 was powered by Nissan's VK45DE V8, producing 340hp (254kW) and 333lb·ft (451N·m) of torque, which it shared with the Infiniti's flagship Q45.
The vehicle was something of a stop-gap solution for Infiniti, providing them with a viable competitor for such cars as the Lexus GS, particularly the V8 GS400/430. When introduced in North America it was using a Japanese design that was already several years old. Its styling was criticized for being generic and homely, and it sold poorly. On the other hand, it did allow Infiniti to expand the use of the 4.5L V8 that had been exclusive to the Q45, thus creating a better economy of scale to cope with that engine's development costs.
For the Japanese version, see Nissan Fuga
Infiniti released a completely redesigned M for the 2006 model year to compete directly in the mid-size sport luxury class, against such class stalwarts as the BMW 5-Series and the Mercedes-Benz E-Class.
The Infiniti M uses a much more rigid version of the G35/350Z/FX Nissan FM platform and is available with the common VQ-series 280 hp (208 kW) 3.5 L V6 in the M35, or the Q45's 335 hp (249 kW) 4.5 L V8 in the M45. This generation of the Infiniti M is sold in Japan as the Nissan Fuga.
While being smaller on the outside than the flagship Q45, the M45 boasts more room, greater performance, a superior rear multilink suspension and front double wishbone, and much friendlier ergonomics, filling in the gap since the previous flagship Q45 was no longer imported from Japan after the 2006 model year.
All-wheel-drive models, called the M35x and M45x, use a sports car derived system (ATTESA-ETS) to send all power to the rear wheels until slip is detected.
The automotive press has awarded the M several best-in-class awards in direct comparisons against its competitors in the midsize luxury segment. Despite entering a very crowded, competitive segment in which almost every member has been recently redesigned, the M managed to claim several key distinctions. Both the M35 and M45 are fastest in their class (0-60 in 6.2 and 5.6 seconds, respectively). The M is near or at the top of its class of every interior dimension, actually breaking into the United States Environmental Protection Agency "large" category depending on which option packages are selected. The M35x has won Consumer Reports top luxury car pick for 2006, taking the spot after the 2005 Lexus LS 430.
The M line was released in February 2005 (almost exactly at the same time as the new 2006 Lexus GS) with first sales in the first months far beyond expectations.
2008/2009 Mid-generational Refresh
For the 2008 model year, minor changes are seen. The M45 gets optional all wheel drive (M45x), all models get new front and rear fascias, and redesigned blue-and-white gauges (as opposed to the previous orange ones). Infiniti is also offering a S type version for the M45/M45x similar to the previous Sport model. Other new features on the 2008 model include touch screen hard drive based navigation, ipod integration, and a music hard drive.
For 2009, the M35 receives a new V6 engine producing 303 horsepower and 262 lb-ft of torque and a 7-speed automatic transmission. The 2009 M35x, M45, and M45x retain the five-speed automatic.
Trademark lawsuit against BMW M series
Infiniti ads often compare the M35 and M45 to their direct competitors in the BMW 5 Series midsize class, notably the 535i and 545i. The flagship of the 5 Series is the M5.
In 2007, Canadian Federal Court Justice W. Andrew MacKay ruled that the use of the letter by Infiniti was in a form similar to that of the BMW's trademark M alone, and its use of M6 as a package for modifying one of its vehicles, cause confusion in Canada between the sources of the wares of the defendant and the wares of the plaintiffs. The court granted injunction that prevents Nissan from directing public attention to its products in any way likely to cause confusion.
BMW introduced its first M series vehicle, M6, in Canada in 1987, and as of 2008 its current lineup consists of the M3, M5, M6, M Coupe and M Roadster. Infiniti registered M45 in 2004 and M35 in 2005 as trademarks for sedans in the middle of its range, and in some advertising has used the letter M alone.
Around 2011, Infiniti is expected to release an all new M, the M37 (VQ37VHR 3.7 liter 24 valve V-6, 330 hp) and the M50 (VK50VE 5.0 liter 32 valve V-8, 390hp (291kW), 369lb·ft (500N·m) torque). The new M is expected to take a design closer to that of the new G37 sedan & coupe.