FM transmitters aren't the best option, but for stereo's lacking an AUX port, it's the only option. With this in mind, the Nulaxy KM18 is the best Bluetooth FM transmitter I tested, despite having an audio quality far worse than each of the AUX-in Bluetooth adapters, like the Anker SoundSync Drive. However, it's significantly better than other FM transmitters I tested. The ease of pairing is superior, as are the controls, making it easy to find the best FM station for reception quality. So, if a Bluetooth FM transmitter is your only option, the Nulaxy KM18 is your best choice.
The Nulaxy KM18 is the most popular Bluetooth car kit on Amazon, and it's not even close. With over 11,500 reviews (twice as much as the second most reviewed product) and a 4.3 rating (tied with other devices for the highest overall rating), the KM18 is clearly popular. The $17.99 price likely has something to do with that, as it's the second most affordable Bluetooth car adapter and the most affordable FM transmitter.
The biggest issue with FM transmitters is the radio technology isn't designed for great audio. Radio signals have a low signal-to-noise ratio – there's a lot of static in the signal. This was clearly evident in the signal clarity test, where the Nulaxy received a D+ grade. When the volume is relatively high, you can trick your ears into ignoring the noise in the signal. With the KM18, the static isn't noticeable unless the songs get quiet or in the lull between songs. In other words, it's tolerable for an FM transmitter. While it's nowhere near the quality of an AUX-in Bluetooth adapter, the quality is a lot better than the Mpow and GOgroove FlexSMART X5 FM transmitters. Both of those devices received D- grades and were nearly intolerable.
The signal strength of the KM18 received an A grade. This is a direct correlation to the signal clarity, because a stronger transmission results in better reception.
The KM18 has a AUX-in port. There are several ways you can make use of this port. First, you use a 3.5 mm audio cord and plug it into your car stereo's AUX port to forgo the FM transmission. However, I don't recommend this option. If your car stereo has an AUX port, I recommend opting for an unobtrusive AUX-in Bluetooth adapter. Second, you can connect other non-Bluetooth audio devices, such as an iPod, to the KM18's AUX port and transmit audio via the FM transmitter.
The call quality isn’t good, receiving a D+ grade. This shouldn't be a surprise though, considering the signal clarity issues of radio. In addition, with studies suggesting hands-free calling is not safer than hands-full calling, this subpar performance means little. Even if you're skeptical about the studies, the National Highway Traffic Safety Administration considers phone conversations, whether hands-free or not, as distracted driving.
The KM18 plugs into your car charger and features an adjustable arm, a similar design to other FM transmitters. The biggest issue with this design is how obtrusive it can be. In my car, it's too close to the gear shift. Even with the short adjustable arm, it gets in the way. Clearly, this isn’t an issue in all vehicles, but it is something to consider.
The controls received an A for ease of use. It's a very simple and intuitive setup. The controls are clearly labeled and the display is big enough to see from the driver's seat, making it easy to navigate radio stations, but it's not so big it becomes more obtrusive than it already is.
The ease of pairing received a B grade. It took me about a minute to successfully pair the KM18 to my phone and music player. This was considerably better than half of the Bluetooth adapters I tested. There were still a handful of faster and easier to pair devices.
As one of the most affordable Bluetooth car kits on the market, the Nulaxy KM18 is a good option for streaming music, especially if your stereo doesn't have an AUX port. The FM transmission isn't comparable to the AUX-in quality, but many older stereos don't have AUX ports. It's bigger than most Bluetooth adapters and has the potential to get in the way, but the controls are easier to use than other Bluetooth FM transmitters.