The Mazda6 has standard Whiplash Reducing Headrests, which use a specially designed headrest to protect the driver and front passenger from whiplash. During a rear-end collision, the Whiplash Reducing Headrests system moves the headrests forward to prevent neck and spine injuries. The K5 doesn't offer a whiplash protection system.

The Mazda6 Signature has a standard 360-degree View Monitor to allow the driver to see objects all around the vehicle on a screen. The K5 only offers rear parking sensors that beep or flash a light. That doesn't help with obstacles to the sides.

Both the Mazda6 and the K5 have standard driver and passenger frontal airbags, front side-impact airbags, side-impact head airbags, front seatbelt pretensioners, front wheel drive, height adjustable front shoulder belts, four-wheel antilock brakes, traction control, electronic stability systems to prevent skidding, crash mitigating brakes, daytime running lights, lane departure warning systems and available driver alert monitors.

For its top level performance in IIHS driver and passenger-side small overlap frontal, moderate overlap frontal, side impact, roof strength and head restraint tests, its standard vehicle-to-vehicle front crash prevention system, its standard vehicle-to-pedestrian front crash prevention system, and its standard headlight's “Good” rating, the Insurance Institute for Highway Safety grants the Mazda6 its highest rating: “Top Pick Plus” for 2021, a rating granted to only 35 vehicles tested by the IIHS. The K5 has not been tested, yet.

The Mazda6's 5 year corrosion warranty has no mileage limitations, but the corrosion warranty on the K5 runs out after 100,000 miles.

From surveys of all its subscribers, Consumer Reports' December 2019 Auto Issue reports that Mazda vehicles are more reliable than Kia vehicles. Consumer Reports ranks Mazda second in reliability. Kia is ranked 9th.

The Mazda6's standard 2.5 DOHC 4-cylinder produces 7 more horsepower (187 vs. 180) than the K5 1.6T's standard 1.6 turbo 4-cylinder. The Mazda6 Grand Touring/Signature/Carbon Edition's standard 2.5 turbo 4-cylinder produces 9 lbs.-ft. more torque (320 vs. 311) than the K5 GT 2.5T's standard 2.5 turbo 4-cylinder.

An engine control system that can shut down some of the engine's cylinders helps improve the Mazda6 (except Turbo)'s fuel efficiency. The K5 doesn't offer a system that can shut down part of the engine.

The Mazda6 has 1.6 gallons more fuel capacity than the K5 LX's standard fuel tank (16.4 vs. 14.8 gallons), for longer range between fill-ups.

For better traction, the Mazda6 has larger tires than the K5 (225/55R17 vs. 205/65R16).

The Mazda6 Sport's standard tires provide better handling because they have a lower 55 series profile (height to width ratio) that provides a stiffer sidewall than the K5 LX/LXS' standard 65 series tires.

For better ride, handling and brake cooling the Mazda6 Sport has standard 17-inch wheels. Smaller 16-inch wheels are standard on the K5 LX/LXS.

The Mazda6 (except Sport) offers available computer-generated active noise cancellation to help remove annoying noise and vibration from the passenger compartment, especially at low frequencies. The K5 doesn't offer active noise cancellation.

To allow full utilization of available cargo room, the Mazda6's trunk lid uses concealed beam hinges that don't intrude into the trunk. The K5's useful trunk space is reduced by its intrusive beam hinge.

The Mazda6's standard rear seats fold to accommodate long and bulky cargo. The K5 LX doesn't offer folding rear seats.

The Mazda6 Grand Touring Reserve/Signature/Carbon Edition has a standard heads-up display that projects speed and other key instrumentation readouts in front of the driver's line of sight, allowing drivers to view information without diverting their eyes from the road. The K5 doesn't offer a heads-up display.

The Mazda6's front and rear power windows all open or close fully with one touch of the switches, making it more convenient at drive-up windows and toll booths, or when talking with someone outside the car. The K5's standard passenger windows don't open or close automatically. With the K5 GT-Line/EX/GT's power windows, only the front windows open or close automatically.

The Mazda6's rain-sensitive wipers adjust their speed and turn on and off automatically based on the amount of rainfall on the windshield. This allows the driver to concentrate on driving without constantly adjusting the wipers. The K5's manually variable intermittent wipers have to be constantly adjusted.

To help drivers see further while navigating curves, the Mazda6 Grand Touring Reserve/Signature/Carbon Edition has standard adaptive headlights to illuminate around corners automatically by reading vehicle speed and steering wheel angle. The K5 doesn't offer cornering lights.

The Mazda6's optional rear and side view mirrors have an automatic dimming feature. These mirrors can be set to automatically darken quickly when headlights shine on them, keeping following vehicles from blinding or distracting the driver. The K5 offers an automatic rear view mirror, but its side mirrors don't dim.

Both the Mazda6 and the K5 offer available heated front seats. The Mazda6 Grand Touring Reserve/Signature/Carbon Edition also has standard heated rear seats to keep those passengers extremely comfortable in the winter. Heated rear seats aren't available in the K5.

Consumer Reports® recommends the Mazda6, based on reliability, safety and performance.

The Mazda6 was chosen as one of Car and Driver's “Top Ten” for 2 of the last 7 years. The K5 has never been a Car and Driver “Top Ten” pick.