GPS navigation and Autopilot—two great driving aids that go great together.

Tesla has offered both features for a long time, and is now rolling out software that combines them, so drivers can set a destination in their cars' navigation system and it will automatically take action to merge and exit highways, and change lanes to expedite the trip.

To be clear, this isn't fully autonomous self-driving. Rather, it's the first implementation of a small sub-set of features that Tesla chief executive officer Elon Musk promised during the company's July earnings call as "On-ramp to off-ramp" self-driving capability.

DON'T MISS: Consumer Reports ranks Tesla Autopilot second among self-driving systems

It's also important to note that drivers are warned when they first enable the feature and then every time you turn it on that the system isn't a substitute for paying driver attention.

Consumer Reports owns several Teslas and downloaded the system to try it out and report on it

The magazine's engineers found that the system does what it claims, but not as well perhaps as it should.

CHECK OUT: Another owner sues Tesla over Autopilot rear-end crash

Most notably, when the navigation was set to go off an off-ramp and the Tesla encountered a slow truck in the right lane, it tried to pass the truck without enough room to return to the right lane and exit. Like a teenage driver, the car had to slow down again in the left lane, impeding traffic,to get back behind the dump truck and get off.

Other times, the car would cut off faster cars as it moved into the left lane to go around a slower vehicle, then wouldn't pull back into the original lane once it completed the pass. Tesla told Consumer Reports that in future updates the car will automatically return to the previous lane.

READ THIS: IIHS: Self-driving systems aren't that, and aren't ready for prime time

"Overall it works best in easy situations with minimal traffic,” says Consumer Reports director of auto testing Jake Fisher. “But as the traffic builds, it clearly displays the limitations of today's technology."

As it is, the system may preview what's coming with autonomous technology, and Tesla says it will improve the capabilities of the system as cars drive more miles on it and the company has a chance to collect more data. 

In the meantime, on-ramp to off-ramp Navigate on Autopilot still doesn't constitute real self-driving, or even drive all that well.