Those 20 and 40 MHz bandwidth modes mentioned earlier really refer to how many channels the router is simultaneously using. 20 MHz mode means it is using only one channel, 40 MHz mode means it is using two and 80 MHz mode uses four channels.
The advantage of higher bandwidth modes is higher throughput. But with only three non-overlapping channels, 40 MHz channels in 2.4 GHz can increase interference with nearby networks, which reduces throughput and, in extreme cases, wireless connection reliability. This is why N routers have an "Auto 20/40" mode.
Properly designed, i.e. compliant with the IEEE 802.11 spec and with Wi-Fi Alliance Certification requirements, routers are supposed to detect any interfering 2.4 GHz networks and fall back to using 20 MHz mode. Manufacturers also have the option of not supporting 40 MHz operation in 2.4 GHz at all, which is the choice Apple originally made, but no longer does.
There are no such limitations on 40 MHz (or 80 MHz) mode operation in 5 GHz. So most AC routers default to 80 MHz bandwidth mode in 5 GHz.
Throughput & Range
Wireless performance is the squishiest spec to pin down, yet usually the driving force behind the urge to buy a new router. Manufacturers are happy to have you confused, because confused consumers usually buy the product with the highest number on the box, which usually is the most expensive. That's why they use link rates and show the sum of the maximum link rates of both radios in simultaneous dual band products. But, by now, you know that number on the box is much greater than the best throughput you'll actually get.
What you really want in a wireless router is the best throughput vs. range. But because of the way Wi-Fi networks work, this number is highly variable. It is highly dependent on the RF (radio frequency) and physical environment the router is operating in, as well as the capabilities of wireless devices using the router.
That's why the wireless performance testing we do for our Charts and Rankers uses a "clean" RF environment and standard test client. Keeping as many things constant as we can, provides the best relative comparison among products available anywhere.
To get you in the right ballpark, however, here's a table of actual measured best case downlink throughput taken from our Router Charts for many N and AC product classes.
|Class||Typical maximum 2.4 GHz throughput (Mbps)
20 MHz B/W
|Typical maximum 5 GHz throughput (Mbps)
40 MHz B/W N devices
80 MHz B/W AC devices
|N300 or N600||~ 75||~ 75|
|N450 or N900||~ 120||~ 200|
|AC1200||80 - 90||215 - 300|
|AC1750||130- 140||400 - 470|
|AC1900||120 - 140||440 - 530|