Currently, PCIe 3.0 is the most common version that you can find on motherboards but if you have older PC, it may actually conform to the older PCIe v2.0. With most new expansion cards conforming to v3.0, it is common to ask: can I use a PCIe 3.0 card in a 2.0 slot?
Basically, yes, you can use a PCIe 3.0 card in a 2.0 slot, but with some caveats which I will cover below.
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A third-generation (PCIe 3.0) card will work in a second-generation (PCIe 2.0) slot because the PCIe standard is designed to be backward, and forward compatible, thus allowing the use of new cards on older hardware and vice versa.
However, as mentioned earlier, there are few caveats that you need to take into consideration particularly in terms of performance.
In the following text I talk about PCIe backward and forward compatibility.
A Review of the PCIe Generation and the Speeds
Before we delve into the topic of backward compatibility, it is worth-it to look into PCIe generation and their corresponding transfer speeds in brief.
As can be seen in the table above, a single 2nd generation PCIe lane has a speed of 0.5 GB/s or 500 MB/s. On the other hand, a single 3rd generation PCIe lane doubles the speed to about 1.0 GB/s.
The point to note here is that each successive PCIe generation doubles the per lane throughput (speed) as compared to the previous generation.
The much higher throughput of the successive generation has a lot of ramification. For instance, a device that would require two lanes on 2nd generation of PCIe interface, would only require a single lane on 3rd generation PCIe interface.
The newer generation also brings better efficiency in encoding algorithms thus reducing the overall power usage.
Both the higher per lane speed as well as efficient algorithms make it possible for the PCIe interface to do the same amount of work at fewer clock cycles with each successive generation.
This in turn has effect on the size of the cards; they can get smaller, use weaker sub components like smaller heat sinks, thus reducing the overall cost.
Newer PCIe generations also allow for highly advanced cards to be designed.
So in short, a card built with the PCIe 3.0 version mind will be more efficient, and have a better performance per dollar ratio compared to the PCIe card built with PCIe 2.0 version in mind.
So Can I Use A PCIe 3.0 Card In A 2.0 Slot?
PudgetsSystems.com has done a comprehensive study on this topic. They tested NVIDIA RTX 3090, a PCIe 4.0 device, and an NVIDIA Titan RTX, a PCIe 3.0 device, on first, second, third and fourth generation PCIe motherboards.
While in certain situation the difference is negligible, in other demanding cases like video editing and post production, the generational gains are too large to ignore.
While PudgetSystems.com did not test games in their benchmark, it can be deduced that tasks that require constant transfer of information to and from the PCIe slots would see impact in performance, this can include games.
So when doing light work, the difference in performance may not be much of an issue, but for more intensive tasks, say playing a demanding game, a top of the line PCIe 3.0 graphics card in a PCIe 2.0 slot can be severely underutilized and you may see a difference in performance.
When to Use this Setup?
If you have an older motherboard running on PCIe 2.0 and are you looking to try out a new hardware that uses the PCIe 3.0 spec, you can go ahead and plug in the card.
While this may lead to under-utilization depending upon how much data the card generation or how demanding it is, this type of setup a cost-effective solution
Another situation where configuring your build like this may be necessary is if all the usable PCIe 3.0 slots are already taken up by other components.
Some motherboards have both PCIe 3.0and 2.0 slots, and if you only have PCIe 2.0 slots available, you can use one of them for your PCIe 3.0 card.
The answer to question, can I use a PCIe 3.0 card in a 2.0 slot is a resounding yes.
While in many cases the performance difference in this arrangement would be marginal, you cannot deny the throttling that can occur.
Nevertheless, changing the entire system just so that you can install a single 3.0 card may not be the most effective way to go about it as it would require a change of motherboard, and in turn, a change of processor as well.
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Therefore, if there is no other way, then go ahead and use that 3.0 card in the older 2.0 slot.