ZBook 17 G3 - Mix memory modules of different sizes

[Mikael1]

My ZBook came with 2x4 GB memory modules and I want to upgrade with additional 2x8 GB.

It is the same manufacturer and specifications, only the size is different.

There are four memory module slots, how do I put the memory modules for best performance?

Maybe: 8 GB in slot 1 and 3, 4 GB in slot 2 and 4

[Huffer]

I have the Zbook 15 G3...very similar if not the same motherboard. I have 2 x 8 gig in slots 1 and 3 and it runs in dual channel so I would say put the 4 gig pair in slots 2 and 4. 

[Mikael1]

I checked with CPU-Z and got the following results:

8 GB modules in slot 1 and 3, it runs in single channel. If I put them in slot 1 and 2 it runs in dual channel.

[Huffer]

Then that is what you should do and put the 4 gigs in the open slots, then and see if they run in dual channel. They should.