Looking on the internet deeply has found these results:

Let's go further together is the most popular phrase on the web.

Let's go farther together 1601 results on the web	Let's go further together 17,500 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
And if you can have the aerobic engine to sustain a higher cadence, you can go farther, faster, longer. He... He always wanted to go farther. I could probably go farther, but I'm too scared. I would start at the beginning, but I think I need to go farther back. Mr. During will need to go farther than that, and so will the press. You have a chance to go farther than any boy in the village. If there are no wolves, I'll go farther. Wouldn't he go farther out?	Ships go further south than this... Look, we'll get a bunch of us together and... if your dad lets us have a Landrover, we should go further up the river than we've ever been before. I support this approach as I believe it establishes genuine, and genuinely strong, relations between the countries to the north and south of the Mediterranean who wish to go further together. No indications he'd go further. We must go further as regards information. Further, let's go further... Training must go further, however. Before we go further, let's be clear on one thing, yes?

Let's go farther together

1601 results on the web

Let's go further together

17,500 results on the web

More popular!

And if you can have the aerobic engine to sustain a higher cadence, you can go farther, faster, longer.
He... He always wanted to go farther.
I could probably go farther, but I'm too scared.
I would start at the beginning, but I think I need to go farther back.
Mr. During will need to go farther than that, and so will the press.
You have a chance to go farther than any boy in the village.
If there are no wolves, I'll go farther.
Wouldn't he go farther out?

Ships go further south than this...
Look, we'll get a bunch of us together and... if your dad lets us have a Landrover, we should go further up the river than we've ever been before.
I support this approach as I believe it establishes genuine, and genuinely strong, relations between the countries to the north and south of the Mediterranean who wish to go further together.
No indications he'd go further.
We must go further as regards information.
Further, let's go further...
Training must go further, however.
Before we go further, let's be clear on one thing, yes?