Differences between using Further and Farther, specifically in Ford's logo

Merriam-Webster on "further" vs. "farther":

For much of their history the words have been used interchangeably. As adverbs, they still are interchangeable when applied to distance (of the spatial, temporal, and metaphorical varieties). Many usage guides will still recommend the aforementioned distinction of farther for literal distance and further for figurative, but there is enough recently published evidence of the figurative use of farther that it is difficult to say it is a mistake.

So you are asking that, if Ford is an automotive company designed to make people go far, why is their slogan using "further," the term more commonly used to describe figurative distance?

I would say that Ford's slogan is not meant to speak to the company's literal purpose, in which "farther" would be appropriate. From a business standpoint, the slogan is meant to make the reader see beyond just the distance that its vehicles cover, and be motivated to go "further" in whatever aspect of life that Ford's services can assist in. The key here is branding - Ford wants you to associate the Ford brand name and its offerings with drive (no pun intended), inspiration, and self-improvement.

To evidence that, here is a quote from a Forbes interview of a Ford executive.

'What we aim to do is inspire behavior,' Matt VanDyke, Ford's director of global communications, told me. "Go Further" is "more than an advertising tagline. We want to institutionalize it as part of our culture.