Como dizer "carro-chefe" em inglês

I'm sorry:
"Bubblegum"

Hi buddies...Howdy?
Mainstay.
Ex: Cattle farming is the mainstay of the country´s economy.
Ta-ta.

Hello!

It's also possible to call it core product.

http://en.wikipedia.org/wiki/Core_product

Regards!

In addition to the above answers (with some "overlaying" here), some way to say that:

The company´s "flagship (product)/main product/the main line product (in the line)
/chief brand/core product/

The main product (flagship product) manufactured by the company.

=
Perhaps you would like to know what they say when there are "dois carros-chefes"

Dreamforce '03: Salesforce.com today unveiled major upgrades to its two main products at its first annual Dreamforce user conference.

Its main products were health food, vitamin, and mineral supplements. Royal Numico ... Bios Life and Bios Life Slim are two flagship products for Unicity.

Dura-Tuff Wear Products- hellocompanies.com
Company.hellocompanies.com › ... › Company‎
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Main Products: Dura-Tuff Wear Products makes thru-hardened grouser bars ... Their two flagship products are Grouser Bars and a Bolt on Finishing Edge ..

Bread and butter nao é a mesma coisa?

Yes, depending on the context. Sometimes it means someone´s livelihood (sustaining income/source of income, the moneymaking, etc), sometimes - less usual - it could be the chief brand of a company, the company´s most important product, the flagship product, etc.

In the following passage, we could say that it is the most important product, but there´s some intersection in meaning. So, it could be understood as the
"hot product" that a company has launched and that its main source of revenue (at least for some time).

Why Dark Horse was really able to survive and do all of these great experiments was having those licensed books—having Star Wars, having Aliens, having Predator—and that was kind of their bread and butter for a long time. (Williamette Week)

Whereas in this one, it means the most "everyday/common" events:

He said “bread and butter” crimes such as robberies, break-ins and car thefts had all declined dramatically on the back of Newman Government laws enabling police to disrupt gangs. (The Courier Mail)

In the following chunk "carro chefe" can be understood as the "highlight" of something (in this case, of Brazil culture and that of Rio Carnival, etc.)

How did the Samba gained respectability in just over a decade, becoming the highlight of Carnival and the prevailing image of Brazil identity?

From the book The Revival of Banned Dances: A Worldwide Study.