The single best credit card for earning rewards

Flickr/Kate Hiscock

The Blue Cash Preferred offers 6% back on groceries and 3% back on gas and department stores.

We've told you about the best credit cards for travelers, college students, and people with bad credit.

But if you're looking for the best rewards overall, here's the card for you: the Blue Cash Preferred from American Express.

That's according to personal finance and credit-card-comparison site NerdWallet, which recently released a study ranking the best rewards credit cards for American consumers.

NerdWallet considered earn rate, the amount of cash back, points, or miles you receive on every dollar you spend, and burn rate, the value of the cash back, or points and miles that you redeem for rewards, when compiling the rankings.

The site also used spending data for US consumers (from the Bureau of Labor Statistics) to determine which cards provide the most value specifically for the average American household.

Read the full methodology.

The Blue Cash Preferred - which offers 6% cash back on groceries, 3% back on gas and department stores, and 1% for all other purchases - came out on top.

"Given that the average US consumer's spending skews towards groceries and gas, it's unsurprising that a card that maximizes rewards on those two categories tops the list," NerdWallet's credit-card expert, Sean McQuay, tells Business Insider.

Another perk: You'll get $150 back, in the form of statement credit, after you spend $1,000 in your first three months.

One word of caution, however: "Spending at discount retailers like Costco and Walmart doesn't count as 'groceries' per American Express' definition," McQuay notes. "If you do the bulk of your grocery shopping outside of traditional supermarkets, I'd recommend looking at the Citi Double Cash which gives a flat 2% cash back, regardless of where you spend."

It's also important to consider the Blue Cash Preferred's annual fee - $75 - and its foreign transaction fee of 2.7%. If you're a light spender, or if you travel frequently, this card may not be right for you.