Imperial, California - So much for thinking we know everything that matters about our spouse or significant other - at least, it seems, when it comes to money.

According to Fidelity Investments' new "2015 Couples Retirement Study," while the overwhelming majority of couples surveyed said they communicate "exceptionally well" or "very well" about financial matters, a whopping 43 percent couldn't correctly identify how much their partner earned - up 16 percent from the last time the question was asked two years ago. And 10 percent of those in the dark were off by $25,000 when they apparently tried guessing.

"We know couples don't always agree when it comes to money, but we were surprised how many missed the mark on the question of their partner's salary," says John Sweeney, Fidelity's executive vice president of retirement and investing strategies. "If gaps exist around basic questions like that, couples might have other opportunities for improvement on the financial front, including how and where to retire and later-in-life issues like eldercare and estate planning."

Feeling a bit smug because you know how much your other half makes right down to the last decimal point? Then go ahead - if you dare -- and try asking him or her these questions to see how you stack up:

* How much do we need to save to maintain our current lifestyle in retirement? (The survey results: 48 percent had "no idea," and another 47 percent -- particularly, alas, Baby Boomers closest to retirement -- disagreed on a figure.)

* How much can we expect in Social Security benefits to help complement what we've saved independently? (The survey results: 60 percent of all couples and 49 percent of Boomers drew a complete blank, even though the government regularly mails the info out.)

* If you add up all our investible assets -- i.e., bank accounts, mutual funds, retirement accounts, and stocks and bonds -- what's the grand total? (Survey results: 36 percent of couples couldn't agree.)

* Traveling the world or staying put? (Survey results: One in three gave conflicting visions of their expected post-retirement lifestyle.)

There are additional interactive "Couples Quiz" questions on Fidelity's website (fidelity.com/couplesquiz) that you might want to try, including a fun one about what you'd do if your favorite store was having a blow-out sale.

Answering them produces your "Financial Personality," and you're encouraged to share the results with your partner and learn how to navigate the retirement process together.

A word of advice: While it pays to be honest, think long and hard about how badly you want to hit that sale.