NEW YORK (MainStreet) — More than a third of mortgage borrowers aren't entirely sure what interest rate they're paying — bad news given that the window to refinance into a sub-4% loan appears to be closing, a Bankrate.com analysis shows.

"Given how far mortgage rates have fallen, people could be missing substantial opportunities to save money by refinancing," says Holden Lewis of Bankrate, which recently commissioned a poll of 1,000 U.S. adults to get their take on mortgage rates.

Princeton Survey Research Associates International found that just 65% of respondents who have mortgages said they feel "very confident" that they know what interest rate they're paying.

Another 21% said they're only "somewhat confident" they know, while 5% are "not too confident," 6% are "not at all confident" and 3% have no idea.

Lewis thinks the true picture is even worse. He believes many of the 65% of respondents who claim to know their mortgage rate "are fooling themselves."

The expert says a mortgage broker recently told him that many would-be refinancers claim to know their current rates, but that a check of their loan papers often shows that they're wrong — although frequently not by much. "People usually roughly know the range of what their rate is," Lewis says.

He believes homeowners often don't know their exact rate because it's typically been years since many bought or refinanced their properties.

"Your mortgage rate is a number that's really important for a couple of weeks, but then it fades out of your memory," Lewis says. "It's not like an anniversary that you have to remember every year."

John Battaglia, a Boston-area mortgage lender and member of the American Bankers Association's Mortgage Markets Committee, believes the rise of online bill payment has only added to the problem. He says that while loan servicers often include your interest rate on your monthly mortgage bill, more and more borrowers pay online and rarely if ever read their statements.

Unfortunately, experts say that even a small misunderstanding about your current rate can discourage you from refinancing despite the fact that interest levels are enjoying perhaps their last run of near-record lows.

Bankrate's latest survey of lenders shows that 30-year fixed-mortgage rates averaged 3.82% in the seven days ended March 31. That's just a bit above the 3.5% bottom that they reached in 2012.

Lewis says someone who bought or refinanced a home with a 30-year fixed-rate loan as recently as five years ago probably has a 5.25% mortgage and can save big bucks by refinancing at today's interest levels.

Example: Someone who took out a $200,000, 5.25% mortgage in April 2010 would be paying about $1,104 a month, but can cut that to around $857 by refinancing the remaining balance at today's 3.8% average rate. That's roughly $247 a month in savings.

Of course, refinancing into another 30-year mortgage would move your payoff date in the above example to 2045 from a current 2040. But you can simply add $94 a month in extra principal and keep your payoff date unchanged while still saving about $153 monthly.

Those who want to lock in today's super-low rates should probably act soon. Many experts believe a firming U.S. economy and the Federal Reserve's expected credit tightening will send benchmark mortgage rates to around 5% by year's end.

Fortunately, Lewis and Battaglia say homeowners who aren't sure what their current mortgage rate is can easily find out by:

  • Checking mortgage monthly bills. Many loan servicers will list your rate somewhere on your statement.
  • Digging out loan papers. Lewis says you'll find your interest rate on a page marked "Promissory Note." (Ignore anything marked "Annual Percentage Rate" or "APR," as that takes your closing costs into account and doesn't reflect the interest level you're paying.)
  • Calling the loan servicer. Your lender can quickly tell you what your current rate is and will probably even want to discuss refinancing.

Princeton Survey Research conducted Bankrate's survey by phone between Jan. 8-11. The poll has a margin of error of plus or minus 3.7%.

— Written by Jerry Kronenberg for MainStreet