Updated 5:05 p.m. ET

FLORENCE, S.C. -- Former Massachusetts governor Mitt Romney said Tuesday that he pays taxes "close to the 15% rate" and that most of his income comes from investments.

Romney made the comments after a town hall meeting here and the day after a contentious debate in Myrtle Beach. In that debate, Romney said he "probably" would release his income tax return in April if he is the GOP nominee.

STORY: Sharper tone at GOP debate

Romney's concession at the debate about releasing his tax returns comes after GOP rivals Rick Perry and Newt Gingrich -- along with the Democratic National Committee -- have pressed him for transparency on his taxes. Gingrich said he would release his tax returns Thursday.

Before he ran for elected office, Romney made a fortune as a venture capitalist. He has a net worth of $190 million to $250 million, according to his personal financial disclosure report.

The acknowledgment by Romney that he "overwhelmingly" makes much of his income from investments is significant because investment income is taxed at a lower rate than wages and salary.

The top federal tax rate for investments is 15%, while the top tax rate for wages is 35% on taxable income over $388,350. Wages are also subject to Social Security and Medicare payroll taxes.

At 15%, Romney's income tax rate would still be higher than the rate paid by most Americans.

The Tax Policy Center, a Washington think tank, projects households making $50,000 to $75,000 will pay an average effective income tax rate of 5.7% this year. When payroll and other taxes are included, that same household would pay an average federal rate of 16.6%.

Romney said he made "a little bit of income" from sales of his book, No Apology, but gave that money away. He said he occasionally makes money from speeches but not much.

According to Romney's financial disclosure report, he made nine speeches from Feb. 26, 2010, to Feb. 20, 2011, and earned more than $362,000 in speaking fees.

"I know that if I'm the nominee people will want to see the most recent year," he said today about his income.

Newt Gingrich poked fun at Romney, saying he'll rename his flat tax-proposal after his rival since the former House speaker is calling for an optional flat rate of 15%.

Romney said he was surprised how "aggressive" Rick Santorum was about an ad by a pro-Romney "super PAC" that claimed the former Pennsylvania senator supported voting rights for violent offenders.

"I thought it was great to have him make it very clear he's somebody who thinks felons should be able to vote," Romney said.

He also addressed a super PAC ad released by comedian Stephen Colbert that refers to Romney as a serial killer.

"I haven't seen it, so I don't know if it is funny or not," he said. "Colbert is entitled to be a humorist ... hopefully it's funny."

(Contributing: Catalina Camia and the Associated Press)