TORONTO - The NDP is warning that Ontario families will end up paying hundreds of dollars more at the pump once the new harmonized sales tax kicks in July 1.

Using a Statistics Canada model, the party says their calculations show the average family with two kids will pay $232 more a year to fill up their car.

NDP Leader Andrea Horwath says the government will collect $895 million through new taxes on gasoline when it merges its sales tax with the federal GST.

She says the governing Liberals have done their own calculations about the impact of HST on families, and should come clean with taxpayers by releasing them.

Premier Dalton McGuinty says his government has released figures about the HST, but the NDP doesn't agree with them.

McGuinty says even Nova Scotia's NDP government understands the importance of the blended tax, which it's raising to 15 per cent from 13 per cent.