GM collected the fuel economy and fill-up data through the vehicles' OnStar system, spokesman Rob Peterson said.
As of Friday, there were about 2,000 Volts in customer hands, he said.
While there's no reason to doubt GM's claim -- 1,000 miles a tankful could be easily achievable if drivers recharge their cars' batteries frequently -- it doesn't necessarily mean drivers are really paying a lot less per mile to drive, said Jeremy Anwyl, chief executive of the automotive website Edmunds.com.
"What's not being stated is that Volt drivers who go 1,000 miles between gasoline fill-ups are also charging their vehicles with electricity each night and incurring the cost of electricity," he said.
The Volt can travel about 35 miles on a fully charged battery, according to EPA estimates. If the battery becomes depleted, a gasoline engine comes on to generate electricity for continued driving.
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