General Motors expects its Chevrolet VOLT electric car to earn a fuel-economy rating of at least 230 miles per US gallon for city driving – more than four times that of Toyota Motor Corp’s Prius hybrid according to a report by Bloomberg News which cites people familiar with the plans.  

The wire service said GM chief executive officer Fritz Henderson will announce the rating and combined city-highway mileage of more than 100 mpg today at a new-vehicle exhibition at GM’s technical centre in Warren, Michigan.  

This would fit with a previous statement from GM vice chairman Bob Lutz who said the Volt would receive “a higher number than anyone would expect.”  

GM said in September it reached an agreement with the US Environmental Protection Agency on a testing method that would yield a rating of at least 100 mpg for the Volt, which will be able to go 40 miles (64 kilometres) on battery power before tapping an onboard gasoline engine for a recharge. 

Bloomberg notes that GM is counting on the Volt’s unproven technology to leapfrog the Prius, the world’s best-selling hybrid, which starts at $22,000. Lutz said in May that the Volt would probably cost about $40,000.

The EPA rates the Prius as the most fuel-efficient car on US roads. The 2010 Prius is rated at 51 mpg in city driving and 48 mpg on the highway, according to the agency’s vehicle-efficiency web site. 

Toyota sold 158,900 Priuses in the US last year, 12% fewer than in 2007. GM is likely to produce around 60,000 Volts annually, once it goes on sale. 

Unlike conventional autos and hybrids such as the Prius, the Volt’s 1.4-litre engine will not drive the wheels but will only power the battery, which also can be recharged at a household outlet. The car is scheduled to go on sale late next year.