DETROIT — General Motors Co.’s changes to the battery of the Chevrolet Volt will extend the 2013 model’s range beyond Toyota Motor Corp.’s plug-in Prius in miles-per-gallon equivalent.

GM said the all-electric range of the 2013 Chevrolet Volt will be 38 miles on a single charge after the company increased battery storage capacity and changed the cell’s chemistry. That’s an increase of three miles. The model’s equivalent gas mileage rise to 98 miles per gallon from 94, the company said .

Toyota’s plug-in Prius gets the equivalent to 95 miles per gallon, and the company has said it is capable of traveling 15 miles on electricity alone. GM overtook Toyota in global sales last year .

“I always like it when our numbers are bigger — when bigger is better,” said Andrew Farah, Volt chief engineer.

The 2013 Volt goes on sale in August and its starting price will be the same as the 2012 version, Farah said. The 2012 Volt starts at $39,145 before a $7,500 US tax credit.

For the Volt’s batteries, “the best way to explain what we’ve done at the cell level is to compare it to a cake batter recipe,” Bill Wallace, GM director of global battery systems engineering, said in the statement. “We’ve done some work at the cell level to modify the ‘ingredients’ to make a better end result.”

The Volt was the best-selling rechargeable auto in the United States last month, with deliveries more than tripling to from a year earlier. That topped Prius and Nissan Motor Co.’s all-electric Leaf.