Edmunds states

the 918 Spyder gets impressive EPA fuel economy ratings of 67 MPGe (miles-per-gallon-equivalent) in all-electric mode, and a still respectable 22 mpg on gasoline. The EPA also estimates it can go 12 miles on electricity alone

I interpret that as the car being able to go 12 miles using electricity alone with no gas. How do they equate the 12 miles to 67 miles per gallon equivalent?

 

This is calculated in Hybrid mode, when the car will use electricity and petrol at the same time.