The trick is, that for the first 40 miles (on average) of any drive, the petrol consumption will be zero.

If you drive less than 40 miles, and recharge before the next trip, you now get to go 40 miles before the petrol consumption starts to count.

If you only did a 40 mile round trip to work and back each day, after 6 days you would have 240 miles on the odometer, and your petrol tank would be as full as it was first thing Monday morning.

GM has factored in that during this theoretical week you did actually use just over a gallon of petrol - hence the theoretical 230 mpg.

Which is somewhat misleading, for they should be required to give the equivalent petrol usage "consumed" by each of the battery recharges, don't you think?

Regards

BE2