I would like to see you drive 640 miles on a full tank before refuelling .
do you mean the range display says RANGE 640 MILES after a fill when you have been cruising motorway .
my V8 says optomistic range after a refuel whilst cruising but always needs refuelling way before I get close to the previously indicated range .
I calculate the actual MPG based on how much fuel I put in against miles covered .