Originally Posted by Frosty
I have a Scan Gauge II, which is a diagnostic device that plugs into the OBDII port of the car and can give you real time feedback as you drive. One of the things you can get is instantaneous gas mileage.
I have found with several different cars that going from driving in gear to coasting in neutral at speed can make the mileage more than double (it would go up to around 60 - 70 mpg in the car I was testing). If you coast in gear, the mileage readout would say "9999", which basically meant I wasn't using any gas at all.
It may be lying to you when coasting downhill in neutral.