It is an interesting topic that most people here would like to know. I can see a lot of complains about bad mpg in this forum. The question is why don't you get the same mpg or better than as claimed by Lexus. The answer is simple, it's because they don't drive the car the way you do. If you are to start off from the same starting point sure you will arrive at the destination a lot earlier than them. The claimed mpg is not just based on Lab Test but it is indeed achievable. They don't test the car through heavy traffic or traffic jam like condition most people experience daily. They drive like granny on a reasonable traffic and road condition with minimum braking. There's always a tolerance say +-5% since fuel comsumption depends on many factors and the characteristic of the car. Every car has its own characteristic, ie. you will not find two exactly the same car (They might look exactly) as the material used in any part might vary slightly and similarly for the assembly line. Even the wheel nuts might be of some degree tighter or looser

You may greatly improve the mpg by following the points below :
1 - Avoid hard acceleration and braking : Drive with very light foot on the accelerator and avoid as much braking as possible. Say you're going at 45mph and the red light traffic is approx 500m ahead, you know your car will get there even if you lift your foot from acceleration. Don't rest your left foot on the brake of an automatic vehicle - the extra drag increases fuel consumption by at least five percent.
2 - Flow with the traffic : Smooth driving can reduce fuel consumption by up to 30 percent.
3 - Watch your speed : By travelling at 90 km/h on multi-lane roads rather than 110 km/h you can reduce duel consumption by 10 percent or more.
4 - Lighten the load : Take off roof-racks and bullbars if they're not needed. Remove golf clubs or tools if you're not using them and reduce fuel consumption by up to 5 percent. Keep you tyre pressure towards the top of the manufacturer's recommended range.
5 - Switch off : When stuck in traffic for any length of time turn the key to the "accessory" position (not to "off") and if an automatic transmission, shift to "park:. Vehicles fitted with this feature as standard typically reduce fuel consumption in urban areas by up to 15 percent.
6 - Open the windows : When the weather is fine, switch-off the air-conditioning. This reduces fuel consumption by up to 10 percent in urban driving.
7. Service your car regularly : Blocked air filters can increase fuel consumption significantly.