When I first got my IS200 I used to think that the MPG reading was useless. Actually, now I think it's quite useful. As long as you're cruising then I think it can tell you something useful, e.g. give you a rough idea of MPG. Though you can get the best feel for it by measuring your mileage and then how much petrol it takes to fill up etc. Fuel economy isn't the car's strong point, but on long runs you can try engaging "Snow" mode - unlike on some other cars this just changes the throttle response (making it less responsive).
It sounds weird though if your MPG figure seems to go *up* when you accelerate, that doesn't sound right at all, your MPG should obviously be decreasing. If the reading is above 40 for most of the time then that also doesn't sound right.