What could be the problem of high fuel consumptions

The first thing is to check for fuel leaks and for restricted intake or exhaust and ensure the routine maintenance is up to date.

It is important to know what the current fuel consumption is in miles per gallon or km per litre. The modern trend of paying set amounts of money gives a wholly false impression of actual fuel consumption.

When checked accurately over a period and averaged to take account of different types of driving conditions, freezing temperatures and so forth, mostly the result is found to be not as bad as first imagined.

If all checks are found to be ok and the accurately checked consumption is still too high further checks will be needed.

A few possible faults are incorrect fuel pressures, bad injectors, incorrect valve timing, "tired" oxygen sensors, bad thermostat, "tired" temperature sensors...

Wheel alignment, tyre pressures and binding brakes can also be factors...

Past due for a tune-up? Dirty fuel injectors, clogged catalytic converter, dirty/clogged air filter, low tire pressure, transmission fluid low...? Dirty fuel filter?

Check things other than the engine like; tire pressure/alignment, brake drag, transmission operation, excessive drag on any of the accessories (PS pump, AC compressor, water pump, alternator). Anything that is significantly hotter than the other side or something right next to it is worth looking into

try running a tank of fuel with an addative in , get one that covers the catalytic convertor as well as the valves' injectors etc , take for a long drive at a highish speed to get the engine to a good working temperature , you may have to repeat this a couple of times but should help with the fuel consumption at the end of it

driving in over drive mode actually is what gives the economy as the engine rpms will be around 300 less than direct drive . Lower rpms = fuel savings
OD should come in around 60 klm/hr and drop out at around 50 klm/hr. When you are accelerating hold the pedal in a set position and watch the rev counter (rpms ) and each gear change you will see the rpms drop and then build back up as the vehicle speed increases At 100klm/hr the revs should be around 3200 rpms or less but if over ( say 3500 rpms) then may be the OD is not coming in. One way to test a box operation is to find a speed shop with a dynomometer and test the gear selections against loads and engine rpms

Oil consumption for any vehicle can depend on a number of factors including engine construction, mileage, type of driving so it's a bit difficult to place an exact figure on it.
In the case of the 3SGTE in the Caldina GTT consumption seems to be average/normal, which equates to a drop of about 1/4~1/3 the distance between full and empty on the dipstick every 5,000km (3,000mi). And that's been my average for all 3SGTE's I've had.
If you have a high mileage (>200,000Km/120,000mi) unit, or drive enthusiastically your consumption may increase, but at that age brittle seals are more likely to be responsible for increased oil loss.

most engines return a good economy when in overdrive (5th)at around 3200 RPM or 60mph. This is the maximum torque band of the engine.

when was the last tune up?

Obvious things first. replace your air cleaner, fuel filter and put in a good quality fuel injection cleaner. do this for at least two fuel tank loads. Clean the injectors and make sure your engine is tuned. It will be fine after that...

make sure when you are traveling on the highway to have it in the highest gear there is it will be the first forward gear on the gearshift the one after neural. also check the plugs and wires and air filter. you might have to go to a dealer for the manual but you may be able to get it from a parts store