Why does my Skyline consume more fuel than a Mark X?

Hi Paul, I appreciate your response to my recent enquiry about changing my Prado TX transmission. In fact I have dropped the idea after I read and understood the cost implications.
However, I have two questions today: Why does my 2001 Nissan Skyline with a 2.5 CC V6 engine consume more fuel than a Mark X of the same engine size when I drive at 80kph.
I need to know how and when to use the semi-automatic mode the (+ and - ) located on the transmission chamber shift and the power button, which lights green and red when you press it.

Thanks, Silva.

Hello Silva, your 2001 V35 Nissan Skyline with a VQ25DD 2.5 litre V6 petrol engine has got close fuel economy and performance statistics with the 2004 Toyota Mark X that has a 4GR-FSE 2.5 litre V6 petrol engine.
You may need to service or tune your engine, make sure the tyres have good tread depth and adjust your driving style (aggressive acceleration and frequent hard braking) in order to improve the Skyline’s fuel economy figures. The Skyline’s 2,495 CC and Mark X’s 2,499 CC engines should give you a close combined driving fuel economy statistic of 12.0
Interestingly Skyline’s slightly lower fuel economy may be due to its higher torque rev curve (270NM@4400rpm) compared to Mark X (260NM@3800rpm). It’s no wonder the Skyline engine is slightly more powerful (215 hp/212 hp) and accelerates a little faster than Mark X (0-100 kph: 7.5 secs/8 secs).
Both Skyline and Mark X have reasonably efficient 24 valve engines with Variable Valve timing and five speed auto transmissions with tiptronic. The Triptronic automatic transmission on your Skyline adopts manual transmission characteristics.
Triptronic allows a driver to bypass the automatic gearbox computer using the torque converter like a manual transmission clutch.
The triptronic feature engages drivers with passion for performance and gives them more control and timing of shift points for more power.
When you select D (Drive) on your gear shift console you can tap the stick shift to the right to slip into steptronic mode where you shift up to higher gears with one gentle tap to the right (+) or shift back to lower gears with a gentle tap to the left (-). The power button on your gear console is used when you want to delay shift points for increased power, while driving in automatic mode.
When pressed down (red light) it will be like delaying to shift from lower to higher gears using the manual transmission.