When checking fuel economy you must take into account any modifications and accessories on the vehicle.
A roof rack, winch, rock sliders, Dual Fuel, rear bar, second or even third battery, shelving, aux fuel tank and lift etc, all add to the weight or wind resistance of your Patrol
Tyre size, type and air pressure must also be taken into account.
How you drive is a big factor.
My fuel economy has always been fairly consistent at around 14.7-15.5/100km.
Off road and towing it can rise to 18-19 l/100 km, or even more, If I hammer it.
I haven't tested it since the 2'' lift.
Hopefully, someone with experience with vacuum gauges will be able to answer your question.