Over the years of reading 4WD magazines and owning a few different cars, these attributes affect fuel economy.
1/ How many cylinders? More cylinders generally use more fuel.
Although, back in the 70s my mates Holden with a 253 eight cylinder was more economical than my 202 six cylinder holden.
2/ Engine capacity. Bigger engines use more fuel.
Again, depends, later models are better at conserving fuel and the vehicle itself is a good part of the problem.
A Ford with a 2.2 litre diesel engine I drove for work used more fuel than my 3.0 litre Isuzu MUX.
3/ Weight of the car.
Yes agreed
4/ Size of the tyres.
Yes agreed
5/Aerodynamics.
Probably the key factor in many cases The coefficient of air friction/ turbulence over vehicles really kills.
Surprisingly/ or not, motorcycles are the worst.
6/ Height off the ground.
See above
7/ Number of gears in the Auto/manual
This makes a difference only in the higher gears while cruising.
But autos used to be less efficient than manuals once, not so much these days (but there is some loss in the torque converter)
So, your common landcruiser/patrol is a 6+ cylinder, 4+ litre, heavy, big tyres, high off the ground AND has poor aerodymanics.
Some people modify them, invariably that makes the fuel economy worse.
It is always going to use a lot of fuel.
Sometimes it'll use a HUGE amount of fuel.
My brother in law has a Nissan 4x4 with a 4.2 litre engine.
My wife has a Suzuki Ignis a tiny 800kg car with a 1.2 litre engine, she gets around 5 to 5.3 L/100k