Pushing a brick through the air isn't going to be fuel efficient. Also, I've noticed in my Kia that there's a sweet spot around 75, which is the new 55 that was around for decades. Going over 75 increasingly drains the tank. The faster you go, the faster the fuel gauge drops.
As the brick moves faster MPG go down but on the positive side - the car directly (and closely) behind you is getting about 100MPG!!
Technical term is "arse drafting"
As mentioned, the Grenadier has the drag coefficient of a brick. Drag increases according to the inverse square law...double the speed and the (aerodynamic) drag /wind resistance increase x4. This becomes significant above 50 mph. At 75-80 mph the rpm is approx. 2100-2300 rpm in top gear which is well into the flat torque curve (1750rpm-4000rpm for the B58 petrol engine), so the vehicle should pull like a freight train in 6th, 7th and 8th gear at those speeds! However, at 80mph the drag is approximately 2 1/2 times the drag at 50mph with a significant increase in fuel consumption. 3 tonnes is a healthy mass to steer at approx. 120 feet per second!
You will be driving a vehicle that has the aerodynamics of a washing machine - so the milage looks abysmal for normal 15MPG and at 70-80 mph it will be much worse. I had a Cayenne Turbo for 3 years and loved it - especially the sound - but the better it sounded (high revs/speed) the milage was horrific....I think I was averaging 12mpg at best...
So the group name for a collection of Grenadiers maybe should be peloton?
Physics.
