There could be a complex, electronic, fuel-supply or engine-management reason for this, and if that's the case, having the vehicle scanned would be a good place to start. Or, it could be a case of human behaviour that is skewing your fuel economy figures.

By which, I mean that when you got the car brand-new, you gave it the kid-gloves treatment, understanding that the brand-new engine needed to be run in and would appreciate a slightly easier start to life. Then, as the novelty wore off and the vehicle began to earn its living, you subconsciously grew a heavier right foot. And it is this that's seen the fuel consumption creep up.

Failing that, you might have an on-board computer that has been silently recalibrating itself, perhaps in conjunction with software updates at the dealership which you may, or may not have, been made aware by the dealer. To be perfectly honest, I'd find 6.4-litre per 100km average on a Colorado borderline unbelievable, and the sort of economy only a dedicated fuel-saving driver could even get close to. Ten litres per 100km, meanw

I'm curious to know what happened when the hill-start assistance kicked in on the flat. Since this system holds the car on a hill so you can move off easily, it should deactivate as soon as you press the throttle to move. Which means, on a flat surface, you shouldn't be able to feel the system at all. Are you sure this is what's happening. Could the problem instead be a brake caliper that is sticking on, or a hand-brake that is dragging?

In any case, the hill-start system is controlled by sensors that detect that you're parked on a slope and then apply the brakes to stop you rolling back. If any of these sensors is kaput or not sending the correct signal back to the computer, the set-up won't work properly. Conceivably it could engage on the flat but, again, I don't know how you would know.

But the solution is a simple one; take the vehicle to a Holden dealer who will be able to figure out what's wrong and then fix the problem under warranty. Your car is covered by a five-year warranty, so make the most of it.

By the way, disconnecting safety gear is not the best way to go as it can (obviously) make the car less inherently safe and, in some, cases even render the vehicle technically unroadworthy.

The HiLux would be my first choice, followed by the Colorado.