well its true that the engines will provide more power at lower air temperatures because there's more more oxygen to burn. To achieve the same Mach number its true that you need more fuel going through the engine (so achieving the same mach number in cold air requires a slightly lower fuel burn). But thats not the whole story - TAS is the important number with respect to how fast you go over the ground, not mach number, and TAS increases with increasing temp. So a cruise of 0.8M at ISA plus 10 is faster over the ground that the cruise at ISA -10 at the same height (I think this is true and I'm quite happy to be told I'm wrong).
So working out the variation in the fuel burn for a given trip in a given time with varying ISA is a bit tough. And it gets more complicated than that because the aircraft will climb better in colder air (because the maximum power available is higher). So you get to height more quickly and therefore benefit from reduced fuel burn earlier.
I think you could work out a really complicated fuel burn schedule for maximum range which would constantly reduce as the weight of the aircraft reduced and also varied with local temp.
taint an easy question..