[embee]

Depends a bit what length journeys you're doing. A lot of fuel energy goes simply into heating up something like 3-400kg of engine+transmisssion+coolant circuit from a cold start. The shorter the trip the bigger the hit from warming it all up. When it cools again all that fuel energy is lost.

As for the ambient temperature effect, assume the normal running temp is around the 90C mark, if you start from minus 10C you're heating it through 100degC, if you start at 25C it's only 65degC.

A very simplified sum, 150kg of alum + 150kg steel (typical family saloon) heated 100degC takes 100*((150*0.9)+(150*0.5))kJ = 21MJ of heat energy . Typical fuel is around 43MJ/kg so half a kg (0.7 Litre) of fuel is required to supply the heat.

If you do a 10mile trip which uses, say, 1.0 Litre of fuel for motion+normal heat loss, plus 0.7 L for warm-up from minus 10C cold start, that's 5.9 mile/L (26.7mpg).
From plus 25C it's 1.0 L for motion+normal heat loss and 0.45 L for warm-up so 6.9 mile/L (31.2mpg).

It's a bit simplistic, but that gives an idea of the effects anyway.

That's not including the greater friction (oil) effects at low temps and a bit more heat loss to ambient too, so fuel for motion will increase and make the effect even worse.