Let's say you keep your TAS the same. Now, up high, you can't go as
fast as down low, full throttle. But at full throtte at 10,000'
DENSITY ALTITUDE say you have a TAS of 120 knots, and a manifold
pressure of 20". You burn, 8 gph, getting 15 miles/gallon.

Now at 5000' DENSITY ALTITUDE, to run at the same 120 Knots TAS, you
need 22" of manifold pressure, and burn 9 gph, getting 13.3 miles/
gallon.

Now, when it gets colder, the same true altitude is a lower density
altitude. So running at the same TAS, when it is cold, you get worse
fuel mileage than when it is warm.

So if you keep the TAS the same, the true altitude the same, the
barometric pressure the same, you will get WORSE gas mileage when it
gets colder.