>How does driving less give you better gas mileage?
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>http://www.boston.com/business/personalfinance/gallery/fueleconomymyths/
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>"At roughly $4 a gallon, many people are desperately trying to squeeze more miles out of each drop. The best ways are usually the most obvious: reduce your load, resist the urge to drive aggressively, and cut the number of trips you make, for starters. However, there are some unusual tips that might surprise you."

Reducing the number of trips basically reduces total distance driven, not mileage - but that translates to less $ spent on gas no matter what your mileage is, and less $ is the main point of the exercise. Basic stuff like getting 2 or 3 days' groceries at the store instead of just 1.

Cutting the total number of trips usually means combining trips. Suppose in a given day you have to go to 3 different places (A, B, and C) and also suppose that at most one of these has to be at a specific time (say, A).

Scenario 1 - separate trips at widely different times of the day.
Distance = Home -> A (to make your appointment), A -> Home, Home -> B, B -> Home, Home -> C, C -> Home (6 legs)
Car must warm up at the start of each round trip, which is bad for mileage

Scenario 2 - combined trips.
Here you get to do a little planning to make sure you make your appointment time at A, and to minimize total driving distance between the individual stops A, B, and C.
Distance = Home -> A, A -> B, B -> C, C -> Home (4 legs). This distance in most cases is less than Scenario 1.
Car must warm up for first trip, but will likely cool off less between legs of a combined trip so overall, less warming up is required.

So, combining trips gives you less distance traveled, and probably better mileage while you're at it.