Read the book. They have graphs of how long it takes a cars coolant to warm up with the heater on and off... and they also have the miles driven vs temp vs mpg.

Another thing I found out is that if you have two cars... say an SVX (fuel pig) and a civic (good mpg), you won't always get the best mpg int he civic. you should ALWAYS take the car that's already warm. If they'r both cold take the civic, but if the SVX was running a few minuts ago, you'll get better mpg with the car that doesn't have to warm up.

Idling is actually the worst thing you can do for your mpg... it uses quite a bit of gas, you aren't moving (hence 0 mpg) and hte low speeds of idling is VERY VERY VERY inefficient at warming up a cold engine. On top of that, even if the engine is warm, the transmission, bearings, CV joints are all still cold, so you're still going to get bad mpg for an even longer period had you just started driving the car.

Seriously, read the book.