Fuel economy -- stumped

[SnowDrifter]

2003 subaru forester 2.5x / automatic

If I get in my car and drive it (let it warm up for 30-45 seconds until the fluids are circulating) I get 17-18mpg.

However... if I start my car and let it really warm up for 7-8 minutes. Enough to let the engine reach operating temperature, I'll get 22-23mpg.

And I'm not reading off of any display. I'm calculating this with the trip meter and how many gallons it took to fill up my car.

I'm not opposed to getting into a cozy car on a cold day.. But I'm wondering now, what would cause me to get such a difference in fuel economy just by warming up the car first. To be honest I figured the warm up would cut my fuel economy since the car is sitting there sucking gas but going no where. Makes me wonder if there could be some sort of underlying issue?

[Trey_Dog650]

Always warm up the car so it's at a good operating temp

[Baydestrian]

When fully cold my car idles at 1k, that's all emissions though, I usually wait til it finishes that cycle and drops to normal idle which is about 2 minutes. Letting your car fully warm up actually wastes gas and is unnecessary, some places even have laws where its illegal to do so LOL.

Check your plugs, I bet they are shot, when they are cold they aren't fully igniting so you are wasting gas but when they are warmed up they are fine.

[SnowDrifter]

Not fully end of the tank. I'm a fill it up at 1/4 kind of guy. But it's still 10 gallons. About a weeks worth of gas. Took the average over 4 weeks

Spark plugs have 8,000 miles on them. All fluids are fresh. I'm a freak when it comes to that sort of stuff. Coolant has 8k on it, diff fluid has 3k on it, engine and transmission fluid were changed 600 miles ago. Air filter gets cleaned every other oil change (about every 7-8k). It's a k&n filter

[Miguels]

cars tend to run rich when you first start them and till it gets to operating temperature.. remember carburetors having a choke? well fuel injected cars do too.. the cold weather happens to mess with your gas mileage too

[Baydestrian]

So you started 4 weeks ago?

Hmm, how long does it take your car to hit operating temps? I'm pretty sure we are now on Winter blend gas too, I've noticed a drop in my MPG by about 2mpg

[SnowDrifter]

This has been something that's been ongoing for the past couple months. Originally started out trying to see how much gas I was wasting letting my car warm up vs. Not (comparing the cost of warm up vs. Electrical costs of an engine pre heater) Ended up getting data like this lol.