this may be well known but the temperature here in the bay area has wamred up a bit - to over 60 (it had been in the 40's and 50's). on my commute, the auto-stop would not kick in until about 10 minutes. now, the auto-stop is kicking in almost immediately - before i get to the end of my street. i surmise this is having a nice effect on my mpg as it has also risen on this tank. anyone else notice this?

 

Neil,
Yes, temperature makes a huge difference. There are quite a few threads on this site that discuss it. It all has to do with how quickly the engine warms up, which of course is dependent on the starting temperature. Hopefully you'll have a long lengthy gas saving summer with autostop doin' its job!

 

It's not just the AS that's increasing your mpg - from my experience so far (driving between -15F and 50F) there's about a 2 mpg difference for every 10F difference. This is independent of AS, as I hardly have to use that at all during my commute.

 

It would be nice to have a day here where the autostop kicks in immediately *sigh*

 

I'm with you. 36 degrees this morning, and I thought we were having a heat wave . I let my block heater cook an extra couple of hours last night and got AS at only 2 miles, but I'm not sure I want to do that all the time....

What I really hate is when I hit that first traffic light just turning red as I approach it with only .7 miles on the trip and the temp gauge setting at about 3 or 4 bars. I sit there and watch my trip mpg drop like a Hummer off a cliff. I never thought I'd actually consider moving south because of the car I drive!

 

A Hummer falling off a cliff would be getting huge fuel economy Until it reached the bottom, of course

 

Good point. How 'bout "a Hummer racing uphill towing an Escalade?"

 

lol!

Why is it that I almost always get a .4ish drop in mpg before I even get down the driveway? The gas it takes to start up can't be that much, or FAS wouldn't work. I guess it's probably just the cold weather- I don't remember whether or not it happened back at the end of last summer, but I'll be on the lookout when it gets warm again.

 

i see the same drop when i first start the car. my guess is that when the engine is cold, the control module is burning very rich fuel (not lean burning yet). as the engine heats up to reach its optimal operating temperature, the richness of the fuel drops down. that's my guess...

 

One thing that I have noticed though is the amount of fuel burned at idle before warm-up and after is extremely different. Based on readings from the instant fuel gauge, running cold appears to use about three times as much fuel at idle as running warm (>0.25 gallons/hour @ 1200-1500RPM vs <0.1 gallons/hour at 650-800RPM), which implies the engine both revs faster AND runs richer than when warmed up.