I was carefully observing my MPG this morning... and the numbers I saw confirmed something I've suspected for quite a while.

My home sits atop a 600' climb from the freeway (over a 1.4 mile distance)... a healthy climb, which generates about 3,000 rpm unless I crawl up. My current tank has about 240 miles on it. The drive from the freeway to my home dropped my displayed mileage for this tank from 51.4 to 51.1. It JUST went from 51.2 to 51.1 as I stopped the car, so it was "in the high ones".

This morning, I turned the car on and immediately left (no warm-ups, temp at 55 degrees, almost no throttle, rpms at 1,200). Within 15 seconds, it dropped to 51.0. Going downhill, it dropped to 50.9 and remained there to the freeway.

Here's where I have a problem. A cold engine on a downgrade at near-idle is consuming almost as much fuel as climbing an 8% grade at 3,000 rpm with a warm engine? This doesn't make sense.

I know a cold engine is far less efficient, but that seems extreme. I'd have to do some math to calculate the number of ounces of fuel consumed in both scenarios (engine volume, rpm, air / fuel ratio)... but a quick (and overly simplified!) calculation says that if two engines consume the same fuel over the same time period... and one is 3X the rpm of the other, then the air / fuel mixture of the slower engine is probably 3X that of the faster engine.

If I remember my shop classes correctly, a normal air/fuel ratio at sea level is 14.7 to 1 ("stoichiometric", as I recall!) . That means for 3X this ratio, it would have to be 4.9 to 1. I don't think an engine could even run at 4.9 to 1.

I'm wondering if the fuel isn't really being consumed at this rate... and perhaps this could be part of our 2 mpg error (displayed vs. calculated)?

Do others experience the same big hits to MPG on startup?