Warm air = better mpg?

[Dave777]

I like TheGman's convoluted logic, but it's sort of assbackwards.

Back to the original question of seeing the MPG increase as the intake air temp increases...

It's totally coincidental. I too have a ScanGauge and it's a valuable tool, but don't misconstrue one variable as influencing the other.

The MPG increases as the car warms up simply because everything is warming up; the tires, the engine and transmission oil, the bearings in the wheels. It's the reason why people who only drive short trips never get good mileage, and why your ScanGauge will eventually peak out on the Current MPG reading, at a steady speed, after 10-20 minutes.

The warmer intake air temperature is slightly decreasing your performance, but the real effect you're seeing is simply the overall warming of the engine and tires. This is also the reason why mileage is better in warmer weather; the car starts out "warmed up" if you will.

[sunnyokanagan]

Originally Posted by Macaw

Maybe I'm just old but cold used to mean running a richer mixture = more gas used and warmer was leaner mixture = less gas used....

You are correct, sir. If your car is driven conservatively, that is. All the hyper-mile clubs will tell you this is so.

For peppier driving, however, we look for lower intake temps.

[SuperGeek]

Originally Posted by Dave777

The MPG increases as the car warms up simply because everything is warming up; the tires, the engine and transmission oil, the bearings in the wheels. It's the reason why people who only drive short trips never get good mileage, and why your ScanGauge will eventually peak out on the Current MPG reading, at a steady speed, after 10-20 minutes.

Yup. In the winter, on our 20 degree days, my MPGs were in the 27-30 range for a 4.5 mile commute. Now warmer, the same 4.5 mile route, etc., I'm back up to 35 average, 40ish on a really good day.

It's really noticeable if the water temp doesn't get above 160 before you get to work and have to shut off. Once it was so cold it never got above 140 - slow steady traffic and I doubt I passed 2500 RPMs more than a couple of times. I think I got something like 25MPG that day, I just gave up. This is why you can't obsess over mileage. Short trips, weather, traffic, all will frustrate your efforts to maintain that 50+MPG average.

[bluedeuce]

My commute is almost entirely highway, about 15 miles a pop at a steady 55-60 mph. With the IAT anywhere below 85 degrees, the scanguage reports low 40s for mpg (after the usual 5-minute warmup period) but when the IAT is anywhere above 95 the mpg jumps into the low 50s. Same route, same speed, same gas. Assuming it's not some quirk of the scanguage calculations, I'd bet on the fuel maps in the ECU leaning out as the IAT climbs from 85 to 95.

[Smart Enuf]

Perhaps what you're experiencing is the switch from wintertime oxygenate gasoline to summertime gasoline? The wintertime gas ends at the end of Feb. or March in many states, so it would help explain at least some of the mileage change.

[bluedeuce]

I thought about that, but it happens on the same tank of gas and even on the same day. The only constant seems to be outside air temperature (the intake temperature is consistently about 30-35 degrees over ambient at cruising speed).