I assume you are after fuel mileage increases, the gage readings are not part of this issue, correct? If so, look for fuel trims on your scanner long term will give you the most info, this time of year, in the nothern part of the US, it has not gone above freezing for 2 weeks, my 3800 Buick is down about 3 mpg. My engine is running well. I would suspect driving conditions, number one, maybe a tired o2 sensor number 2. There are no live fuel trim readings in freeze frame, don't use this mode to check live data such as fuel trims or 02 sensor activity.n Also does the engine reach 195 degrees fahrenheit in a reasonable warm up time?