2006 Saturn Ion poor gas mileages

1. Misfires / needs a tune-up
2. Dragging brakes
3. Coolant not reaching proper temperature

With all the electronics the insane engineers are forcing on us, this may no longer apply to your car, but in the past, the dash gauge and the Engine Computer used two different temperature sensors. Sensors for the dash gauge are notoriously inaccurate, but all that's important with those is you notice when there's a sudden change from what you're used to seeing. Sensors for the Engine Computer are much more accurate, and since there will never be two sensors exactly alike, the computer will learn its characteristics by comparing it to other sensor readings and operating conditions.

The only way to know for sure if the engine is reaching proper temperature is to connect a scanner to view live data. That will show you the signal voltage and the corresponding temperature that the computer has calculated. Normal temperature will be around 195 to 220 degrees. If the thermostat is opening too soon, you'll see somewhere around 170 degrees or less.

There's all kinds on eBay. I use the Chrysler DRB3 because it works on all brands of cars but that uses an older computer language. I think you're going to need the "CAN BUSS" system. There are a lot of scanners that cost less than $100.00 but they are very slow. Professionals don't have time to wait for the display to update, but they work okay when you just need a little information.