We've had our 2007 Equinox since October of 2008 and it currently has almost 178,000 miles. I very rarely use any fuel additives. I can probably count on one hand how many times I've done so. I don't think any of them make much difference in how the engine runs or affects fuel economy. I put in a bottle of Chevron Techron fuel injector cleaner for a couple tanks to clean the pickup on the fuel sending unit because the fuel gauge was inaccurate and it worked.

In cool weather (below about 40°F), 17.6 mpg seems about right for mixed driving. Most days we get about 18-19 mpg city and about 22-23 mpg highway. Sometimes it's a bit more. We have achieved as much as 27 mpg on a couple long trips. We always use E10 (10% ethanol) fuel and synthetic 5W30 engine oil. I follow the oil life meter for oil change intervals which are usually around 5500-6000 miles. The engine is usually a half quart low between changes, but I think the engine has a minor leak around the front main seal.

All of my fuel economy numbers are based on actual calculations - not what the DIC (Driver Information Center) tells me I'm getting. The DIC is generally accurate to within 1 mpg, and usually indicates slightly better fuel economy than what I calculate.