Buick Rainier Forum : “what could be causing me to only get 12-13 mpg ”

According to www.fueleconomy.gov, the Ranier is only rated at 14 city, 20 highway.  These are EPA estimates.  In the real world, those numbers are usually slightly lower for SUV's.  While 12-13 seems painfully low, it is not terribly unusual for this model.  If you have close to 100,000 miles, a tune-up consisting of spark plugs, air filter, fuel filter, and upstream oxygen sensors will restore any fuel economy that may have deteriorated over time.  However, if your car has less than 100,000 miles, it is not likely to benefit greatly from this work.  Unfortunately, I do not think that your numbers are terribly out of line for what can be expected for that car.