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.

Depending on your mileage I would think it is time for a tune up. If you have had one of those recently then it could be your fuel injection, air filter, or sparkplugs. I have a 2006 Rainer as well and get around 18-19 miles a gallon. You should try changing you spark plugs to the bosch platnium plus 4.

well it depends a lot on your driving habits as well. Try changing the way you rev up or shift your gear. Another cause can be screwed up engine, which can for various reasons. http://bautos.net/

modern engines use cam and crank sensors to tell the engine control module what the engine is doing. I know you have a straight 6 but on 3800 V6 Buicks, if the cam sensor fails, the engine goes into "limp home mode" and that usually affects the mileage. Take it to AutoZone and have them run the codes. You can then get on line and decode the codes and probably track down the problem