The cadillac DTS is a heavy (roughly 4000 lb), high horsepower car. Aggregates show around 15mpg city and 23mpg highway. If fuel economy is high on your priority list, I would suggest shopping around for a more fuel efficient vehicle. Here is a short list of some of the better sedans available with good fuel economy:
With 24 mpg city and 33 highway, the Toyota Camry has respectable gas mileage. Although plain, the vehicle comes with a stellar track record of reliability, a 5 star safety rating from NHTSA, and an affordable price. (A low milage 2012 model runs around $14k)
The Honda Accord boasts a 27mpg city and 36mpg highway fuel economy rating. The big drawback is power and comfort, but the Accord also has a 5 star safety rating, excelent reliability, and runs very similar to the Camry in price.
Gas milage is rated at 27mpg city and 39mpg highway, which is undoubtedly impressive. The power is nothing to crow about, but the Altima offers moderate comfort, a 5 star safety rating, and competetive pricing (A low mileage 2015 model with frills runs around $13k)
If luxury is more your style, then these cars might not be on the top of your list. If you want to shell out a bit more for some of the finer things in life, consider the following vehicles:
Gas milage suffers in the luxury line, but the TL still has a moderate 20mpg city and 29mpg highway. Simple luxuries such as heated seats and leather interiors come standard, and with 280 horsepower in the base model, the TL has some get up and go power. For the years 2009 - 2011, the TL has excellent reliablility ratings, with other years being respectable as well. The 4 star safety rating isn't stellar, but it is respectable. The big drawback is price. It's difficult (but not impossible) to find a TL with less than 50,000 miles that is less than $18k. If you're patient though, it can be done.
Gas milage isn't great here, but at 19mpg city and 27mpg highway, things could be worse. a 328 horsepower engine will certainly make things interesting, and the comfort is characteristic of luxury sedans. Safety data is more sporadic than usual, but what ratings there are indicate good, if not great crash protection. JD Power predicts an underwhelming reliability though, so if that's a major concern, you might want to avoid this vehicle. As with most luxury cars, price is quite a bit higher. If you want this model with less than 50,000 miles, be prepared to go right to the top of your $18k budget.
These are just a few of the many great cars available to purchase today. Hopefully though, this list will establish a base line and help you get started with your search. If you want my personal opinion, I'd go with the Camry. Reliability is huge in my book, and the Camry has proven over and over that it is one of the most reliable cars, period. If you'd rather have that extra power, tech, and comfort, it would be difficult to go wrong with the TL.
As always though, my advice for whatever vehicle you choose is to buy from a reputable dealer with a warranty, or have a certified mechanic that you trust give the vehicle a thorough inspection before purchasing. This will buy peace of mind, and offer some buyer protection against the many vehicles currently on the market with hidden mechanical issues. Good luck and happy buying!