Wy am I suddenly getting terrible gas mileage?

You may have tire pressure issues, fuel injector build up, even a plug not firing - when was the last time you had the injectors cleaned? Take a look at your service records and suggested service calendar, and also give your mechanic a call if you don't do this work yourself. Talk it over with them as well....

It may just need a tuneup (changing the spark plugs and spark plug wires.) The spark plugs are always in fire and literally burn up, then they can't explode the gas in the engine with as much power. Idling doesn't generate much power to start with, bad plugs reduce even that and Presto--not enough power to even keep the engine going by itself.

This needs to be done every 25,000-30,000 miles, or you will lose power, burn more gas, fail emissions tests, possibly ruin the catalytic converter because of the unburned fuel passing through it (like I did) and have it die on you unexpectedly. If you can't remember when it was last done, I'd start there. And write it down and put it in the glove compartment, and write it on your calendar twice a year to check to make sure you don't forget it. but of course, you should be getting a 30,000-mile service at 30k-60k-90k miles that includes it...

My 87 Civic went 240,000 miles, dying only because I failed to change the timing belt, and whenever I thought it was dying it just needed a tuneup.

Best regards,

Mike

When I encounter a car that suddenly drops to half its normal fuel economy I always think of the thermostat first. If the thermostat is not closing the engine will never warm up and cold fuel consumption is right at twice the fully warm consumption. Testing is easy enough. Feel the upper radiator hose as the engine warms up. It should stay cool until the engine reaches operating temperature, then should quite quickly become too hot to hold. If it gradually warms up as the engine warms up the thermostat has to be replaced.

2003 Cadillac Cts Mpg

Bad gas, clogged air filter, fouled plugs. Go and have a diagnostic done by a shop you trust.

Maybe there is a leak in the gas tank and you are leaking gas. I would go to a garage and have it checked out.

You would have something wrong with the fuel system. I would take it in and have it checked right away.

2003 with 100 K plus miles?...tune it...