Poor gas mileage

I have a 94 cadillac STS. The car runs great but it only averages about 15 MPG. My wife has a 98 Deville that gets around 20 MPG. What could be causing
the poor mileage on the STS? Thanks
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causing
1) Different engines. The STS is a "sports car" with a high performance engine. Gas mileage was not at the top of the design specs. The SLS I have now and the STS previous got maybe even a tad less than that around town and about 26 on the highway.
2) Different driving styles. Your foot a little heavier?
3) Are YOU using premium gas? Believe it or not, this might increase your mileage, if you are not using premium already. (This difference might be noticeable only on the highway, however.)
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Chevy Man wrote:

Not sure what engine your 94 is but I had a 99 Northstar that I notice the gas mileage had dropped recently. If you have O2 sensors in that engine and it got 100k miles its time to change them. I changed the front O2 sensor and went from 20 to 22mph in Atlanta city driving. It gets about 28-31mph Hwy depending on weather its hilly or not!
I have also heard that a coollent leak can damage O2 sensors. I did have a busted head gasket causing coollent to leak into the exhaust. This may have caused the O2 sensor to go bad!
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I agree with the poster above, The first thing I would check is the 02 sensor. That should help out a lot. I am sure you have changed the air filter also. Getting fresh air into the engine always helps.
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