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I just picked up a '99 V6 SLE last week and I'm a little conerned about the 18.5 mpg I am experiencing. What is everyone out there really getting?

Before I bought it, I did some research and found that it should get around 20/city and 28/hwy. This was one of the main reasons I got it because I wanted to save some money - both in mpgs and gas price (my Z was running about 20mpg combined on 91 octane).

The day I got it, the CEL light went on due to some bad O2 sensors so the previous owner had them replaced so it would pass emissions. I was hoping this would help, but it doesn't seem to have any impact. Would it take longer than a week for anything noticeable to happen with the mpgs?

I'm going to take it in to a shop nearby to have it checked out and didn't know if I should have them lookinto anything in particular in this area,

Thanks

Yeah, definitely something wrong. I usually get about 22 mixed driving and 25+ freeway.

my 99 se v6 5 speed 168k miles, get about 26.5 mpg mix and 28 to 30 on highway, no check engine light.

18MPG on freeway? there is a few majr things that can screw up mileage like:
-dragging brake caliper (may be pulling at low speeds)
-wheels out of alignment (car is pulling at high speeds)
-any CEL related to EGR, EVAP or running LEAN may impact MPG as well
-Engine Coolant Temperature sensor bad and cheating ECU on engine's current temperature (affects Air Fuel Ratio)
-severely dirty/imbalanced fuel injectors
-serious vacuum leaks (usually you could hear air hissing and idle RPM is high)

also when we talk about city driving only then max you can get is 17MPG when driving at WARMED UP engine.
if you do lots of short trips with car always being cold, then MPG in such scenario can drop easily below 15MPG (car uses more fuel in warm up phase). Short distance (.e.g. less than 2 miles one way each time) city driving is a double MPG killer.

You may want to get el cheapo OBD2 scanner for a PC laptop (ELM327 clone from Hong Kong ebay) and licensed (full version) copy of software like ScanMaster ELM and get some live data readings when car is running to see what's going on inside.
Also Air Fuel Ratio sensors should be replaced together for best results. Get OEM Denso brand off ebay, not worth paying extra $$$ to dealer for same part, they sell those real cheap ~$130 each on ebay ($330 at dealer).