I know this has been covered ALOT, however after some searching I'm still stumped. I bought my car about a month ago now, with 88k on it. I've been getting ~20 MPG with a 50/50 highway/city. I've changed the spark plugs, pcr valve, oil/filter, fuel filter, air filter, seafoamed the car, and checked tire pressure. I'm not getting a CEL so i've no clue why my mileage is so low. The car runs and idles well, and i think if my o2 sensors or EGR valve are bad, then my CEL would come on.. i don't think its the coil packs or wires since i can't feel any sort of misfire.
Off topic, after seafoaming the car via the brake booster line, my oil pressure is now lower, around 30 PSI while idling, 40-50 under acceleration where as it used to be 40-50 while idiling and 60-65 when accelerating. Weird.

'98 3.8 5speed

 

That's not low, it's normal. I've had my 94 3.4L 8 years, and always gotten 21mpg. Even threw all the gas/mileage data into a giant spreadsheet for trend tracking- nope, always 21mpg. Sometimes it'll push 30 on long trips at highways speed, but not commuting to/from work. I'm about 50/50 city/highway, too, btw.

Only time it changed appreciably is when I went a month driving with 5 cylinders. You'd think it would be 18% better mileage, but nooo....

 

That's just what they get. I pulled off almost 30 once driving from my house into town, but it's all downhill from my place to town, and there's another way back home that's less of a grade. I can litterally go most of the way to town in neutral. And that's how I did it. Otherwise, I get about 23, and a little over 30 on the highway.

 

Update: My latest calculations show that i'm getting a dismal 15MPG. No WOT, or hard acceleration, keeping RPMs below 3k. Suggestions? Intresting thing, when i'm in fifth at a low RPM (i.e. 45 in 5th) and i give anything more than 1/4 throttle, the car starts stuttering badly...

 

RPM doesn't mean ****... It's all in your foot. And my 1999 3.8 gets about 16-18 city driving if I don't get excited... About 22 on the highway... This is with 93 octane fuel as well...

 

How many miles does your car have on it?

in simple terms more RPM's = engine spins more = more fuel squirted for combustion

RPM is important, a car with _ cylinders travelling 70 MPH @ 1,500 RPMS will get better fuel economy than a car with the same __ cylinders travelling 70 MPH @ 2,500 RPMS.

RPM's are important for gas mileage.

 

Not enough gas, too low of RPMs.

 

20 is average for the cars, and bad, i agree that RPMs are important, but there's a point where lower RPMs are worse for MPG than higher RPMs. my car gets best gas mileage if i shift around 2800 and cruise around 2200, any higher it gets worse, and lower and it gets worse than higher RPMs.

 

Yep that is true, but 90% the time you will get better gas mileage 750rpms below "max efficient" than 750rpms above that max efficient rpm


20 is average for city...highway should be more than that

that is why often LS1's will get equal if not better MPG, because they produce more power at low rpms than V6's

4 cylinders, DOHC (think the new ions are DOHC) are alot different than a OHV V6

 

It would be sad if 20 mpg was average... That's what my AWD 5.0 Mounty gets. My Camaro gets 25+ mpg on a regular basis. Of course I know how to keep my foot out of it.