My 2000 civic has 126xxx miles on it. It's getting about 28-29 mpg highway @ 70 mph. I have new tires inflated to specs, new plugs, new air filter, and a new (non Honda) pcv. The check engine lite is not on and it runs excellent. I was wondering if it shouldn't get better mileage than that.....My question is could I have a bad o2 sensor and it not cause the check engine light to turn on, also which sensor (up or down stream) would be more likely to cause poor mileage? Any other suggestions?
thanks

 

If the CEL is not on, and if it works (comes on when you first start the car, then turns off), then it's probably not your o2 sensors.

Mileage depends on a lot of factors. What size rims and tires do you have? Are there extras in your car, like a subwoofer enclosure with a couple of subs in the back? Is the car modified at all?

One thing you might try doing is going to your local Autozone or Advance and buying a can of this stuff called Restore. Get the 4 cylinder can. Toss that in at your next oil change. Drive about 200 miles and see if you notice a difference in fuel economy.

 

The cel does come on with the key on. The tires and rims are the original size.
No subs and no modifications, everything is stock. I've used restore in other vehicles and have had good luck with it so I'll give it a try.

 

I also have a 2000 LX and using Restore gave me +3 MPG, bring me up to about 31 MPG. My engine is just worn, though I only have 130K miles on mine.

 

That mileage my be about right.
You'll get your best gas mileage if you keep it under 60.

 

yeah what Hobo said. A constant 60 for best mileage.

 

Drive no more than 65 mph on hwy.
Air up tires to max tire wall pressure. (I run 45 psig on my 99 Honda Civic EX)
Do partial grilled block up front.