Am I wrong to worry about this?

New '06 XE, 2WD, 4cyl, 5sp.
Just under 1000 miles now. Filled up at different levels several times. Gas mileage has consistently been about 18.5 mpg. That's not too good for the 6, but for the 4 cyl, I'm thinking I got a problem. My '01 always got 22-25, and this one is supposed to get about the same. Any ideas/suggestions? When I first picked it up, it had 199 miles on it because the dealer brought it from elsewhere. Could anything have been done in that initial break-in to damage mileage or is there actually a chance that my mileage will still improve that much. I've read reports where people said that their mileage was consistent from the start and seen others say it gradually improved, but never that much. Help.

I have a '05, V6, 6 speed SE 4x4 and it was advertised as being 18 city 21 highway. I calculated it out and I was only getting between 15 and 17 on average. Maybe its because I drive it like I stole it. I don't know about the kind of milage you used to get, but I've heard that the manufacturers get these numbers with the vehicle sitting on a dyno at 40 mph. No real world numbers are used because that doesn't sell. Hopefully in your case it IS something that can be fixed but as for me, I just live with my sh*tty gas milage.

Two things that will help. Read up and read about the K&N Filtercharger on the counter of most auto parts stores(or make your own modified cone filtercharger). Get rid of sucking the intake air from the LF wheelwell. 2) I also removed stock muffler and added a Flowmaster exhaust, quit and efficient, which I bought narly new on ebay of course. On my V6, 5 speed I ave 20-22 mpg. There are a few other tricks which may however void the warranty.

Igbar is right about how manufactures get these estimated mileage ratings. They're taken the same way they get the results of their emissions tests. They sit on a automotive "tread mill" , if you will. Maybe great for emission testing but not very realistic for gas mileage. From what I have read lately. The government is about ready or has stepped in to force more realistic tests. But,all that said. 18.5 is a little low for a 4 banger. My 92 gets consistent 23-25. But my 86.5 got 25-28. Go figure. Maybe your Frontier is heavier??

same thing here, 03 XE I4 5 speed, it gets 20 mpg, don't matter hwy or city.
that's way off from the manfacutor rating.

I installed a cheap air/fuel ratio gauage.

that thing reads normal when I just start the engine,
then as the engine warm up it moves to too lean,
but anytime when the rpm goes higher then 3400,
it would just jump to too rich.

I have no clue, maybe the gauage isn't working well....

Before I found this website I did a search on Google for "problem with the 2005 Frontier" and came across this review it might help answer your gas-milage question

(not promoting the site, just an interesting review)

http://www.automotive.com/2005/12/nissan/frontier/reviews/index.html

I have a 05 4 cyl auto i usually drive 62-65 mph have a toneau cover about 2500 miles on it and i am getting 21 -22.3 I did get a 23.4 but had some tail wind on that tank and I use a little known additive 2 oz acetone per 10 gal

I have a 2003 King Cab with the 4 cyl. Been getting around 22 - 24 MPG for around 50,000 miles, even with the air on. Real happy with it.

the 06 Frontier has the new 4.0 engine VQ40DE Motor where your 01 had the 3.0 VG30E motor. Gas mileage to believe for 06 frontiers are right around what you are getting. 18 to 20mpg . Also If your enjoying the power of the 4.0 engine that could be causing the poor gas mileage also :)