[JNR]

Another factor is the new gas you're getting may be have more ethanol in it (pure ethanol has 2/3's the energy and mpg of gasoline, so a 10% blend would probably be around 6-7% less mpg; although that would be ~1 mpg at most I'd think).

Check your tire pressure, since changing altitude will change your actual psi (here's a good explanation)

Don't rely on the ECU to give you 100% accurate MPG readings...even logging it manually *can* vary, since the pump doesn't always shut off same exact point on every fillup and when it's colder it likes to shut off sooner. Still, variations are probably minor and over a long term should be close enough for comparison.

Other factors could be dirty air filter, plugs (?), carbon build-up, etc...

Are you sure you were getting 20 mpg in the city though? Seems awfully high, unless you drive like a total grandma and even then...

Anyhow, here's a cut and paste for ECU and other resets:

Nissan Frontier (2005+)

Please read all instructions and be familiar with them before any attempts.
A stop watch or an analog clock with a second's needle will be useful.

ECU Resetting Procedures

The Easy Method:

After experimenting with different timing, and reading posts by other members, I've come up with a condensed method of doing this reset. This method works EVERY time, even w/o a stopwatch. I've used this method successfully a dozen times, or so, now w/o a watch. After a few times, you won't need a watch or instructions. You'll get so darn good at it, you'll find yourself doing it while waiting at a traffic light.

Timing is not as critical with this method...no stop watch needed. Just count in your head...one-one thousand, two-one thounsand, etc, or use stop watch. The most critical step is step #3....wait 10 sec, instead of 7.

1. Ignition "ON", count to 3.
2. Depress/Release pedal 5 times...hard and fast.
3. Count to 10 (not 7!) and fully depress pedal
4. Count to 12 (SES should be blinking), release pedal
5. Count to 10, depress pedal
6. Count to 10, release pedal.
7. Turn off ignition, then start engine. Done!

Accelerator Pedal Release Position Learning
Operation Procedure

1. Make sure that the accelerator pedal is fully released.
2. Turn ignition switch ON and wait at least 2 seconds.
3. Turn ignition switch OFF wait at least 10 seconds.
4. Turn ignition switch ON and wait at least 2 seconds.
5. Turn ignition switch OFF wait at least 10 seconds.

Throttle Valve Closed Position Learning
Operation Procedures

1. Make sure that accelerator pedal is fully released.
2. Turn ignition switch is ON .
3. Turn ignition switch is OFF wait at least 10 seconds.
Make sure that the throttle valve moves during above 10 seconds by confirming the operating sound.

Idle Air Volume Learning (Throttle Position Learning)
It is better to count the time accurately with a clock.
Operation Procedures
1. Perform Accelerator Pedal Released Position Learning .
2. Perform Throttle Valve Closed Position Learning .
3. Start engine and warm it up to normal operating temperature.
4. Turn ignition switch OFF and wait at least 10 seconds.
5. Conform that accelerator pedal is fully released, turn ignition switch ON and wait 3 seconds.
6. Repeat the following (steps 7a, 7b) procedures quickly five times within 5 seconds.
7a. Fully depress the accelerator pedal (HARD)
7b. Fully release the accelerator pedal.
8. Wait 7 seconds, fully depress the accelerator pedal and keep it for approx. 20 seconds until the Check Engine Light (CEL) stops blinking and turned ON.
9. Fully release the accelerator pedal within 3 seconds after the CEL is ON.
10. Start engine and let it idle.
11. Wait 20 seconds.
12. Rev up the engine two or three times and make sure the idle speed and ignition timing are within the specifications.

 

[JNR]

if you don't have a spacer, don't think you'll be able to get the coils out on the passenger side, just by looking at it. Probably be easier then to plan on removing it if you don't have a spacer (if you get one, you'll need to remove it for that, lol).

Yeah, I hate RTV if I can get away with it and most of my previous (older) cars the gaskets were one-time only (they get compressed and develop a memory), but since this is an extrude rubber, it's OK to reuse, within reason and don't use RTV on a setup like that, since the flat area that mates is less and not designed in this way.

for reference, I don't know if you dealt with carbs, but on holley's, the 'paper' fuel bowl gasket sucks, so you replace them with a 'rubber' one and if you put some vaseline on it, you can use it over and over again, which is nice when you have to keep changing jets

 

[JNR]

Yeah, we have COP (coil on plug) setup on these, so no ignition wires (except for the small AWG 'signal' wires) and the odds of a coil(s) going out is probably slim, so you should be alright. If these are like most COP's, then there is a spring inside the boot...if that gets corroded, then I could see some performance loss; think of it as the spark plug wire, fwiw.

Most oem plugs state they are good for 100k and this may be true (normal driving conditions, especially highway), although I've always changed mine at 55-60k regardless on my other vehicles that had similar plugs and the oems still look decent.

The only 'bad' part about going to a normal plug, is you do have to change them more frequently, but can still get 25-30k out of a set, I'd imagine...plus, these plugs are so easy to change, I don't mind doing it more often.

Least we don't have 16 plugs like I did on my Hemi (WK)...talk about $$ is you go with a premium one, lol.

I'm finding out it's worth waiting to 110k. I found a write up and some guides and it looks like the intake manifold needs to come off and gaskets will need replaced. I thought it was a simple untighten/replace/tighten job. I wanted a project to do to keep me busy and I figured the plugs would be good with the next oil change. Guess I need to look elsewhere for something to upgrade.

I've heard with a plenum spacer you don't need to remove the upper intake manifold, but that is hearsay...the upper manifold is not that difficult to remove, but does add another hour, let's say...it uses an extruded rubber (not really rubber) gasket that sits in a groove, so you don't *have to* replace them, although if you go thru that much trouble it probably is not a bad idea to use a new one. I've taken off and put on my manifold like 3 times already with the same gasket and no issues, fwiw.

 

[JNR]

Yeah, don't forget proper heat range too on the plugs I think I've tried the more premium (platinum?) NGK's once since they were on sale and they worked fine...I did however have a bad experience with a 'fancy' tipped plug where that little piece of metal came off somehow and needless to say, didn't work too well, lol...once last thing about tightening is do not overtighthen, or risk cracking ceramic and that'll make the plug pretty much useless...finally, always a good idea to apply anti-sieze on the threads on aluminum heads before install.

65 psi?!? holy crap, ha ha...I liek to go on the higher side, but that's crazy high. The bad part about too much psi is not only do you get a rougher ride (esp. on low profile tires), but you risk wearing the centers and can even decrease your contact patch with the road...40 psi won't do this though (not in my experience at least), but can imagine 50+ could.

 

[JNR]

The psi shown on the tire is generally maximum, so wouldn't use that...The sticker on the door is a good reference, but don't be afraid to go a little higher, provided you don't exceed maximum rated (on tire). I think general concensus is that ~35 psig is a good all-around pressure, although want to say 32 is what is shown...Personally, I like to do 38 psig cold.

Keep in mind that when doing a pressure reading/fill on a cold tire that it will increase a couple-few psi once it travels and gets warm (from friction)...I'd allow 2-3 psi on anormal ambient (say 65-70 F) day, when the tire's cold.

As for plugs, most any good brand works, although I do like NGK's nowadays and always had good luck with them. I always use standard plugs and gap them to spec. No need to get the fancy plugs as the only advantage to *some* of them is a little more mileage out of them, but despite marketing ploys, you do not get more power out of any plug, provided both of them being compared are gapped properly, tightened and ignition (coil(s), wires used) is apples to apples.