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.

 

altitude WILL affect the fuel economy. im not sure if higher altitude would be benficial though as its typically thinner air. perhaps its the change in temperature from a colder air to a warmer air that did the change. or...at least contributed to it. i would do an ECM reset, it cant hurt. there are some threads out on the forum that give step by step instructions on how to do this yourself too...i know theres one related to the phenolic spacers, could start there in your search. its a long thread though so id start at the end and work backwards. that thread should take you to a build thread, and in the beginning of the build thread, they show how to reset ecm. hope that helps.

 

I found it. Thanks for the directions.

I also figured the altitude would make for worse mileage as higher air is thinner air and thinner air doesn't compress well. And actually it has been colder here in Vegas than it was in Colorado. Maybe the humidity has a slight affect? Who knows. Can you do an ECU reset by just pulling the ECU fuse? I checked both fuse boxes under the hood and could only find an A/T ECU which I'm assuming is for the transmission. Am I looking in the wrong places?

I'll try the pedal reset trick, but I'd like to avoid the battery reset because I don't know if I have all my tools with me. All of my stuff is still in Colorado.

 

youre welcome for the directions. i would try the pedal trick first. i dont believe pulling the fuse will reset the ecm..its possible, but i believe that until the ecm is commanded to do a reset it wont. pulling the fuse just makes it power down and may give you a check engine light if the ignition should be bumped over into on position, the ECM receives power from multiple locations, so pulling the one fuse probably wont have the effect youre looking for. battery reset may though. disconnecting both terminals and touching them together for a couple minutes should discharge all the capacitors in the system enough that the ecm would have to relearn after having lost its memory due to the hard reset.

id have to look at the boxes myself to tell you which fuses do power the ECM. its either in the IPDM module (the big box by the firewall on the passenger side) or its a mega fuse thats at the positive battery terminal

 

i hadnt thought of that yet myself either. the change in altitude will definately affect your tire pressures and if you have decent mileage on the motor, the throttle and air filter may be "soiled" and need cleaning the throttle and replacing the filter. lots of little things go into overall MPG....and i have to agree with JNR that 20 city is high for these trucks...my pathy gets 10.5 city right now in Ohio.

 

I put a brand new K&N air filter in a few days before the trip. I still have the other air filter in the truck in case there were problems during the trip with the replacement K&N. Mileage stayed the same with the K&N.

I didn't even consider the tire pressure, but I'm going to check that out this weekend. Should we go by the pressure stated on the tire or by what the truck suggests on the sticker?

Spark plugs are another project for another time. I don't have the tools at the moment to get too deep into breaking things.

-edit-
not to get too off topic, but what spark plugs do you guys recommend? NGK has always seemed to be a favorite for my Toyota and I hear it's a japanese car favorite. Does the same apply with the Frontier?

-second edit-
Nevermind. Found a good link: http://www.clubfrontier.org/forums/f8/what-best-sparkplugs-our-trucks-67818/

 

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.

 

NGK should be the ONLY plug you ever put into a nissan. period. the iridium tipped are my personal favorite, but any performance NGK plug will do. (provided they were machined for that motor)

as for tire pressures, go by off the door sticker. you want your tire pressures set to what the vehicle wants, not just want the tire can handle. had to explain this to my cousin the other day when i found out he was running 65 psi in his frontier just cuz his tires can handle it.

its good practice to adjust tire pressures using a known good tire pressure gauge when the tires have sat and cooled to ambient air temperature. if its about 60 degrees and the tires havent been driven in the last couple hours id say to set them to 35/36 psi. when driving on highway speeds they should increase to 37/38 psi, on hot days, even 39 or 40. that being said when cold set to 35/36 you shouldnt exceed the tire's manufacturerer recommended max pressure of 44 unless youre doing burnouts in 110 degree weather.

when/if it drops below 60 degrees and the tires are cooled, you can expect a 1 psi drop per 10 degree drop. so at 50 degrees when the tires are cold it will be about 34 psi. 32 psi is the dealer/manufacturer preset for the tire light indicator. it can be changed with a scan tool (i believe) but they come from factory set to illuminate for low pressure at 32. so avoid that cusp, or you'll have a light on every morning. i usually set all my customer's wheels and my own to 36 to avoid any tire light hassle or overinflation problems.

 

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.

 

he bought some offroad/work tires that were rated to 80psi max pressure and assumed that thats what he needed to fill them too but was worried, so only went to 65....i learned this after having watched a video of him getting stuck in minimal mud in PA..i had to laugh at him a little and educated him on the notion of airing down and proper tire inflation per the tire placard.

for the plugs, im not aware that NGK makes a multi tipped plug for use like the bosch plugs. should be single tip, thats the way to go. every manufacturer likes different things.

had a ford escape on the used car lot some years ago. the salesmen flooded it out at -40 one winter. we ended up having to change the plugs because we couldnt clean em up enough. NGK's were in stock, so we used them. it flooded again the next morning. Put in autolite plugs ( ngk were 9$ a pop and autolites were $1.50 a pop) and it never had a problem afterwards. every manufacturer has their recommended plug brand. dodge is bosch, GM is ac delco, Ford is autolite, and surprisingly, most imports just take NGK

 

NGKs are what I've always used in my Toyota and the Subaru. I was definitely looking into the iridium tipped plugs. My truck deserves all the best.

Do the wires/coils need to be replaced with the plugs? Or should I leave those alone?

 

should have coil-overs....ive never replaced them unless they were testing faulty. theyre not like wires that get changed with the plugs. theyre long lasting, and freakishly expensive to replace them all everytime u do plugs. FYI, i believe the plug interval for recommended service is 110k miles..if youre not there..id check them before replacing.

 

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 think I'll give it a go without removing the intake and if it comes to that then I'll just stop and get a dealer gasket. I've had bad luck in the past with trying to use old gaskets and it never worked out, which is why I became an avid RTV fan. On this truck, though, I want to keep it clean. So no RTV here.

 

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

 

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

Thankfully I never worked with a carbed engine. They've always been fuel injected with minimal computer control. This newer Frontier will be a new, uh, frontier, in my learning how to work on engines.

 

ive got the hood up on my pathfinder right now...it needed a battery, but it looks like on the passenger side, only one of the plugs should give you any real trouble, with the right flexes and extensions. and thatd be the...er..4 plug i believe. its the middle one on the passenger side. the forward coil and plug look like they'll clear the intake through the spacing that is created when the intake loops towards the front of the engine bay, and the rear coil looks easy easy easy.

 

Yeah, I'll definitely give it a go towards the end of the month. Luckily I can walk anywhere I need to be and if I break something I have some flexibility for the downtime.

 

keep in mind when wiggling that center coil out, that the top portion is stiff, the lower extension that actually touches the spark plug, is flexible to a point. its just a rubber boot with a spring inside, so you'll have some flexibility there too, just avoid too sharp of an angle so you dont crush or compress the spring.

 

I'll bring this one back from the dead just because I feel like it lol.

We moved to Co a few months ago and noticed the increase in mpg right away. Less air pressure means you get less power (3% for every 1k over sea level so here we get about 21% less power) like you have a smaller engine basically and the air is thinner so less resistance when driving means you get better mpg at the expense of acceleration. I've had my truck for almost a decade and never noticed the other valves kicking in at 3500 rpm until we moved here and I'm constantly revving higher just to get going.