Horrible MPG continues. PLEASE HELP

[USMC5811]

A little while back I posted that my average MPG dropped from around 19 city/highway to 14 city/highway, which in my opinion is unacceptable for our cars, especially since I drive slow most the time. I noticed the drop in MPG immediately after installing new tires and getting a wheel alignment done. I went from Toyo Proxes to Bridgestone Potenza. I know since I have an "Ultra High Performance Tire" that i will notice a decrease in MPG, but it shouldn't be 5 MPG. My PSI is exactly the same as it was before as well. Could it be a bad alignment?

Also, my SEL came on recently and stayed on for about a week and then went off. When I ran the code I got the dreaded P0455 "EVAP Emission Control System Leak (Large)" which made me start to wonder if maybe there actually is a problem going on that could be causing the decrease in MPG. But since the light went off on its own, I'm guessing that the gas attendant didn't close my gas cap tight enough since I live in Jersey and we don't pump our own gas. I noticed the car does smell like its burning extra rich in the morning, but when its 15 degrees out i would almost expect that.

Does anyone have any possible ideas? It's not the tire pressure, its not the wheel hub bearings, its not the way I'm driving.

[EniGmA1987]

Well you said you are driving slow, the lower gears dont get the greatest MPG. I found that a 50mpg cruising speed got the best MPG because the car is able to get into 5th gear there but RPM is also pretty low still. If you are under that speed on streets then you are in lower gears with higher ratios. I also found that accelerating somewhat quickly up to speed so that you can be at cruising speed for as long as possible was far better for MPG than doing what 99% of the population does with taking an the entire time to the next signal in a constant slow acceleration state.

Heavier tires can also decrease MPG, but Id only expect maybe a 1mpg difference from tires. I noticed that going 3lbs heavier on each rim actually dropped my MPG by 2. A bad alignment could also drop the MPG a little bit. Really there are just too many variables to tell you that "the problem is X", and most likely it is a lot of little things adding up to a lot.

[USMC5811]

Well you said you are driving slow, the lower gears dont get the greatest MPG. I found that a 50mpg cruising speed got the best MPG because the car is able to get into 5th gear there but RPM is also pretty low still. If you are under that speed on streets then you are in lower gears with higher ratios. I also found that accelerating somewhat quickly up to speed so that you can be at cruising speed for as long as possible was far better for MPG than doing what 99% of the population does with taking an the entire time to the next signal in a constant slow acceleration state.

Heavier tires can also decrease MPG, but Id only expect maybe a 1mpg difference from tires. I noticed that going 3lbs heavier on each rim actually dropped my MPG by 2. A bad alignment could also drop the MPG a little bit. Really there are just too many variables to tell you that "the problem is X", and most likely it is a lot of little things adding up to a lot.

I really appreciate the feedback! I feel like I've tried everything, and I can confirm that getting up to cruising speed quickly definitely makes a positive impact on MPG, especially since I have an "X". Most of my miles that I'm putting on my car are city miles in Philadelphia, going to and from work, but theres also some highway miles once i get in to NJ. If i calculate my mileage while only driving in the city I average around 12MPG. I just didn't think that 12MPG was normal even though its all city driving since the car is rated for 18MPG city I believe. My previous vehicle was an Escalade so I'm not really complaining about getting horrible MPG because i don't want to spend the money. I knew when i got this car that it wouldn't be an astronomical difference in MPG and since gas prices dropped big time I'm still happy with what I'm spending now on gas compared to what i was spending. I'm more or less just worried about the car and if something is really wring or not. I am meticulous about my cars and I baby them big time, so whenever anything is wrong with them it drives me nuts and i fix it right away. But this is a noticeable problem I've been facing ever since I got the alignment and installed new tires because I dropped 4MPG.

[CPJ LB]

Just curious, do you change out your air filter on a regular basis? also other maintenance upkeep will keep the car running good. oil changes, PCV valve change, spark plugs (also check for the dreaded valve cover leaks where oil gets into the spark plug tubes), replace gas cap if you have high mileage on your M (with only OE Infiniti M gas cap), clean MAF (mass air flow sensor)...

I add Redline fuel system cleaner in my car at every oil change and take the car out on the hwy for some quick runs to help burn out the gunk...I feel the difference afterwards.

My car is modified and I'm averaging close to 17-19 mpg (city/hwy combined) while in my "aggressive sport" tune. And I keep close to the maintenance as possible.

[USMC5811]

Just curious, do you change out your air filter on a regular basis? also other maintenance upkeep will keep the car running good. oil changes, PCV valve change, spark plugs (also check for the dreaded valve cover leaks where oil gets into the spark plug tubes), replace gas cap if you have high mileage on your M (with only OE Infiniti M gas cap), clean MAF (mass air flow sensor)...

I add Redline fuel system cleaner in my car at every oil change and take the car out on the hwy for some quick runs to help burn out the gunk...I feel the difference afterwards.

My car is modified and I'm averaging close to 17-19 mpg (city/hwy combined) while in my "aggressive sport" tune. And I keep close to the maintenance as possible.

I keep up with the required maintenance pretty well. I change the oil with full synthetic every 3500 miles and i also change out the PCV and the air filter pretty often. I put new plugs in about a year ago, there was a tiny bit of oil inside one of the tubes, but i don't believe it was significant enough for me to change the valve cover. I have also cleaned the MAF, although i haven't tried replacing the OEM gas cap. Could a new gas cap make any difference though?

[CPJ LB]

^ good work on your car!!

Yes, a new OE gas cap will work perfectly. I went thru a similar experience and I replaced the old OE gas cap with an aftermarket gas cap -- about a few weeks later I noticed that it wasn't sealing tightly each time I would put in gas. I picked up an OE gas cap from the dealer and no issues/codes... In fact, I have to use effort to twist off the new gas cap each time I go to fill up

big difference...I still had the old OE gas cap in my garage and compared the old vs the new...the rubber gasket on the new gas cap looked more sturdy and was thicker.

[USMC5811]

^ good work on your car!!

Yes, a new OE gas cap will work perfectly. I went thru a similar experience and I replaced the old OE gas cap with an aftermarket gas cap -- about a few weeks later I noticed that it wasn't sealing tightly each time I would put in gas. I picked up an OE gas cap from the dealer and no issues/codes... In fact, I have to use effort to twist off the new gas cap each time I go to fill up

big difference...I still had the old OE gas cap in my garage and compared the old vs the new...the rubber gasket on the new gas cap looked more sturdy and was thicker.

Thanks a lot for the advise! I don't mean to sound like an idiot but i just want to clear one thing up that I'm a little confused with lol. So I understand that a newer OEM gas cap may seal tighter and that a loose gas cap is probably what caused my SEL light to come on. But the light went off on its own and hasn't come back on, so the SEL light isn't the issue. So are you saying that replacing the gas cap with a new OEM gas cap could potentially increase MPG and/or be some of the reason my MPG is suffering so bad?

I just want to make it clear that I'm not disagreeing with you, I'm just trying to verify that a new OEM gas cap could possibly increase MPG, since I can't tell if you're advise is on the topic of my SEL or my MPG decrease. Thanks for the help!!!

[CPJ LB]

^ you're welcome!

Yes on both accounts - a new OEM gas cap will help fix the check engine light from coming on (also use a SCAN tool to reset / delete any evap codes), also a new OEM gas cap will help improve your gas mpg's. Having a tight seal after refueling is the primary goal. If not, gas vapors are escaping as well as air being sucked in....causing the mpg decreasing issue. I found a short video for reference, however there are many write ups on this issue - pretty common.

https://www.youtube.com/watch?v=1tmXfTMJ5U0

[keithhr1]

my gas mileage was getting so bad that I suspected someone was siphoning gas from my 2008 M35, so I purchased a locking gas cap and it plumeted to below 9 miles per gallon, 8 plus to be exact. Since I filled up last time I have been coasting downhill watching my fuel economy info as I drive. I must say that I am retired, live in a hilly area next to san francisco and all my driving is virtually stop and go, the worst possible driving conditions, hills, stop signs corners and last tank of gas I went 164 miles on 15.0 gallons of gas. I filled up today and my new fuel estimate registered 188 miles directly after my fill up. I was advised by an Infiniti service tech that my engine may be carboned up and need to be cleaned chemically. First thing I will do Friday morning is get an oem gas cap for experimenting purposes. My mileage hit 10.83mpg today after fillup and I guess my locking gas cap must be judged on some other criteria. Fact is, car mileage should be evaluated on other criteria such as fuel consumption at idle , partial throttle and full throttle, not in MPG. If I drive for 30 minutes on the freeway I can go approximately 34 miles. If I drive for 30 minutes in San Francisco I might go 11 or 12 miles at most so mpg is a really stupid way to measure fuel consumption. Fuel burned at different throttle levels would be far more accurate, I figured this out 30 plus years ago and have always wondered why nobody has suggested similar theories.

[ross6200]

I am having the same problem. My average MPG has dropped from 17.5 to 14.5, but my daily driving routine has not changed one bit (mostly city driving). I did put on different snow tires this winter, so I'm curious to see if my MPG improves once I put the summer tires back on. Other than that, I recently did a major service at 110,000 miles, including new filters and spark plugs. That did nothing to improve my mileage.

[TDot]

Other than that, I recently did a major service at 110,000 miles, including new filters and spark plugs. That did nothing to improve my mileage.

I'm glad you said that, because although I'm far from an engine and performance guy, I can never buy into those too things making a significant difference from an overall well maintained car that is not pushed hard. I guess it could help in the office really health of the engine, but not make a difference like in erasing 2mpg or so. I'm wondering if all the guys with 16+ mpg have tuned cars.

[fat3oy]

Yall X models do me a favor - and kill your battery for a night... reset everything... my fuel problems are self-inflicted, but I have noticed this issue on certain performance awd nissans... the attesa learns a default state for the rolling diameter of your previous tires - willing to bet that shifts are no longer efficient with a quick change of the tires - wouldnt explain it getting worse as a pattern, should get gradually better, but no telling as those transmission's are freaky magic...

but on all my logs, a 500-1000rpm difference in average shift times made me drop nearly 5mpg... i'm hypothesizing that the learning process for our trannys is acting all retarded with changed in RD, randomizes shift times, and jacks up the MPG (ESPECIALLY in city, since the learn process keeps having to reset.)

[fat3oy]

oh and drive on a hwy for 30 minuets over 45mph after resetting the battery.

[TDot]

So I was arguing with some coworkers and none of the would believe I was getting 12-13 mpg. All said I had a hole or something, being stupid and asked if I was driving a Lamborghini, or comparing my car to theirs and because theirs is more powerful or a truck it was simply impossible that my mpg was this low (it was actually starting to annoy me and I really don't let things bug me). I told them I knew plenty of others with this car with the same issue and it went in one ear and out the other.

Anyway, I started to try and remember when my mpg started to drop this low, and the best I can remember is when I changed my wheels. I'm starting to think its all self inflicted because when I use to fill up I'd see 224 miles, now I see 192 miles to empty. Not that averaging 14mpg is any good, but it is substantially better than 12. Maybe in time I'll throw my factories on for a months test, but I think that is the major culprit...at least for me. Also researching I see treads have a significant impact (1-2mpg) on fuel economy which I just don't get. So that will be a significant part of my research on my next tire purchase.

I forgot about disconnecting the battery, I'll do that next week when I have to fill up again and see what happens.

[Brian35XR]

i get 12mpg in the city and its pretty freakin pathetic!! i am driving on 22s though so that might be a factor... i think before the 22s i was only getting around 14mpg though so not much better.