Just recently picked up a 2nd car for the winter - 2008 G35x with 140k miles all stock

After a few fillups i've noticed terrible mpg....getting around 220-230 miles to a tank and averaging 12-13mpg on calculations, all city driving, just normal driving not ripping on it or anything like that. My G37 does about 19mpg with the same city driving so this is pathetic.

Air in the tires is good, put brand new air filters in, fresh oil change etc....what else can I look at or might be the problem here?

 

Though 12-13 is crappy. You're comparing a rwd 3.7 7speed to a heavier AWD 3.5 that who knows the kind of life it had.
The 7speed is the key to the 3.7s better mileage. Your best bet is to start with some maintenance and see where this leads you.
A bit at a time to check what works and what doesnt. At 140k youre due for plugs. Who knows if they ever ran premium gas.

PS:
And this applies to ALL vehicles. Storing more than 8 gallons of fuel is a waste.
With a full tank you're carrying about 110lbs of gas. Since I also live in the city and see myself pulling to gas stations for other things I use that time to fill up.
By carrying about half of that you should see better numbers. I get about 170miles for just 8 gallons which is a mixed driving with some spirited runs. I would average 19mpg on a full tank however about 21 on half tank.
If you dont see yourself stopping as much as I do go ahead, I personally feel it's a waste for city drivers.

 

My thoughts are with the o2 sensors given the mileage. As they age they become lazy and maybe imprecise. Probably need to do some logging to determine if that's the issue.

 

12-13 mpg all city driving? Sounds about right.

 

you think so? But yes 99% city driving

 

I have owned/leased 3 different Gs, they all get around 14mpg in the city, I don't have a heavy right foot either.

For road trips they do get about 25mpg.

 

My 07 G35 with the 5AT got 17-18mpg city if I drove like a grandma. Closer to 15 if I drove more enthusiastically. I would say 12-13 is too low. The AWD does reduce it a bit, but a difference of 5 is pretty big. Is this block to block city driving we're talking about? That's the only way it would make sense besides a plugged cat or bad o2/maf sensors.

 

I’m at 150k and recently changed all of my fluids, plugs, and got the throttle bodies cleaned up really well. I ended up going from 15.5 mpg to almost 16, lol. The plugs were pretty shot and the throttle bodies were overdue for a cleaning so I was hoping for a little more improvement than what I got, but I drive about 75% city so that seems about right for these cars.

 

Which city? All the ones I've been to are pretty wide open right now. Its almost like driving in the burbs.

Are you allowing the car to warm up every time you drive? Or idling a lot? Its cold out. Some people warm their cars...idk

Winter blend gas also....

 

I’m in Fort Worth so no need to really let the car warm up for that long. I usually let it run just long enough for the idle to drop back to normal off of a cold start.

Car gets great mileage on the highway, but I’ve got about a dozen traffic lights that I always seem to be stopped at that don’t help much.

 

I'm on my third Infiniti since December 2005.
All three including my current Q70 get 25 MPG + on my trips from NJ to Florida.
17-18 MPG on short trips
I have never used premium in any of my vehicles and get excellent MPG using regular 87 octane.


You can see the severe drop in MPG during short frequent stops while in Florida

 

Ah...so you're in the burbies then.

Idk...try not warming up the car. its pointless anyway.

Maybe you're just making short trips?

 

The ECU will set a CEL if some emissions related part is out of spec. Changing your O2 sensors prior to a CEL will result in SLIGHTLY better MPG because of the faster sensor readout. But VASTLY better MPG because you'll be driving more gingerly to try and justify the few hundred $$ you just spent on O2 sensors. You'll be subconsciously or proactively trying to get better MPG by using better driving habits. Right?

95% of the time, MPG issues are caused by driving conditions and habits. That's the biggest factor.

Rarely, some sort of malfunction like a dragging caliper or low tyre pressures are causing some SMALL drop in MPG. If your brakes are causing a big drop, my hope is that you've smelled them and/or at least wondered why you've had to extinguish the flames from your wheels every time you stop.

Air filters don't directly impact closed loop MPG. As long as airflow isn't completely or almost completely blocked...the car is going to run as close to 14.7:1 as possible in closed loop. The less air it sees, the less fuel it sprays.

If air flow is bad enough that throttle tip in doesn't match air flow...you get (take a guess).....

a CEL

The performance efficiency will go up with a free flowing air filter, though. Basically, you get more power and your MPG while you're giving it a lot of gas pedal will go up. By some likely small amount. Unless your ECU is mostly ignoring the MAF and MAP and just ignorantly spraying fuel to try and make power (nope its not the 80's).

However.... you get a small MPG increase with new filters by going into open loop less times the car isn't gasping for air and you're getting less pumping loss.


You can make up some small increases via fluid changes and general maintenance, as well as reducing rolling resistance.

The car advertises 17 MPG "city" (whatever that means). You're getting 12. Optimizing and maintaining the car is gonna give you back like 1 or MAYBE (probably not) 2 MPG assuming the same habits.


TLDR?

Its almost for sure just driving habits. Change that for free, as much as you can.

Air filters, plugs, O2 sensors, fluids, etc will improve MPG slightly. Maybe 1-3MPG all put together. Assuming some consistency in your % of open vs closed loop driving.

 

I would switch fuel stations first off. Try a Chevron Techron Concentrate as well. If you can run a tank of Ethanol free fuel and get out on the highway for a long drive. My car does the same as yours in the City and then gets alot better on the open highway which are normally 200 mile open stretches in Wyoming on I-80 at 85. Its just the way the Trans and ECU are in this car. For me I could care less if I am having fun with this car.

 

Bottom line, the g37 is decent enough on long distance highway trips. I can get 500 miles on a tank. I averaged almost 29 mpg on my most recent trip. However stop and go city driving will be around 12 mpg with conservative driving.