Why am I getting such horrible MPG?

General Questions
#1

The car in question is a 2004 Infiniti G35 sedan with manual transmssion. The revised EPA MPG ratings are 17/24. I purchased the car, used, in 12/2009 with 82K miles; it now has 91K.

I’ve gotten 25 MPG once on a single trip that was all highway driving. Pretty much every other fill up (with mostly city driving) has yielded between 13.5 and 15 MPG, and while I haven’t filled the tank in the last couple of weeks, judging by the gas gauge (which admittedly isn’t accurate, but it is consistent) it seems to be getting worse.

Here’s some relevant info:

[ul][li]Tires are in good condition and are kept at manufacturer-recommended inflation (30 PSI)[/li][li]I usually upshift at around 2500 RPM, downshift at around 1500, and I don’t ride the clutch[/li][li]I don’t have a lead foot[/li][li]Air filter is clean[/li][li]The “check engine” light has not come on since I purchased the car; in fact, the car has been 100% trouble-free[/li][/ul]

Additionally, I’ve driven probably over 150k miles with a manual transmission and haven’t had this problem with other cars I’ve owned (so I’m reasonably sure it’s not something I’m doing wrong, although I’m willing to listen to ideas).

Any suggestions?

#2

Are the brakes dragging/parking brake not fully disengaged?

#3

Try upshifting sooner. That car’s engine should be torquey enough to handle 1800-2000 rpm shifts.

#4

The city mileage doesn’t mean that’s the mileage you will get in city driving, rather it means that it’s the mileage the car gets in the EPA’s city driving cycle. If you’re doing more stop and go driving than the cycle, or if you’re taking really short trips, it is very likely you will get worse mileage. What kind of of mileage did you get with whatever you drove before and have you had a change in commute that was coincident with getting this car?

In highway driving (assuming no traffic), you should be able to hit the EPA mark pretty easily because the highway cycle is basically just running constantly at highway speeds. So in general if you’re getting the EPA rating during highway trips, there’s probably nothing wrong with your car.

Oh yeah and, YMMV. :slight_smile:

#5

We have a 2004 G35 coupe with auto. About 25/26 mpg on the highway is about all. Around town; her shopping, exercise, grandson taxi trips; get 16-18 tops. The short trips are killers.

I’d run the tires at 35psi. A number of years back, the tire and auto companies had to sign a consent decree?agreement? not to put low inflation numbers on the cars (collusion that increased tire wear and replacement revenue for the companies). I notice inflation pressures creeping down again. The original Brigstones were real stiff in the sidewalls making the extra pressure a little lumpy. New Michelins are more comfortable at the higher pressure. Mileage is up enough to notice, unfortunately so are the gas prices. Premium gas is recommended but we’ve had no problem in the past 4 years on the mid-grade.

#6

2005 Infiniti G35 Coupe, 6-speed manual checking in (297hp, 19" rims and aero/tech packages)

23-24 mpg on an 75/25 highway-to-city mix, w/ semi-spirited driving. 25mpg if really, really trying on smooth, clear highway per the engine comp (record was 26.5).

When lugged through the city, the mileage plummets into the teens. 15-17 would not surprise me if it were bad city traffic. Heck, even coming close to the EPA city rating means it is normal and nothing really alarming about 15-17.
.

#7

Your numbers are right in line with the EPA’s. How long are the “short trips”?

#8

Pretty sure they’re not - the car rolls nicely when it’s out of gear, and when I replaced the front rotors last summer I had no problems getting the calipers off.

#9

Best mileage actually comes with a bit higher revs on the Infiniti/Nissan Q engines.

1800-2000 is a little low.

#10

Have it inspected for any external fuel leaks.

Have it checked by a sharp driveability expert mechanic – someone who knows about fuel trim, oxygen sensor scope patterns, mass airflow sensor speed density testing, etc. Components with suboptimal performance don’t always set trouble codes.

Google around for others’ experience with this particular model.

#11

And this is most easily determined by driving awhile and then getting out and feeling each wheel cover. A dragging brake will make the cover very hot.

#12

It’s a mile here, a mile there. We’re in a small (25K) county seat. Pretty compact but no lack of stop signs and signals down town. At least we don’t sit for 10-15 minutes at a time just to move a half mile like some major cities.

#13

I didn’t think about it before, but smithsb’s first post reminded me-- isn’t this a “premium recommended” car? If you’ve been running regular gas, it might be worth trying a couple tanks of premium to see if it helps. Or if it helps enough to be worth the price difference.

#14

Premium is recommended for the coupe, not the sedan.

Since it seems like smithsb’s driving habits are similar to mine, and he’s getting better mileage, it does seem like something is wrong.

Gary T, how do I find such a mechanic?

#15

Got vibration?

#16

Bud Bundy is siphoning off the gas for Al :slight_smile:

#17

According to the owner’s manual posted here: http://www.infiniti-techinfo.com/refgh0v/og/infiniti/g35/2004-Infiniti-G35.pdf

The coupes require premium, but Infinity still recommends premium for the sedan. So that means your car might still benefit (in mileage and otherwise) from running premium. I’d say it’s at least worth a try, certainly before you start racking up a bunch of diagnostic hours.