Silkk

iTrader: (11)

So, lately I've noticed a significant decrease in MPG - both on my digital guage and calculated manually. The car is an 03 enthusiast with all bolt-on mods (less headers) and the AAM reflash. Car also has 115k miles.

Even after all these mods (reflash last mod done 6 months ago), I was getting a consistent 26.0-26.5MPG with a mix of town and highway driving (about 25/75). This is up until, literally, 4 weeks ago. Lately, it's down to 22.5 - 23.0 - I consider 3-3.5MPG significant taking into consideration I'm doing nothing differently than I was a month ago. I drive casually 6 out of 7 days I drive the car, one spirited day a week on average and this is how its always been, still the same mix of town/highway driving. Can't think of anything too different other than:

A) it's much colder when I drive the car now here in PA winters (10-30F), then when I had the car tuned or when I normally drive it (at 65-85F)?
B) i don't drive the car as consistently (take other vehicle due to snow/crap roads) - so it sits for a few days or a week between drives

Could one of these be the cause? Could I have carbon buildup or something? Some other issue due to cold and sitting for longer periods? The car definitely runs rich with the tune anywhere below redline (as evidenced by black soot on the rear/tailpipes). Things you can do to resolve it, if these are the issue? Fuel treatment, etc.

Or is it lack of maintenance? I haven't:
A) cleaned the Pop-Charger filter in 15k+ miles
B) replaced spark plugs in about 75k (replaced them at around 35-40k somewhere) - although the manual says they are good to 105k
C) injectors probably have 40k on them - replaced them once due to a clogging issue

What order would you attempt to solve the problem in/pinpoint the problem? I suppose I should resolve all the issue eventually, but anyone had a similar experience?

Thanks in advance...

DavesZ#3

iTrader: (26)

Did you change trans or diff fluid?

Did you change tires?

Silkk

iTrader: (11)

Tranny and differential fluid haven't been changed in about 65k or so. I plan on doing that with plugs, soon. What impact would this have?

Speaking of tires, I have recently changed from 275/30/19 rear, to a 275/35/19 rear... This might throw off the guage? But my manual calc also showed a decrease lately.

phreaktor

iTrader: (22)

Are you driving any harder with the new mods you have?

Silkk

iTrader: (11)

Mods aren't new - I've had pretty much all of them for years. The reflash is the newest mod and I've had that for even 6+ months I believe.

Not driving any harder. Probably even less harder than summer considering I don't like driving a cold car hard. And it's been COLD.

The tires are a good guess though - only thing that changed around the same time as the mileage decrease. Could this 5mm (10mm total) bigger tire really have that much effect? I guess its more tire to turn, raises the car a bit (more drag), etc. but man... never expected it to effect that much?

sabrefanpc

iTrader: (4)

check your tire pressure maybe?

DavesZ#3

iTrader: (26)

When I went from OEM trans and diff fluid to Royal Purple, I saw my MPG go UP about 2 mpg. It's slicker and more efficient than regular oil so there is less power loss through the drivetrain.

Tires can have the same effect. Going with wider tires or a compound/brand that has a higher rolling resistance will cause your MPG to drop. Changing the tire size will have a real effect on MPG but won't have an "apparent" one in the gauge. The speedometer is based on a certain tire size. If you change the OD of the tire, it will take more or less revolutions per mile, hence your true MPG varies. Since the computer doesn't know that you changed your tire size, it still thinks "X" number of revolutions have taken you "Y" miles. Your miles per gallon should read the same on the display.

DavesZ#3

iTrader: (26)

I doubt it would make that much difference unless the brand/weight/compound were significantly different. I went from OEM 18" tires (225/245) to 19" 255/285 and saw my MPG drop about 3 mpg. The wheels/tires weight less, the estimated rotational inertia is about the same, but the rolling resistance of the significantly wider rubber is the likely culprit for the drop.

As mentioned in my previous post, switching to RP in the trans and diff bumped my MPG back up by 2.

gti303

iTrader: (15)

wow...I get mainly around 17mpg around the city...am I doing something wrong? lol

blastoff99

iTrader: (5)

My prime suspect here is the cold weather and winter-blend gasoline. I know my motorcycle mpg can drop as much as 25% this time of year -- that's an air-cooled engine, so it's not a terrific comparison to the Z, but some of the same ideas probably apply. None of my vehicles have *ever* gotten as good of fuel economy on winter gas. Also, I wonder about vehicles, especially those driven short distances, not getting warm enough. Cold engine = EFI dumping more fuel in = decrease in mpg.

That said, it probably wouldn't hurt to do a little preventative maintenance.

Silkk

iTrader: (11)

Thanks for the ideas fellas. I'm sure its one or a combination of all of them. I plan to do the maintenance as soon as I can get to Function:Tuned or somewhere reliable. I'd much rather have them do plugs, fluid changes, etc. than the Nissan dealer... I need a good weather weekend here. Soon...

Jay'Z

iTrader: (118)

Take it to function.. Problem solved..

davidv

iTrader: (2)

Are you sitting for 5 minutes in the morning and letting the car warm up?

Cold weather decreases tire pressure. Check tire pressure.

Freezing road surface means decreased traction and decreased milage.

Speedometer error is insignificant. At 100 MPH your speedometer reads 100.39 MPH.

3fittyBiH

iTrader: (1)

that cold air will drop your MPG down.thats probably the reason.

Voboy

iTrader: (58)

did you do an oil change recently? +1 on tire pressure.
if your 350z burns any oil... after an oil change your mpg may decrease.

terrasmak

iTrader: (8)

your MPG is better with the shorter tire , but you wearnt actually going the distance you car showed. Now your actually going the right distance but you MPG is slightly lower. No big deal.

r_seng

iTrader: (9)

weather.

bailey bill

Y9ou will always get lower mileage in colder weather.

1) The ECU sets a richer A/F as temps drop

2) Winter blend gasolines also result in increased fuel usage.

bill

Britx

iTrader: (2)

^^
Ditto

singh

iTrader: (2)

I do. Is there something wrong with that?