Serious Fuel Economy Thread!!!

[Sentientbydesign]

Ok, before I start. PLEASE REFRAIN FROM COMMENTS LIKE:

It's a performance car, you're not supposed to get good fuel economy.

Go buy a XXXXXX (Insert prius, smart car, insight...etc)

It must be your driving.

You're doing good, I only get XX miles per gallon cuz I drive it like I stole it.

We've gone down that road plenty of times and I'd like to seriously diagnose what's going on with my G.

To start. I have a 2005 G35 6mt coupe. Mods include grounding kit, K&N drop in filter, aftermarket RPS clutch/flywheel, and a modified lower non rev-up lower plenum.

Since I bought this car, I've consistently gotten poor fuel economy out of it. It doesn't matter if I drive like a maniac flooring it from stoplight to stoplight or drive mild mannered, I can't get good fuel economy out of it.

Average mixed driving 70/30 City/Highway yields 16.5-17mpg. Reverse the ratio and I get somewhere in the mid 18s.

Best all city fuel economy was 19 mpg. This meant shifting at or below 3k rpms. Coasting when available, very gradual acceleration (most economy cars would pass me).

Best Highway fuel economy was 23 mpg. This was with moderate driving at or below 70 mph keeping large space "cushions" and only braking when absolutely necessary.

Typical highway fuel economy is around 19 if I'm lucky. I typically cruise at 80 just about everywhere I go unless the flow of traffic exceeds that speed.

I just spoke to a friend of mine who said that his SUPERCHARGED 350Z (6mt) averaged 21 mpg highway. What's the deal???

I know the weight and gearing on the sedans yield better fuel economy, but my coupe shouldn't be doing so horribly with the way I drive. Do I need to do some magic-jacko-mods to get better fuel economy?

I bought this car for it's styling, make, and performance potential.

[liquidmice]

Well ok then... I don't think you're crazy. I have a 2008 G35s 6-speed and I can EASILY average 23mpg... barely trying. If I hypermile, I can average 26-27mpg. And I do get stuck in traffic. That said, it's all about long stretches of road isn't it? I average a 70-mile commute every day. A significant portion (1/2 - 1/3) of the commute is early-morning, no traffic driving which can mean I'm coasting a lot and focusing my driving on mpg. 3 days a week I pick up the kids and drive home which means no crazyness. I TRY and have weeks where I'm good (that way when I'm lead-footing all the way home, it seems like that much more of a treat.

It certainly sounds like you know how to drive and take advantage of the 6mt (coasting etc.) Do you use cruise control? Cruise is not very efficient, you're better off trusting your foot. So, it seems like either your vehicle is abnormally poor on fuel economy, or you're doing something wrong. Here, read this link on hypermiling and see if it is counter to your driving habits: http://www.hypermiling.com/car-mpg.html

Other than that, it does not seem to make sense. The mods you listed don't seem like they should effect MPGs... at least not negatively. Good luck in solving the problem!

[Sentientbydesign]

Welcome to NICO!

Thanks for the input. Unfortunately, we can't compare cars because your engine is what my engine strives to be lol.

The VQ35 comes in 3 major variants:

Two DE engines:

The 6mt Rev-up (this engine SHOULD have it's own designation instead of "VQ35DE Rev-up"). Single intake, with poorly designed plenum and a 7k redline.

The regular VQ35DE which is in all of the automatic VQ35s with the FM (Front Mid-ship) chassis built before 2007ish. Single intake, with the same poorly designed plenum, but optimized for low/mid torque instead of high end HP. 6600 RPM.

Then the beauty you have in your G. The VQ35HR. Dual intakes, higher redline (7500), much better heat resistant plenum and stronger internals.

Your G was advertised to have both better power and better fuel economy because of the aforementioned aspects AND the improve variable valve technology.

Has your brain popped yet?

I'm going to try to drive a little slower on my trip to OC tonight and estimate my Highway fuel economy. I'm sure I won't pop 23 mpg

Enjoy the forums!

[smockers83]

I haven't really read the OP, but I figured out that you're getting poor highway mileage. If I cruise at 80, I'll get anywhere from 26-28 MPG. A more in depth post is for later.

[awdjdmtalon]

Nate.

You ARE getting poor fuel economy.

I have very close to the same mods as you do. I get 16.5-17 mpg city. I shift around 3k. Some fast starts, and some WOT stop light to stop light.

On the HWY w/ AC on econ, i get 26-28 mpg. It varies if I am around where I live. Mostly mountians. Or up north where it is flater.

[tollboothwilley]

I think a large part of the reason that you get bad gas mileage is due to the fact that CA has a lot of hills and the gas + ethanol mixture. Back when I lived in Corona, CA and worked all over the OC I averaged 2 MPG lower in my driving, and I was driving a lot more freeway.

What is abnormal to me is your All highway driving.

My guess is that with your mods are you running "fat"...didn't you get your car dyno'd before? Did they do an A/F at the time? Was that with the same mods?

The ECU could be compensating for the added air in by dumping more fuel but without an exhaust it might be reducing the power and gas mileage.

[biggie]

I was getting an average of 23.5 or so, not much hwy driving. And over 25mpg when a lot of hwy driving was done. After putting on the Z tube and JWT Popcharger I've noticed a drop to 20.5mpg. Not sure if my foot got heavier or the ECU needed to be reset.

I recently unhooked the battery for about 15 hrs to reset ECU (this past weekend) so hope to see it improve.

[joe603]

I noticed a big drop in fuel economy when I installed my CC lightweight flywheel/clutch combo. I was getting 20-21 city...now I get 17-19.

[Sentientbydesign]

What is abnormal to me is your All highway driving.

My guess is that with your mods are you running "fat"...didn't you get your car dyno'd before? Did they do an A/F at the time? Was that with the same mods?

The ECU could be compensating for the added air in by dumping more fuel but without an exhaust it might be reducing the power and gas mileage.

The last time I got the car dynoed, I had the JWT flywheel, not the RPS, my A/F looked bad (in my unexperience opinion). I remember ratios from 11:1 down to 16 or 17:1 and the torque curve was anything but smooth.

Getting an Osiris and a few tunes is very tempting, but I'm not sure I can part with $650 in order to save 2-4 cents per mile. If I start commuting from OC back to work, I guess it would be worth it if I can get my hwy average around 25. Then the tune would pay for itself in 8 or so months.

I noticed a big drop in fuel economy when I installed my CC lightweight flywheel/clutch combo. I was getting 20-21 city...now I get 17-19.

This makes no sense, BUT it may be the cause. I've always had the poor fuel economy since I got the car and it's always had an aftermarket flywheel. Maybe it has something to do with rotational inertia. Get a massive object in motion and it stays that way better than a less massive object?

[tollboothwilley]

Joe

would you attribute that to your driving style with the new clutch being different or do you think it is just because its lighter?

I would think that lighter wouldn't change gas but only driving style.

[pfarmer]

This makes no sense, BUT it may be the cause. I've always had the poor fuel economy since I got the car and it's always had an aftermarket flywheel. Maybe it has something to do with rotational inertia. Get a massive object in motion and it stays that way better than a less massive object?

It may make some sense although to me it would be hard to quantify to the point of stating that it makes 'x' amount of difference.

One of the big items with hypermilers is maintaining forward motion at all times, slowing up way before traffic lights turn red if they will be turning red, slowing up while red so you have forward motion when they turn green, elimination of stop/go etc. in traffic and so on.

In between power pulses of the engine it seems like you may see the same thing only in little tiny events. This could well add up. But I would have a hard time seeing that it would add up to that much since at the same time you are tied to the rest of the drive train which in itself is absorbing these power pulses.

I believe a lightweight flywheel provides little benefit except in spoolup. The amount of horsepower it takes to place and keep the flywheel in motion is not all that much. If it affects transmission wear then the transmission may well make a lot of difference in parasitic loading even if the wear is not all that noticable by noise.

Perry

[joe603]

The only thing that I changed is that it takes more RPMs to start. I thought that my MPGs would increase. I guess the only other thing that changed is the transmission was replaced at the same time my clutch was done. But it was just an OEM transmission...

[tollboothwilley]

Nate.

I think you need a tune. But, you would want to get exhaust before then to take full advantage. If your AF went from 11s to 16s that would explain a very sloppy torque curve as well as bad gas mileage. 16s are very very lean. 11s are very fat. In addition to running better you would find GOBS of power if you are really running that fat/lean.

There is a place there in OC (talk to Vivek) where he gots his Osiris & dyno time for $500.

[Sentientbydesign]

Nate.

I think you need a tune. But, you would want to get exhaust before then to take full advantage. If your AF went from 11s to 16s that would explain a very sloppy torque curve as well as bad gas mileage. 16s are very very lean. 11s are very fat. In addition to running better you would find GOBS of power if you are really running that fat/lean.

There is a place there in OC (talk to Vivek) where he gots his Osiris & dyno time for $500.

Yeah, I spoke to him about that. Got to wait for him to get back from India and I'll get the place's details.

I'm soooo curious what I/E and a tune would yield to the wheels. $2k isn't really an option to blow on my car right now unfortunately. Anybody want a kidney?

[zozoka1212]

I know my G has the HR engine. When I went to the East Coast I had a chance to see what I can get on pure HWY driving. I've got around 24-25 mpg with lead foot no steady droving. In city lately getting around 19-20. But my wife gets better than me for sure so I know there could be room for improvement but I just love driving it spirited. On my 07 I've got worse but I was pushing that one harder.

Tire pressure, spark plugs, type of oil, type of fuel,etc can change the whole thing. How long the tires and brakes lost for you? That indicates a little how you drive. On my 07 I needed new tires and brakes lot earlier than on my 08. I've can see that on my mpg improvement.

[tollboothwilley]

I bet you'd pick up a good 25-30 RWHP with a proper tune and a good exhaust.

you should look for a used exhaust...save you some $$

I'm sure you can find a good exhaust used for $500-700.

If you get the tune for the same price as Vivek paid then you'd be about half of the $2000 you're thinking.

MD is the best exhaust though...I was skeptical at first but Tony really knows his stuff. Top notch in quality, design and performance. Did I mention sound???

[Sentientbydesign]

I bet you'd pick up a good 25-30 RWHP with a proper tune and a good exhaust.

Wouldn't this make you cry? J/K

[terrycs]

Yeah, I spoke to him about that. Got to wait for him to get back from India and I'll get the place's details.

I'm soooo curious what I/E and a tune would yield to the wheels. $2k isn't really an option to blow on my car right now unfortunately. Anybody want a kidney?

GTM is near you.

[leeG35]

I have a 06 Coupe with automatic and 40,000 miles on the clock. Fuel economy data to date gives me an overall average MPG of 22.3, with a low of 16.1 and high of 26.9. I consistently achieved 26 MPG during a 1,600 mile road trip cruising at a steady 70 mph.

[telcoman]

I haven't really read the OP, but I figured out that you're getting poor highway mileage. If I cruise at 80, I'll get anywhere from 26-28 MPG. A more in depth post is for later.

I see similar results.



Here are the results of my study



My G loves regular 87 octane

My tire pressure is 35lbs when cold using OEM Bridgestone Turanza tires.

Mileage is calculated by setting odometer A to zero at each fillup. Total miles traveled is divided by gallons to fillup which equals MPG.Any other method is not accurate.

Telcoman
Modified by telcoman at 5:24 PM 9/17/2009

[Sentientbydesign]

I have a 06 Coupe with automatic and 40,000 miles on the clock. Fuel economy data to date gives me an overall average MPG of 22.3, with a low of 16.1 and high of 26.9. I consistently achieved 26 MPG during a 1,600 mile road trip cruising at a steady 70 mph.

My last fill-up averaged under 17MPG for the 300+ miles.

I'm also still consuming oil and 4drmadness has my ghetto catch can. I'm curious how much oil is going through the PCV valve during a 3500 mile interval.

[telcoman]

My last fill-up averaged under 17MPG for the 300+ miles.

I'm also still consuming oil and 4drmadness has my ghetto catch can. I'm curious how much oil is going through the PCV valve during a 3500 mile interval.

Nate

I'm not sure what you mean when you say you averaged under 17MPG?

300 miles divided by 18 gallons = 16.666 MPG

Your low fuel warning light would have been lit

300 miles divided by 17 gallons = 17.647 MPG

How many gallons did it take to fill your tank after the 300 mile trip?

[Sentientbydesign]

Low fuel light was on for 30 or so miles.

I believe it was 307 and 18 gallons which is actually just slightly over 17 MPG (I originally calculated it at 300 miles, not 307).

I decided to use the Mobil 1 0w-30 fuel economy oil and I think the engine went through it faster than the synthetic blend that is usually in there.

[tollboothwilley]

How much oil are you consuming? Enough to do the OC test for the REVUP?

Are you still under warranty?

[Poyzinous]

My 04 G35X gives me between 16 and 17 depending on A/C usage and temperature. I drive fairly hard.

[Sentientbydesign]

How much oil are you consuming? Enough to do the OC test for the REVUP?

Yes.

Are you still under warranty?

Warranty is over.

I'm on my second engine and still consuming oil. It's my fault for not pushing the issue more with the second engine. On one of the OC tests, I measured 15mm at home. The service advisor came out to my car at the dealership and measured 18mm.

Then the tech measured it again once it was in the shop and they only measured 10 or 12 mm which didn't qualify. I'm like WTF? I measure 15mm, the SA measures 18mm and the tech only finds 10-12 missing?

On the next OC test, I fubarred it by not coming back in time.

It looks like the revised engine isn't the answer as there are still numerous revised rev-ups consuming oil.

[joe603]

Try using a heaver weight oil...I use 10W-40 RP and don't consume a drop.

[tollboothwilley]

Might be a good time to trade the G in. Unless you plan on removing the engine and replacing the rings on your pistons, that or just dealing with adding oil on a regular basis.

[QX1]

Ok I am driving 06 G35 sedan 6MT mods, high flow air filter, Y-pipe Tanabe an Magnaflow high flow cats. My highway fuel consumption 30mpg and city is 20mpg.