J30 Overdrive function question...

Post by **alexhho** » Mon Sep 09, 2002 6:15 am

Can someone explain more about the overdrive function? I'm not getting good milage at all, and am wondering if it's more economical to drive the J30 in gear 3, instead of D, while driving under 35 mph. Thanks.

Post by **Mayhem_J30** » Mon Sep 09, 2002 6:15 am

Well, for one, the J doesn't have an OD. It may feel like it though, but that's the torque converter kicking in. I typically drive around town in 3rd gear unless I hit a constant 55+mph. This helps saves the tranny through fewer shifts, but I doubt you'll get better MPG. Nissan claims the J gets 23/18 mpg Highway/City. So even at it's best it's not that great. The J's transmission control unit is designed with economic reasoning, so keeping it in Drive and letting it do what it wants to will most likely yield the best mpg.

Post by **alexhho** » Mon Sep 09, 2002 6:15 am

Mayhem_J30 wrote:Well, for one, the J doesn't have an OD. It may feel like it though, but that's the torque converter kicking in. I typically drive around town in 3rd gear unless I hit a constant 55+mph. This helps saves the tranny through fewer shifts, but I doubt you'll get better MPG. Nissan claims the J gets 23/18 mpg Highway/City. So even at it's best it's not that great. The J's transmission control unit is designed with economic reasoning, so keeping it in Drive and letting it do what it wants to will most likely yield the best mpg.

]

Thanks Chris, for replying. I get 16 mpg around town while in D drive, on flat roads. On the back of the owner's mannual it states that there is overdrive in D drive, and that one should drive in 3 if not going too fast. Of course, the mannual does not state what "too fast" is. I always accelerate slowly from stop, so I'm hoping that the poor mileage is not due to some other problem. Thanks for your inputs. YOu always seem to be around. Take care.

Post by **Mayhem_J30** » Mon Sep 09, 2002 6:15 am

alexhho wrote:]

Thanks Chris, for replying. I get 16 mpg around town while in D drive, on flat roads. On the back of the owner's mannual it states that there is overdrive in D drive, and that one should drive in 3 if not going too fast. Of course, the mannual does not state what "too fast" is. I always accelerate slowly from stop, so I'm hoping that the poor mileage is not due to some other problem. Thanks for your inputs. YOu always seem to be around. Take care.

hmmm...that's interesting that the owners manual states that about the overdrive. I don't remember all the things that have been going on with your J, but a little engine maintenance might be able to pull the mpg up a little. Air filter, fuel injector cleaner and so on and so forth. I know mine gets just as bad mpg as my 4.3L v6 chevy truck if not worse. Next fill up I'll track how many miles I get out of the tank so we can compare. Of course I drive in 3rd most of the time so it might not be a fair comparison.

Post by **Q45tech** » Mon Sep 09, 2002 6:15 am

Generally the term overdrive applies to a tranny that has a top gear that is less than 1:1 both the Q and J transmissions have a 0.694:1 ....4th gear so we call it overdrive..

The Q turns 333 rpm per 10 mph vs the J 383 rpm per 10 mph.6 cylinders like 4 cylinders must turn higher rpms as they have few power pulses per rpm and need higher engine rpms to develop enough torque to move the car.The gear ratios are the same in the tranny just the J's rear diff ratio is higher than the Q 3.538:1

Fuel consumption is related to rpm and load, acceleration consumes 5 to 5.5 times the fuel as steady state cruise!

How you get to 60/35 mph is very important , a slow easy long time acceleration say twice as long as minimum 8/4 seconds vs 16 seconds will use less fuel, taking 32 seconds to get to 60/35 will use even less. I am speaking in grams of fuel used per hour/minute/second not miles per gallon.

The Q assumes that if you keep the rpms below 2500 you want best mileage, if you rev beyond this you don't care. My guess is the J does something similar but at 2900 or 3000 rpm

Your foot on the throttle controls it all you determine whether 2 millisecs of fuel is squirted in or 10 millisecs worth per cylinder cycle....one drop or 6-7 drops x 3 x rpm.

Generally the 4th gear is unable to be used below say 40-45 mph as the engine multipled by the gears just doesn't have enough torque to move the car without lugging and bucking.

16 mpg is good around town with little heavy acceleration I would have guessed 14 mpg.

Nice fresh egg on the back of throttle or taped to bottom of shoe will improve mileage tremendously.

Post by **PalmerWMD** » Mon Sep 09, 2002 6:15 am

I would consider the J tranny having an overdrive due to its long legged top gear.

It mighjt be fair to call it a 3+1 gear tranny (just like in my Q) to signify the "hiway fuelsaver gear" which is what an overdrive is.

One can leave it in "3" when in town to minimize excessive shifting and to keep the engine in it's powerbad , however this will not save fuel (on the contrary).

If performance and avoiding engine and tranny lugging is important to you, than the tranny should not be left in"D" but rather in "3", at least in town.

For best economy stay in "D"

Even my torquey Q engine I keep in "3" in town, even if on an intown hiway ( up to 60 or so) unless I will stay on that hiway for a longer distance.

Fred...

Post by **Q45tech** » Mon Sep 09, 2002 6:15 am

At 60 mph 3rd is 2612 rpm if you normally get 24 mpg [on a Q] at 60 in overdrive no AC, you would be reduced to 16.6-17 mpg in 3rd on a Q, the 30.5% reduction should be the same percentage on a J as the tranny gear ratios are the same.

Post by **alexhho** » Mon Sep 09, 2002 6:15 am

So if my driving is mostly under 35 mph, I will not be saving fuel if left in 3rd gear? The owner's mannual states that one should drive in 3rd gear if "not going too fast," though.

Never mind. YOu were right. I just checked the mannual and all it says is not to leave it in 3rd gear when going over 60 to save fuel. I just assumed that the 3rd gear is designed to save fuel at lower speed, but this is not mentioned any where in the owner's mannual.

Post by **juiceman** » Mon Sep 09, 2002 6:15 am

I live in a semi urban area of tampa, FL No hills really and most driving is 30 residential and 40 to 55 local roads. I average around 21 mpg on my 93 J30. I am not really a fast driver nor do I gun it alot but sometimes I get the need and haven't noticed a major drawback except for around a 1 mpg loss. I always keep it in "d" I guess from what I hear this is good. Don't get me wrong we have alot of lights especially in the major town area. Actually I curse everytime they put up a new one. But with all the accidents (mostly caused by the young and elderly) We have alot of elderly thats why florida is call "gods waiting room" and the state flag is a half steering wheel with knuckles and the top of a grey head

Post by **Q451990** » Mon Sep 09, 2002 6:15 am

It seems like some J owners have reported significant increases in MPG after replacing their O2 sensors... you might think about doing that if you have over 75-90K on your car...

Heath

Post by **Mayhem_J30** » Mon Sep 09, 2002 6:15 am

Q451990 wrote:It seems like some J owners have reported significant increases in MPG after replacing their O2 sensors... you might think about doing that if you have over 75-90K on your car...

Heath

do you think the O2 sensors actually go bad or just get crudded up and need a cleaning on any wiring harnesess and so forth? i've read on the Z sites how just going through with an electronic cleaner and cleaning up all the wire connections(for almost all wires) it seems to fix a lot of problems.

Post by **juiceman** » Mon Sep 09, 2002 6:15 am

I have heard from some self proclaimed experts that choosing the correct O2 sensor is important. The oem is selected on a cost vs effiecency. I have heard there are some brands that can improve your milage significantly but at a significant cost.

What do you think?

Post by **alexhho** » Mon Sep 09, 2002 6:15 am

Q451990 wrote:It seems like some J owners have reported significant increases in MPG after replacing their O2 sensors... you might think about doing that if you have over 75-90K on your car...

Heath

I have a 97 J30 with 51,000 miles. It's been well maintained. On a recent trip, with a 75 mph average over 350 miles, I average an astounding 31 mpg. In the city, however, it's 16mpg--which is a big discrepancy in my view. I don't think anything is wrong with the O2 sensor, though. Thanks for your reply, Heath.

Post by **Q45tech** » Mon Sep 09, 2002 6:15 am

The ecu memory knows the approximate amount of gasoline to inject for each rpm and load [percent of throttle opening].

This is checked against the MAF voltage to correct for air temperate and elevation and humidity.

Then the O2 sensors are consulted to make sure that the calculation producees the correct 14.7 to 1 ratio......this is the short /long term block learn a fine trim to correct for injector clogging or crud on the intake valves limiting the air flow.

The O2 sensors only affect the mixture by 15-20% so typically they can only affect the mileage by this amount.

The O2 doesn't measure the exact ratio it just goes above 0.5 volts when it is richer than 14.7 and goes below 0.5 volts when it is leaner......at cruise the sensor is switching back and forth rich to lean [hopefully fast 10 times per minute or more].....the faster it switches the faster the ecu is adjusting and fine tuning the mixture.......old O2 are slow [SLUGGISH] in the rate they identify the mixture error........so the ecu goes rich when it doesn't get the signal fast enough!

The ecu always goes rich as a protective measure any defective sensor [loss of signal] will make it very rich.

Post by **Mayhem_J30** » Mon Sep 09, 2002 6:15 am

i got 20.847 mpg out of my J this week. That's with driving in 3rd through the city. There was also a can of BGK44 in the tank, so that might have affected it somewhat. I'll report another one next week since i just filled up today.

Post by **alexhho** » Mon Sep 09, 2002 6:15 am

Mayhem_J30 wrote:i got 20.847 mpg out of my J this week. That's with driving in 3rd through the city. There was also a can of BGK44 in the tank, so that might have affected it somewhat. I'll report another one next week since i just filled up today.

Thanks for the update Mayhem. Did you have to change your gas filter when you added a can of BGK44 to your filled up? I wanted to try a can, but couldn't find the location of the gas filter in the owner's mannual. And it only makes sense that if any product is used for dissolving gimes in the fuel system that the fuel filter had to be changed. Love you have your thought on this. Take care.

Post by **Mayhem_J30** » Mon Sep 09, 2002 6:15 am

alexhho wrote:Thanks for the update Mayhem. Did you have to change your gas filter when you added a can of BGK44 to your filled up? I wanted to try a can, but couldn't find the location of the gas filter in the owner's mannual. And it only makes sense that if any product is used for dissolving gimes in the fuel system that the fuel filter had to be changed. Love you have your thought on this. Take care.

I would change my fuel filter but I just changed it about 2 months ago. If your fuel filter is original it's a bit of a pain to remove. It's between the left side of the engine and the passenger side strut. Just follow the fuel lines from the top of the plenum(there should be two) and they'll connect to the fuel filter eventually. It's aluminum colored and no bigger then a baseball. Make sure your fuel lines are depressurized before removing or there'll be a big mess. I ordered my BGK44 from http://www.infinitipartsusa.com . Check it out, you can get an even better deal by purchasing both the fuel filter and the BGK44 at the same time. If I remember I'll take pictures of it when I get home and post 'em.

Post by **PalmerWMD** » Mon Sep 09, 2002 6:15 am

I am also all for changeing the filter too.It makes sense and it's what the wizards at T3 highly recommend.

Fred...

Post by **Mayhem_J30** » Mon Sep 09, 2002 6:15 am

fuel filter location....and a small oil mess courtesy of jiffy lube.

Post by **Mayhem_J30** » Mon Sep 09, 2002 6:15 am

close up

Post by **alexhho** » Mon Sep 09, 2002 6:15 am

Thanks, Chris. You're the best. Have a good weekend. Take care.

Post by **gbridger** » Mon Sep 09, 2002 6:15 am

Overdrive on J30? Of course. Just because it does not get activated like the OD on the M30 (with a button) doesn't mean it isn't an overdrive in fact.

Mileage on my 94 J30 for the past 9433 miles is 22.4. This is a mixture of short trips, a 12 mile commute, and a few open road trips. Car has 125,000 miles and is well maintained (but not up to Q45Techs's standards).

For you PDA users, AutoBase does a real nice job of tracking these things on my Palm. For free.

Post by **Mayhem_J30** » Mon Sep 09, 2002 6:15 am

hmmm...either my trip meter messed up or I didn't fill up the fuel tank all the way. My last mpg was 16.405!!

I'll let this thread die now.

Post by **alexhho** » Mon Sep 09, 2002 6:15 am

Nah! Keep posting, Mayhem. I'll post once my tank is up. I have been using the D3 gear since your last post of getting 20+ mpg. Take care.

Post by **PalmerWMD** » Mon Sep 09, 2002 6:15 am

I also drive in "3" in town, I get 14-15mpg (91 Q45).

Which is exactly where it should be for me under thses conditions.

Fred...

J30 Overdrive function question...

J30 Overdrive function question...

Nissan Forums

Infiniti Forums

Datsun Forums

Technical Forums

Classifieds / Marketplace

Nissan Motorsports

General Forums