Lost 5th gear, Better MPG? WHAT!

[nogassershere]

Believe it or not, it's true!

My wife drives our 2000 Jetta TDI as a daily driver, with a 40 mile commute each way. Her driving style coupled with the conditions and traffic usually net her around 38-40 mpg, In almost two years that we have had it, she has never done any better. Recently, due to a mechanical condition that I would rather not discuss, she was without fifth gear for several days, (over a full tank of fuel.) I drove it on a weekend, filled it up for her, and checked the mileage as I usually do. That tank which was driven entirely in 1-4th gear got 46mpg! the best since we bought the car! I checked and rechecked and came out the same. I finally got the shifter fixed, and it is back to the same old 38-40. Has anybody else experienced anything like this? Her commute is mostly flat, with no mre than two miles of city driving, the rest is 55mph country roads. Thoughts, comments.......................

 

[jcrews]

Use only 1-4 gear a couple more tanks under as similar conditions as possible, and see what happens. Once just isn't enough information. Keep track of total miles/gallons across several fill-ups to dilute any imprecision in fill-ups.

 

[jobob307]

Are running a .658 fifth? I just put one of these in my car and I don't understand how you could ever run one without heavy mods.

 

[supton]

Does seem odd, maybe it's a fluke tank. The only thing I can think of is that 5th gear had too much drag--bad bearings on just 5th, maybe.

When she did not have 5th, did she still drive 55mph, or did she limit herself to like 40?

 

[jobob307]

Come on you have to tell us why fifth was missing

 

[yatzee]

we won't judge or anything.......

 

[Trog thou Meatier]

I get slightly better economy in 5th than I do 6th but there's not much in it. Besides, it's lovely and peaceful in 6th.

 

[jettawreck]

Believe it or not, it's true!

My wife drives our 2000 Jetta TDI as a daily driver, with a 40 mile commute each way. Her driving style coupled with the conditions and traffic usually net her around 38-40 mpg, In almost two years that we have had it, she has never done any better. Recently, due to a mechanical condition that I would rather not discuss, she was without fifth gear for several days, (over a full tank of fuel.) I drove it on a weekend, filled it up for her, and checked the mileage as I usually do. That tank which was driven entirely in 1-4th gear got 46mpg! the best since we bought the car! I checked and rechecked and came out the same. I finally got the shifter fixed, and it is back to the same old 38-40. Has anybody else experienced anything like this? Her commute is mostly flat, with no mre than two miles of city driving, the rest is 55mph country roads. Thoughts, comments.......................

Fluke tank/innacurate fill. Not possible unless you never exceed 45 mph in 4th.

I'm not sure why your "usual" mileage is so very low. My 40 mile daily commute is fairly similar and average 50+mpg in the Jetta. Mostly 55mph country road stuff would get me 55+ easy.

 

[Ski in NC]

She might have driven slower without fifth. Engine sounds like its screaming at the higher rev's (even though it is perfectly happy there) and she might have just run slower. Running slower makes for better mileage mostly due to less wind drag.

 

[n1das]

Come on you have to tell us why fifth was missing

Yes, please do tell. I can relate to it becasue I've got a similar story to share although it didn't happen to me.

A co-worker of mine recently lost 5th gear in his 2000 Jetta TDI with 300k miles on it due to a mechanical failure inside the tranny. He was cruising along in 5th and then all of a sudden it was like he was in neutral. 1 thru 4 were OK but 5th was definitely gone. He ended up replacing the tranny with a used tranny, installed by TDIclub guru Chris Hill ("mrchill"). According to CHill, the gears for 5th were still intact and I think the mainshaft snapped after 4th, leaving 5th disconnected. The tranny still had gear oil in it but it was really baaaaaaaaaaadddd (it had never been changed or level topped off).

I don't know how he did for MPGs but he babied it for a while until the tranny swap work was done.

 

[dogdots]

The EGT's seem higher accelerating in 5th versus 4th at 55mph, so with the higher gearing you are using more fuel to reach same speed, assuming you are rolling in and not stomping. Is the commute level or hilly?

 

[validius]

depending on the driving cycle the car may be more efficient turning slightly higher revvs. Just because the parasitic HP is higher doesn't mean the engine is more efficient. My best guest is that your wife drives like a granny and with 5'th available she never was on boost. Boost can increase combustion efficiency.

 

[DPM]

I've found that sixth gear is about useless in the Subaru. Much better mpgs at the slightly higher revs/lower throttle position...

 

[nogassershere]

Thanks for all of the comments people, to answer a few questions, the car is completely stock, we live in south Alabama, and about 30 miles from the highest point in Florida (345 feet above sea level,) so her commute is mostly flat, and no, no, triple no, she does not drive like a granny! She would probably rear end your civic and then pass you with her finger out the sunroof if she heard you describe her driving like that!
I don't know why her mpg is where it is, seems to be normal for TDIs around here, even on the road across the country, its not any better. (check out my post on the road trip page).
Oh, the trans issue, just a loose nut on the top of the shifter input shaft right below the battery, and like a dumb a!#, I kept forgetting to tighten it! Got it now though.
My bet guess is just less of a load on the little oil burner in 4th, but can you use less fuel and still turn more rpms?
More L8R

 

[CTD&TDI]

My bet guess is just less of a load on the little oil burner in 4th, but can you use less fuel and still turn more rpms?
More L8R

yes it is possible...think of this, when you take your foot off the throttle, a diesel burns zero fuel. has the intake/egr ever been cleaned? lower rpms, it could really be choking, and the higher rpms pumps more air through (theory I just came up with....idk if it mechanically is tru...)

 

[MayorDJQ]

If she's only getting 38-40mpg there's something wrong with the car. I never got worse than 45mpg from either of my 5 speed ALHs, even with a 12 mile 4th gear commute that was up & down hills.

 

[nogassershere]

Well, there are no obvious issues, only about 45k on tb. Suggestions on where to start?

Let me correct one thing..... I stated earlier that the car was completely stock, I forgot about the ventectomy that I performed the day I got it home from the dealer......

 

[Journier]

yes it is possible...think of this, when you take your foot off the throttle, a diesel burns zero fuel. has the intake/egr ever been cleaned? lower rpms, it could really be choking, and the higher rpms pumps more air through (theory I just came up with....idk if it mechanically is tru...)

you are sorta correct.

Im not going into it but if the factors are the same in a different gear your good to go.

 

[meetis]

If she's only getting 38-40mpg there's something wrong with the car. I never got worse than 45mpg from either of my 5 speed ALHs, even with a 12 mile 4th gear commute that was up & down hills.

I would not make that assumption with how he described her driving habbits earlier. There is a sort of magic range for tdi's on rpm's and speed. Seems that at least for me the my TDI gets the best MPG at roughly 2k rpms where the car is not bogged down due to gear ratio and yet its not working very hard to go fast. My SGII reports roughly 60mpg at about 2k rpm's on the highway which for my car also is about 60mph (very tall 5th forgot the ratio but it was tallest that tdiparts.com offered) If she floors it on most gears that can drasitcally reduce average MPG. Though look at it like this if it was a gasser she would probably be averaging 20mpg with similar engine and driving habbits.
With highway speeds being 55 in your area i would not really expect the car to work in the optimal rpm's to mph range for very long so 5th gear in your area does sound to be optional for the most part. She may be shifting into 5th too early in some places where the engine would accually work harder than it needs to compaired to stayign in 4th gear. Kind of a funny thing with diesel's or probably cars in general shifting too early is just about or sometimes worse than shifting late.

For what its worth for me now workign in RI and haveing a very tall 5th gear i often times dont shift into 5th in RI (assuming i am going the limit on 95 /195 ) because the car has to work so hard just to not stall on a hill. Soon as i am in MA and the limit is 65 i am into 5th and at about 60 where the rpm's are semi low but not so low where it may stall and getting great mpg's.

 

[grizzlydiesel]

very interesting, i had been planning a fifth gear swap myself which would make fifth gear kinda useless for me around town. So i had been toying with the idea of only using 4th gear and modifying my speed (no more than 45-50, and seeing what that did to my mileage. i filled up yesterday, and no time like the present. guess ill see what happens. im currently getting 44mpg on my daily commute, with a little bit more city driving than you see. ive got my tires pumped up to 42psi, and dont drive fast, but i dont hypermile either. I ride a fixed gear bike with no brakes, i try to drive my car the same way. less attention on the gas, and more attention on trying to avoid the brake pedal. carry momentum as much as possible, rev match coming out of a turn to avoid lugging the engine in too high a gear. that kinda stuff.

also, there may be something to this boost thing. ive found my boost begins to build around 1800rpm. so i changed from crusing at 1200rpm to cruising at 1800-2000, and have noticed a few mpg increase.

 

[thebigarniedog]

There really is not enough information presented here. What would be helpful is knowing the typical MPH she drives on her commute and whether she deviated from that when 5th gear was lost (and if so, how so). Additionally, as previously mentioned a scangauge is an excellent tool to show the variable MPH driven and resulting MPG associated at such speeds and in what gear.

My scangauge shows that my ALH will return high 50s low 60s MPG if I "could" drive a constant 45ish mph. Never going to happen as I usually have to run from one proverbial fire to another and I like to use that right pedal .

 

[grizzlydiesel]

My scangauge shows that my ALH will return high 50s low 60s MPG if I "could" drive a constant 45ish mph. Never going to happen as I usually have to run from one proverbial fire to another and I like to use that right pedal .

in which gear???? 4th or 5th at 45mph. 5th seems like it would be lugging the engine pretty bad, and would certainly be below the boost threshhold.

 

[nogassershere]

I am kinda wondering bout the whole fourth gear thing, it would be nice to see other people's results. It would provide a wider range of conditions to compare. It's not real likely hat she changed her driving habits at all during the fourth gear days. There was also about a half tank of fuel run through the car in fourth gear besides the full tank, although not the "stellar" mileage of the full tank, it ws still better than our average MPG.
keep the ieas rolling.....

 

[pdh]

In my experience with my '06 Jetta TDI w/Scanquage I have found the engine to "lug" based on the SG engine load readout if I'm in 5th @ 55mph (on speeedometer) or below. I do not use 5th gear untill I hit a speedometer reading of 60 mph. I believe the mileage improvement may be related to improved turbo boost @ the higher rpm. Only real proof would be dyno testing.

 

[BrianCT]

I was always told that rpm vs miles per hour determines mpg. Your car consumes "X" amount of fuel per stroke at rpm vs actual speed.

You can not get around the fact that the stroke of each piston demands "X" amount of fuel.

 

[grizzlydiesel]

I was always told that rpm vs miles per hour determines mpg. Your car consumes "X" amount of fuel per stroke at rpm vs actual speed.

You can not get around the fact that the stroke of each piston demands "X" amount of fuel.

diesels are fuel throttled, even if they are air throttled, air/fuel ratios do not remain constant. but especially with a diesel that is fuel throttled. producing Y ammount of power, can be done at a higher rpm, with less throttle input, which would put less X ammount of fuel in the engine, with a leaner burn.

this can be done, as described, by running higher rpm, which spools the turbo, and builds boost, allowing less fuel to be necessary for the same power output.

at least in theory. Ive started neglecting my fifth gear today, running fourth at 45 to and from work. Ill report what turns up fuel economy wise over a half tank.

 

[thebigarniedog]

in which gear???? 4th or 5th at 45mph. 5th seems like it would be lugging the engine pretty bad, and would certainly be below the boost threshhold.

Uhm, this thread is about no 5th gear, so what'd ya think it is .

 

[BrianCT]

I've started to neglecting my fifth gear today, running forth at 45mph, to and from work. Ill report what turns up "fuel economy wise" over a half tank.

I've done that for 225,000 miles. I never dump 5th until seeing freeway posted speed limits [more like 65 mph]. We do this to both the PD 2006 MKV and the MKIV, which are both manual transmissions. No record breaking mpg marks between us in 4th and nothing to write home to on 5th.

 

[HopefulFred]

I was always told that rpm vs miles per hour determines mpg. Your car consumes "X" amount of fuel per stroke at rpm vs actual speed.

You can not get around the fact that the stroke of each piston demands "X" amount of fuel.

Like grizzlydiesel was saying, diesel can run as lean as it wants. The way I think about it, you only burn as much fuel as you need to drive and accelerate as fast as your right foot requests - gearing is basically irrelevant in the ranges where all the fuel in burned.

What that logically leads to is that if enough air is running through the exhaust to keep the turbo on boost, and you're not making smoke, it doesn't matter. 2k in fifth or 2800 in fourth.

I'm not sure where advance is maxed out and there begins to be insufficient time for complete combustion, but that would be the engine speed where economy starts to drop.

That said, I have never consistently done better than 41mpg. That was a 35 mile, mostly highway commute. Now that I do more stop and go for only 13 miles, I'm down to about 37.

 

[McBrew]

I was always told that rpm vs miles per hour determines mpg. Your car consumes "X" amount of fuel per stroke at rpm vs actual speed.

You can not get around the fact that the stroke of each piston demands "X" amount of fuel.

Certainly not true. You'll be running at a given RPM at a given speed in a given gear. That doesn't take load into the equation, though. Going down a hill in 5th gear at 2,000 RPM will probably use no fuel (overrun). Moving on a flat road in 5th gear at 2,000 RPM will be the same speed... but will have to use a small amount of fuel to maintain that speed. Going up a hill at 2,000 RPM will be the same speed but will require more fuel to keep speed up on the hill. Towing a trailer at 2,000 RPM up that same hill in 5th gear will still be the same speed, but will require more fuel to move that trailer.

A generator is a good example, too. They typically run at one speed all the time. You'll notice that fuel usage of a generator is based on load (i.e. 1 gallon per hour at 50% load).

FWIW, in my experience a gasoline engine works the same way, but to a lesser degree. Diesels have the ability to run much leaner than gas engines and can run with a lot less fuel at a given RPM. This may be changing with new gasoline engine technology, such as direct injection and more advanced valve-lift (rather than throttle) technology.