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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old October 5th, 2005, 19:16   #1
david_594
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Fuel Economy: 51.7/48.2/40.7
Default 5th gear upgrade. .681 success 11% RPM drop

Edit: The important details.... A .70 and a .681 fifth gear sets that should fit the stock 5 speed manual tranny with the TDI were found. The .70 was never tested by anyone and I installed the .681. So now there are both a .681 and a .717 5th gear set that are verified to fit the 02J transmissions of our cars. The .717 provides a 5% reduction in RPM's while the .681 provides an 11% drop in RPM's. Either swap will provide for a much more pleasant highway experience. Rough estimates on the fuel economy savings from either swap is guestimated to be roughly half the drop in RPM's but this is not thourghly verified. If your intersted in doing this swap please read through the rest of the thread.

dave

Original stuff....
The info on the .681 gear is in my last post down this page somewhere.

So its a topic that seemed pretty popular 5 years ago and myself and another club member were considering it a bunch as of lately and found some new information. First off the ALH tranny comes with a .756 5th gear set and the original upgrade was to swap it out for a .717 from older diesel AAZ engined diesel available in canada. This provided an RPM drop of rough 5.16% and the people who did it claimed a small increase in fuel economy. In the old threads on this subject there was mention a few times of a .681 gear set from an Audi which would have provided roughly a 9.92% drop in RPMs, but no one ever provided what vehicle or tranny this was from so for as much as anyone knows it doest exist.

The new news

Doing some searchign, supposidly the new 100hp BEW PD's come with a .74 5th gear ratio with 29/39 teeth. This seems fitting with the added HP of the newer 100hp PD motor. (I got this info from a VW tech who looked it up at work)

Also, a .70 5th gear set(28/40 teeth) was found from a euro eurovan 02A tranny. From whate I have gathered the gears from the 02A and 02J are interchangable so this should be a feasable upgrade for all our A4 TDI's. This would provide a roughly 7.41% drop in RPMs for an ALH tdi or 5.41% drop in RPM's for a PD.

Here are the part numbersfor the .70 eurovan gears.
02A 311 361 AM
02A 311 158 AF

So who wants to be the guina pig? This wont be cheap, probabily somewhere close to $400 as the parts would have to be shipped from europe. But at the same time if you get your original gears out in one piece you should be able to sell them to a gasser for around $150 to recoup some of the cost as our gears is an even larger upgrade for them.


Here is a link to the how-to hosted on the tdiclub pictures server:



Follow the link, login to the pics server, scroll down and click on the image that says "multimedia" and it should download the PDF for you.

Please ignore the numerous typos as I clearly didnt proof read it when I made it. Also take the warnings seriously. When I first wrote this up ~10% of people had major complications when doing this mod.

Last edited by david_594; July 11th, 2012 at 18:25.
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Old October 5th, 2005, 19:25   #2
JettaJake
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Default Re: 5th gear upgrade. .70 ratio!

group buy?
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Old October 5th, 2005, 19:44   #3
cevans
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Default Re: 5th gear upgrade. .70 ratio!

Dave,

Just be the trial subject yourself!

Lets see if we can get a vendor to get a definite price!

cevans
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Old October 5th, 2005, 21:54   #4
nicklockard
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Default Re: 5th gear upgrade. .70 ratio!

I second the motion: group buy!
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Old October 5th, 2005, 22:38   #5
DrSmile
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Default Re: 5th gear upgrade. .70 ratio!

Excellent research, please let us know the results. At $400, even assuming an optimistic 10% increase in mpg and assuming a $3 a gallon diesel price, it would take 4.5 years or 70K miles to recoup the cost, but I could claim a 50mpg average. Quite a dilemma...
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Old October 5th, 2005, 22:46   #6
david_594
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Default Re: 5th gear upgrade. .70 ratio!

My assumtions on the cost/benefits of this was actually a modest 5% increase in economy. Also if you resell your original 5th that puts you at roughly $250 out of pocket. Roughly a 5% savings and you would have to spend 5 grand on fuel before you break even. At current prices of $3/gal were talking 1666 gallons roughly. Figure 50 mpg and your only talking 85k miles or so to pay off this mod.

Its all relative though. There is some cool factor to this also that its hard to put a price on.

Cevans,
Of course I'm planning to do this to my car. But like all my mods im a lazy bum. It could take me a long while to source the parts; then an even longer while to install them. If people want to wait for me to be first on this one, thats cool, if they want to try it before me, thats cool to .
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Old October 5th, 2005, 22:53   #7
lumberjackbob
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Default Re: 5th gear upgrade. .70 ratio!

If someones interested, I know of a salvage yard that has a used 5sp tranny with about 70K for $750. The yard is located in SE Washington. If I didnt have a wedding to pay for I might be all for this.
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Old October 5th, 2005, 23:33   #8
DrSmile
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Default Re: 5th gear upgrade. .70 ratio!

You don't need a spare trans. 5th gear on a transaxle sits on top of the case and doesn't even require transmission removal from the car to swap out the gear. Labor is minimal, really the major cost is gears and transmission fluid.
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Old October 5th, 2005, 23:47   #9
TheCheat
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Default Re: 5th gear upgrade. .70 ratio!

Someone confirm the gears are indeed interchangeable and I'll pony up in a couple months.
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Old October 6th, 2005, 00:11   #10
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Default Re: 5th gear upgrade. .70 ratio!

Contact TMtuning.com They can get any parts you need--tell them you are interested in a group buy and they will work with you--they like giving me personal deals because I have them listed on my "sources" page
Sounds like fun though--wish it worked on an A3!
-BB
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Old October 8th, 2005, 18:40   #11
david_594
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Default Re: 5th gear upgrade. .70 ratio!

So I am going to be order a set of these. I found a guy from the vortex in Europe who is getting them shipped to me. So hopefully we will know in a few weeks.
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Old October 8th, 2005, 19:38   #12
Mike_M
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Default 5th gear upgrade. .70 ratio!

WOW! Here's some numbers...assuming the ring & pinion (final drive) remains stock. Assumes 2150rpm @ 60 for stock .756 gear.

1900rpm (torque peak): 53mph before, 57.3 after
2000rpm: 55.8mph before, 60.3 after
3000rpm: 83.7 before, 90.4 after!

65mph: 2329rpm before, 2156 after
75mph: 2688 before, 2488 after
85mph: 3046 before, 2820 after!

Huge improvement from just a 5th gear upgrade.

Now, if I have time to go look at my Bentley manual (to find out the stock ALH final drive), I'll be able to calculate if you also swap in watercop's beloved 3.156 final drive...

Mike
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Old October 8th, 2005, 19:51   #13
david_594
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Default Re: 5th gear upgrade. .70 ratio!

final drive is 3.389
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Old October 8th, 2005, 21:04   #14
QuickTD
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Default Re: 5th gear upgrade. .70 ratio!

Quote:
My assumtions on the cost/benefits of this was actually a modest 5% increase in economy.
I don't think you'll never see a 5% increase in fuel economy. It still takes the same amount of energy to propel the car at a given speed. The only thing that is gained with taller gearing is a slight reduction in friction losses. A 5% increase in gear ratio will not improve fuel economy by the same amount.

That said, I regeared my AAZ to a 0.71 fifth gear from the stock 0.75, about a 5% increase. My goal was to improve my sanity on long highway drives. The taller gear ratio dropped the engine rpm at cruising speed out of the "buzzy" range and improved my comfort quite a bit. If I said it improved my fuel economy it would be a lie, it didn't change one bit.

Just make sure you're justifying the expense properly, you won't ever recoup it in fuel economy, but you can tell your wife that if you like.
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Old October 8th, 2005, 21:11   #15
david_594
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Default Re: 5th gear upgrade. .70 ratio!

No wife, Im a college student though. Its not just the friction losses you are reducing. The main advantage in terms of economy should be the fact that you are keeping the car closer to its max torque. And the max torque should also be the where the car is nearest its maximum mechanical effeciency. This is what your gains would be from, not friction losses.
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