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  #1  
Old 11-14-2008, 09:42 AM
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Better gear ratio for W123 Diesel What axle would fit ??

15" wheels should help the MPG on my 300TD I mounted 15" steel rims with 205/75 tires. The RPM dropped by 10% so I hope I get some better than 20MPG. The ride is super nice on the highway and the tires are stiff enough for stability.

1. What differential could I use for better gear ratio??
2. In what other models can I find that differential ?

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  #2  
Old 11-14-2008, 09:44 AM
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mmmmmm Diesel...
 
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2.88 that was used on the '85 w123 and w126. You will need the '85's speedometer as well.
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70's Southern Pacific #5608 Fairmont A-4 MOW car

06 VW Jetta TDI 133K miles-replaced the other '06 that quit after 300k miles. (did not catch the bad cam in time)

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 195K

90 Dodge D250 5.9 Cummins/5 speed. 400K
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  #3  
Old 11-14-2008, 09:46 AM
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What ratio does my 1981 300TD wagon have now?
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300 TD Wagon 1981 (Daily Commuter Tractor)
911 MFI 1973

www.vintagemotion.com
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  #4  
Old 11-14-2008, 10:00 AM
rrgrassi's Avatar
mmmmmm Diesel...
 
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I'm guessing 3.07. It is stamped on the bottom of the rear diff, I just do not remember if it is the left or right corner. You can see it just below the rear diff cover. Of course it needs to be cleaned to read it.
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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

06 VW Jetta TDI 133K miles-replaced the other '06 that quit after 300k miles. (did not catch the bad cam in time)

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 195K

90 Dodge D250 5.9 Cummins/5 speed. 400K
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  #5  
Old 11-14-2008, 11:38 AM
ForcedInduction
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If you're getting only 20mpg then you're either driving it VERY hard or there are problems dragging down its efficiency.

Search the forum for suggested maintenance points.
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  #6  
Old 11-14-2008, 11:49 AM
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I don't think mpg is as related to rpm on a diesel as it is on a car. It takes a set amount of POWER to move the car x distance at y speed based on the rolling resistance of the tires and aeordynamic drag. That power requirement when factored with the efficiency of the engine gives fuel economy.
Increased resistance in any way (dragging brakes, added roof racks, who knows what else) will decrease economy, so will decreasing the efficiency of the engine with things like bad thermostats.
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  #7  
Old 11-14-2008, 11:53 AM
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mmmmmm Diesel...
 
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The same diesel engine will use more fuel running 3000 rpm vs 2500 rpm, going the same mph.

All the other stuff stated will also factor in.
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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

06 VW Jetta TDI 133K miles-replaced the other '06 that quit after 300k miles. (did not catch the bad cam in time)

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 195K

90 Dodge D250 5.9 Cummins/5 speed. 400K
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  #8  
Old 11-14-2008, 02:28 PM
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I couldn't imagine only getting 20 mpg.
I think the worst I have ever gotten is around 25 in town
with a lead foot.

I think I would look into the maint. things too
Brakes dragging, injectors, tire pressure, etc.
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  #9  
Old 11-14-2008, 03:13 PM
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Quote:
Originally Posted by curtludwig View Post
I don't think mpg is as related to rpm on a diesel as it is on a car. It takes a set amount of POWER to move the car x distance at y speed based on the rolling resistance of the tires and aeordynamic drag. That power requirement when factored with the efficiency of the engine gives fuel economy.
Increased resistance in any way (dragging brakes, added roof racks, who knows what else) will decrease economy, so will decreasing the efficiency of the engine with things like bad thermostats.
The efficiency on internal combustion engines is highly depended RPM. If to low the efficiency is bad and if to high the efficiency is bad. I commute 35 miles each way and a speed on 75-80 MPG. The engine refs 3000-3500 RPM.

Next week I will try my MPG going no faster than 65...we will see.

Still hoping to find a 2.88 axle ....would a 300SD axle fit?
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Good Patina Comes With Good Miles Driven.
Drive more polish less

220SEB 1963 (excellent driver with no rust but far from pretty)
300 TD Wagon 1981 (Daily Commuter Tractor)
911 MFI 1973

www.vintagemotion.com
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  #10  
Old 11-14-2008, 06:59 PM
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Quote:
Originally Posted by MercedesOtto View Post
The efficiency on internal combustion engines is highly depended RPM. If to low the efficiency is bad and if to high the efficiency is bad. I commute 35 miles each way and a speed on 75-80 MPG. The engine refs 3000-3500 RPM.

Next week I will try my MPG going no faster than 65...we will see.

Still hoping to find a 2.88 axle ....would a 300SD axle fit?
not sure this works out, at least not for me. I dropped my RPMs by a whole gear and I gained 2-3 city(so far) 5 hwy(estimated due to past difference, I will know when the snowbirds go home) MPG going from the 3.07 to the 2.47.

the 2.47 I have is from a w126, you just have to flip the rear mount. also, when you pull the rear make sure you keep the axles shims as they are sized to the diff.
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Last edited by lutzTD; 11-14-2008 at 07:09 PM.
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  #11  
Old 11-14-2008, 07:32 PM
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you need to calculate your effective gear ratio. if it actually reduces rpm 10% then the actual mileage is 22. likewise the speed will be higher so if it reads 60 the actuak would be 66.

the 81 is na so the diff would be 346.
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  #12  
Old 11-14-2008, 10:21 PM
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Look in the resources tab above and click on DIY articles. A good write up on R&R a Diff.

See Post #2 above for 2:88.

Charlie
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  #13  
Old 11-14-2008, 10:57 PM
ForcedInduction
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Quote:
Originally Posted by t walgamuth View Post
the 81 is na so the diff would be 346.
For the 300D/CD, yes. The 300TD was the first non-300SD turbo in 81 so it should be 3.07.
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  #14  
Old 11-15-2008, 12:00 AM
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Actually, according to my mb illustrated buyer's guide the cd was turob in '81 too. I wouldn't be surprised if it was a mid year change over.
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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.
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  #15  
Old 11-15-2008, 12:21 AM
ForcedInduction
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Quote:
Originally Posted by t walgamuth View Post
the cd was turob in '81 too.
I have yet to see a genuine one.

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