I have heard rumors that the Euro versions of my 1991 350SD have taller rear end gears.

Is this true?

If I change the gearing would I get better mileage?

Could I go faster? (not that I would do something like that

Would I have to change something with the speedometer?

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1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside

Taller gears, such as 247 from the 500sec or 224 from the euro 500 sec or sel would slow the engine at highway speed and return better fuel economy at the expense of acceleration. YOu would need to adjust the speedo too to match it. On that car its an electronic drive so I don't know how you would need to do that.

Tom W

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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.

No such thing as a diesel W126 in Europe. A 1991 S350 would be a W140, not a W126.

If you want taller gears swap in the rear from a 560, but if you have to go up any hills its going to suck. However if you live in a pretty flat part of the US it could work well.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

I live on a mountain, so the taller gears might suck.

My thinking was that they slowed the cars down in the US because of the 55MPH limit.

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1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside

Operating the car at the peak torque of the engine will produce the best economy..

and there is a happy medium about Rear end gears, and what they will do for economy.. too tall and you are always hunting for a lower gear.. too low and you are buzzing the motor to keep up with the Yugo's of the 80's.. but you may find that most people want to get great economy, at 78MPH, which may happen, but it would become fantastic economy if they would slow down 20 mph...

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46 WDX Power Wagon

84 300TDT daily driver

[QUOTE=JDmills;1824519]Operating the car at the peak torque of the engine will produce the best economy..

I respectfully disagree. The best economy will be achieved when you have the gears so that the engine turns the slowest possible rpm at the cruising speed you desire without down shifting.

That could coincide with max torque output or not.

Tom W

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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.

Neither. The engine has a BSFE (brake specific fuel economy) curve, the most efficient operation is where the cruising speed coincides with the top of the BSFE curve. Without knowledge of where the engine produces its (b)hp most efficiently, you cannot gear it for the most efficient operation.

Usually this is in the low-end of the engine's useable range.

I suspect that taller gears would yield better fuel economy at today's highway speeds.

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Gone to the dark side

- Jeff

True.

Operating it at peak torque will provide maximum efficiency not economy. And that's mechanical efficiency not fuel efficiency.
You don't see people revving their Honda S2000's to 7000 RPM at every stoplight getting good fuel efficiency.

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1985 CA 300D Turbo , 213K mi

Ok, I am over my head here.. all I was trying to say goes something like this: If you want to get close to your best fuel economy, try driving the car at, or near peak torque, which often is close to the most efficient engine speed, for fuel economy..

changing rear end gears, or even changing the hight of the rear tires will change the effective final drive ratio.. then you can go ahead and find a set of rear gears that suit your desires.. and get a map of the BSFE, and figure out how to drive by it's suggestions. BUT for tomorrow's commute, the RPM at which peak torque occurs, is a good place to start.. Or it seems to work for me in my driving... of course that effectively limits my top speed to about 55 mph in the '84 300TD... just as MB planned it (Or so I choose to believe).

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46 WDX Power Wagon

84 300TDT daily driver

Again, that's wrong. Peak torque is around 2500 rpm. You will get better mileage at 2000 rpm than 2500 rpm.

What about a Corvette where the torque maxes out at 4000 RPM or so? Cruise at that and you'll get 15 mpg. That's why they gear them so high, at top gear they're turning 1500RPM at 70 mph giving the best fuel economy.

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1985 CA 300D Turbo , 213K mi

Gassers and Diesels do not use the same gearing and are apples : oranges when talking about cruise RPM. Gassers want a high a gear as the driver can stand because they take in fuel to match the air ratio.

Diesels need to be geared to run in their optimum torque range because that is where the engine is using the most energy out of the fuel. Our torque range is 2400-3500rpm so thats where they should be geared to run at.

I don't agree Forced. If you compare the BSFE curve on a big CAT to the torque curve, they are not at all the same. I'm open to being proven wrong if you have some data to support your claims though, I've been wrong before.

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Gone to the dark side

- Jeff

A 603 should spin about 3k at 70. If you gear it down so that its spinning what 2.2kish with the 560 rear than you will be off boost and be forced to constantly downshift if you want to do anything. If you live in FL where its flat as can be you could probably get away with it.

Around here where we have hills that sometimes require you to floor it at 70 if you want to hold that speed, NFW you will be crawling up at 50 in 3rd.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

It is just how much fuel is used per mile. Obviously there is some dispute as to what will work or not. My reasoning is that the car was built for 55mph in 1990 and now everyone drives 80mph around here. That seems stupid because of the fact that wind resistance increases with the cube of the velocity.

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1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside

Maybe it is because I drive a 350, but I don't have to floor it now to go 70, or even 100, over the mountain pass (I64) next to my house. On the flat at 80mph I barely touch the pedal.

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1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside