I have read all of the threads I could find on this and I am beginning to understand the issue with horsepower limitations in improving fuel economy through a rear end swap to a 2:88. What about going with something less drastic? I am content to cruise at 55 if I have to to get the best mileage possible but since I plan on keeping my China Blue baby forever, I feel that even small improvements will pay for themselves over the life of the car, especially with B100 at $3.50/gal!

Best,
Rich Mason
1980 240D Manual 200k

I would normally be completely against such a swap considering the limited horsepower already available from a 616.

Very simply:

How do you consider the acceleration of the vehicle currently? Is it acceptable to you? Is it marginal? Would you be OK if the vehicle was 15% slower in acceleration?

Personally, I cannot imagine a 240 at 15% slower than it already is, however, if it will work for you, then you could consider a 3.07 axle from a 617 powered vehicle.

You would achieve somewhat better fuel economy from running the engine slower, however, I would be very surprised if it was more than 7% on the highway. There will be no improvement in fuel economy in the city.

I went to the 3:07 but bear in mind that my 240D now has a 300 turbo in it, so I didn't feel the acceleration issue. Shift points and fuel economy are more favorable.

On my old China Blue 240D, I wouldn't have switched.

Don

__________________
DAILY DRIVERS:
'84 300DT 298k (Aubrey's)
'99.5 Jetta TDI IV 251k (Julie's)
'97 Jetta TDI 127k (Amber's)
'97 Jetta TDI 186k (Matt's)
'96 Passat TDI 237k (Don's
'84 300D 211k Mint (Arne- Undergoing Greasecar Conversion)

SOLD:
'82 240D 229k (Matt's - Converted-300DT w/ 4 speed

Acceleration is not an issue for me and I would gladly give up some around town speed for improved efficiency. For a few hundred bucks I could always switch back if I didn't like it I guess. Is there someone out there who can do the math for the best 65mph rear end to balance horsepower and torque?

Much thanks,
Rich

I believe the point is moot, Rich. AFAIK, the tallest axle that you can put in the 123 would be the 2.88:1 from the 1985 vehicle. This would offer an engine speed of about 2600 at 65 mph. It also will limit your top speed to something like 70 mph.

With this axle, however, you could always shift down into third and run the vehicle at about 4000 rpm. This would provide ample power for an upgrade at speed. In fact, you might even get more speed from the vehicle in third gear at 4500 rpm than you can in fourth gear!

How would the swap to a 2.88:1 in a 123 (1945 300d turbo) limit top speed to 70 mph if 2600 is 65mph?

It's in response to a 2.88 with a 616 engine. Not enough torque at the lower engine speed to accelerate any more.

__________________
'83 240D with 617.952 and 2.88
'01 VW Beetle TDI
'05 Jeep Liberty CRD
'89 Toyota 4x4, needs 2L-T
'78 280Z with L28ET - 12.86@110
Oil Burner Kartel #35

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Since I have a 1980 240d Manual also... and have thought about the situation due to a lack of hood insulation making the revs very noticiable...
I would suggest keeping your car stock... and just being sure it is as efficient as possible in the usual preventative and tune up sense...
Most of us 240 owners do not rev the engine to the marks on the speedo meant as shift points anyway....
Shifting much before them... or running the car in a speed over them for long periods may be wearing out the engine sooner than it would otherwise....
Lugging an engine is not good for mileage or engine wear....
And then there are the relationships in the governor/throttle/load areas and stuff which makes it very hard for a DIY fix....
On the other hand... if the Factory put out some change noted in their Tech Service Bulletins that might be worthy of spending money on.. check the titles in Alldata.com..
You listed several figures... but not what you are getting for mileage.... have you checked it carefully....and done the easier and more usual tuneup stuff ?

Old300D is correct.

We are discussing the swap in a 240D n/a.

I am guessing that the available horsepower at 70 mph in high gear (at 2800 rpm or so) will exactly match the horsepower required to push the vehicle through the air at that speed.

It might achieve 75 mph, but, it would take forever to get there.

I'm currently getting low 20s for mpg. I've ordered some LM purge and I'm going to go through everything else before I swap the rear end. I've had the car about 3 weeks although this is my 4th 240 (3 115s before this). I have no service records so I have no idea of the timing chain history (200k miles on car), age of injectors, timing adjustments, etc. I do know that even when its warm outside I get alot of unburnt diesel out the tailpipe for the first minute before it warms up then no smoke in exhaust. Oil consumption is negligible. The car positively flies on the interstate from 55-75 making me think there is some room for negotiation with the rear end there.

Thanks for all of your input. I'm sure I'll find all of the information I need for making a complete assessment and all adjustments in the archives.

Rich

I would think the same, Rich.

What would you say this vehicle has for a top speed with the current 3.69 axle?

That axle swap is a 23% rise in axle ratio, what you might try is a 205/75/14 tire that will give you about 15% rise in axle ratio and might be cheaper & easier to do, not to mention easier to swap back. The standard axle ratio of 3.69 in the 240D would then become almost 3.00. Remember this car and gear ratio was factory with tires from 185/70/14 to 205/70/14, which is on average a 2 inch difference. These larger tire will easly fit as it was designed to except a 15 inch tire in 3rd world use, let me know if you want info about those tire sizes.

I have the 205/70's and I can hit about 80 mph on the flat and with long enough run maybe 85 mph. With 185/70's I could push 85 mph. I average 31/32 highway at 65 mph (gps) and love the car. Mater of fact I have decided not to move up to a turbo 300.

__________________
'10 Chrysler T&C Stow-N-Go White. Grandpa's ride.

'13 Chrysler 200 Touring Candy Red. Grandma's ride.

Age and cunning will always over come youth and vigor.

Navy, the change in tires will only offer 7.8%.

A 185-70-14 tire has a diameter of 24.2
A 205-75-14 tire has a diameter of 26.1


Engineers..........

Key word here is about. Man Brian give an old fart some credit for guessing. Sorry I just guessed as I had to run and do something at the time. The 15" rims would probably get him in the range he's looking for, but that's a guess Brian. My guess is the cost of this change would probabley take average person 10 years to recoupe in fuel savings. If it was me I would do the 205 or 215 jump in tire size when it comes time for new tires. 195/70 was biggest on US cars from MB and those had alloy wheels, lots of dealerships upgraded them from hubcap style at buyers request to the alloys with larger tires.

__________________
'10 Chrysler T&C Stow-N-Go White. Grandpa's ride.

'13 Chrysler 200 Touring Candy Red. Grandma's ride.

Age and cunning will always over come youth and vigor.

On that guess, Navy, we are in agreement.

Unless he gets the 2.88 for free, of course.

Oh, BTW, Navy, you can guess at stuff anytime you want.

Just don't guess wrong.