Even though my –84 SD always has gotten a fairly good fuel economy (especially in comparison to similar sized vehicles of its own age), I (too) wanted to improve it. Having done all the usual stuff (setting engine internal timing, replacing injectors etc) I wanted to go out a little further in “ecofriendliness”. What next?
So I started thinking about a rear differential swap especially since several folks here have reported good results with a 2.47 rear. Unfortunately, obtaining a suitable 2.47 (for the early W126) over here in Finland was going to be difficult (in my pricerange, at least)… What I did happen to find was an affordable 2.24 (early euro 500SEL/SEC) with a matching speedo so I decided to go for it, even though I was really concerned that this would be too much for poor old 300SD accelerationwise. WELL – having now testdriven the car for a few days, I’ve become a believer in the good things that such a rear swap brings along:
a) As one would expect, the
RPMs drop greatly! For example, whereas before at the speed of 120 km/h (75mph) the RPMs were up to about 3400 (IIRC) now they are around 2500 – this surely has an effect on the fuel economy. (BTW, it was nice to notice that math didn’t fail us here as: 3400 x 2.24/3.07 = 2480)
b) With the RPM drop the
noise level in the car has dropped also greatly
c) I was also worried about
in town driving – not any more – the SD pulls fine with ability to accelerate even on 4th gear in lowish speeds. (at 45 mph the RPMs are somewhere around 1400RPM
d) The most amazing thing is the
accelaration off the line – it is NOT that bad. Really, I do notice a change, but much less so than what I expected. It still moves nicely even with this 2.24 diff. (We are not talking about a rocket here to begin with
)
e) Others have reported the same thing – the
shifting (auto tranny) has
improved. Yep, didn’t believe what others claimed, now I do. (Maybe the old worn tranny just simply benefits from the change. My tranny certainly was tuned to perfection with the 3.07 (Probably NOT!) - I dunno what the reasons for this are)
The only other downside (besides the acceleration) that I’ve noticed is
coasting to a stop (engine breaking) – this is more difficult – the car just wants to roll and coasts to a stop much more slowly (makes sense, of course, just didn’t see it coming). So, one has to plan ahead or just use more brakes…
Well, the most important thing is still missing – the hard numbers concerning fuel mileage. I will report on this thread once I get some driving and fueling done on the car. Like I said, just got the car back on the road so… And beware, all of what I’ve said is still “initial experiences” based on relatively little driving.
The terrain around here is not very hilly, so if that is the case where you are at then I would not hesitate doing this swap… Probably a different story if you live in the Alps or some other mountain area (I don’t think the climbing would be the problem - but not being able to engine break could cause…well…discomfort).
Anyways – my point is – it is not a terribly bad idea to swap the 3.07 for a 2.24. Furthermore, based on what I’ve read from others, I don’t think the difference between a 2.47 and 2.24 is that noticeable (in a diesel). So if you happen to get your hands on a 2.24 go for it…
makes you love the SD even more, I think!