D vs. S for city mileage - PeachParts Mercedes-Benz Forum

seafood54 · #1 12-09-2004, 11:13 PM

I've been wondering about a non-scientific mileage test I started. I know this isn't the way a proper test should be performed, as in I shouldn't start with a presumed result and perform a test to prove it, but...

Feeling that the peak in the power curve for our diesels is somewhere around 2800-3200 rpm, I've begun doing city driving in S vs. D, thereby keeping the engine opporating closer to that band, rather than letting 4th lock in and drop the rmps down and out of that band.

I realize that this will also bring the effect of the turbo into the equation, and have read that that will have a negative effect on the mileage.

Any thoughts?

Franko in O..Hi..O

pawoSD · #2 12-09-2004, 11:16 PM

Same here, going to start doing that. I hate going 25mph when it locks into 4th gear at 22mph, there is NO acceleration in that gear at that speed, its so annoying, I see no use for D/4th gear in the city anymore, I am going to use S from now on

starting tomorrow....bleh to mileage

The endless kickdowns (especially when the engine is cold) are so annoying, and it can't be good for the tranny either in the city, with constant going between 3 and 4 like that....so S it is. There's a lot of hills on my daily route too, so growling up those hills can't be good for it either....eh, I'll see how it goes.....

seafood54 · #3 12-09-2004, 11:22 PM

Keep a loose track of the effect on your mileage and let us know.

Franko in O..Hi..O

pawoSD · #4 12-09-2004, 11:26 PM

Yeah I'm going to fill it up tomorrow, I always watch my mileage, I have noted that even using D, as my foot gets heavier (usually it just stays heavy) my mileage falls

....a lot. I got 19.4 to the gallon on my last tank, rather...bad. I've had as high as 26 in the city....so I know all that revvvy driving was using a *bit more* fuel.......

AlexTheSeal · #5 12-12-2004, 05:25 AM

My experience may be a little different since I have a 603 and everyone else who's posted here has a 617... but I always keep my 300D in "3" (equivalent to "S" in the older cars) except when going over 55 or so on the freeway. I also have my Bowden cable from the accelerator linkage to the tranny adjusted very tightly so that it downshifts with just mild tip-in on the accelerator at freeway speeds.

I've read that lugging is even worse for diesels than for gassers---to minimize wear, you want to keep the revs up off idle when there is any load at all on the engine. I believe I've also read that, unlike a gas engine, a diesel gets better mileage with the turbo spinning than without. That's why our cars are geared so that at a comfortable freeway cruising speed of 75 or so, the engine is well into the boost zone.

pawoSD · #6 12-12-2004, 08:32 AM

I tried out driving around in S, and it does certainly give a little more accelerative power, but its running over 1000rpm higher than if it was in D, so I often slip it back to D if I am going to be able to hold the speedlimit for a while, I sometimes use S to downshift it, as well as when I am in a real slow area, and don't want it to keep going between 3 and 4. I have kindof mixed it up, using S when its practical, at 30mph+ D still gives sufficient acceleration, its just the 25 and below range where it does not. I also use S in the morning when the engine is still frozen, as keeping it at a higher rpm for a while helps it warm up a lot faster, then I switch to D.