What's the most efficient speed and or RPM for the diesels?
 

I've been wondering what point in the engine's and the car's speed that it is the most fuel efficient. I get the impression that it's right about the 2000-2500 RPM range in 4th gear going 45-50Mph. The car runs the smoothest, the AC blows nice and cold, and it just seems to really like 2000-2500 RPM very well.

Efficiency?
 

My benz has no tach but I get 31 to 34 mpg at 50 to 55, at anything over 60 it drops to 28 or less.

Sorry, Nate! I rarely drive below 70 mph, but it seems that 55-65 mph is the right range. Remember, though, that this car was designed for autobahn at 100mph +, so anything is possible. the only time I noticed a substantial drop in mpg is with a lot of starts and stops (heavy traffic). I just drove on New Joyzee TPK from exit 10 to Delaware memorial bridge at an average of 75-80 mph and fuel consumption at about 34mpg (1-2 mpg better than daily commute). :D

My 79 300SD seems to like slightly higher speeds...around 70 or 80 mph. It's very smooth in that range, and it doesn't feel that fast. This could be bad for my driving record! :( Whenever I'm on a road-trip or just on the open highway for any length of time, it just seems very comfortable at such speeds, and wants to "creep up" there if I'm not paying attention to the speedo. Don't know about the difference in fuel mileage...never calculated it that closely.
Mike

My 300D's a lot like that. It wants to creep up even if I'm doing 70mph. On long drives, I actually get uncomfortable 'cause the car wants to go that fast and I wanna do the speed limit, and the speed winds up varying quite a bit and I get fatigued early on. I need to get the cruise control working...with that, long trips are easy. Fortunately, the CC worked for most of the trip back from Texas. :)

I've gotten mine up to 94Mph, but I let off when I got that fast because I knew that it would take a lot longer to get much faster, and I didn't want to get pulled over... no way to explain that you were just seeing how fast the car could go... :)

Wouldn't engine temperature also give you a good idea of how well the engine and all its system run at their best? That 45-50 mph 2000-2500 RPM range keeps the best engine temp (right over 80C) with no fluxuation like I get when I run at 70+ on the highway, it will get closer to 100C on the highway and even a bit higher with the AC on. At 45-50 mph the engine temp doesn't move w/ AC on or off.

I dunno though.

Nate, again, think autobahn and no speed limit. I'm not sure how it works in diesels, but in general, Americans are used to large, low rpm engines. i know a guy who told me that his CRV was kinda slow. I drove it and disagreed. He then admitted never pushing the tach past 2500 rpm. I don't think he believed me when I told him he is killing his engine. Same story with a lot of WRX owners. I can tell you that my 190 feels the best at about 3500-4000 rpms. the temp creeps up to about 95C and stays there. i think you may have another problem. I KNOW I have blown head gasket, and yet the car cruises at 75-80 mph (3500 rpm) with no ill effects. Realax and just floor it!!!:p

I was amazed when I found out that the 617 engine doesn't red-line 'till 4500 or so RPM. My Navistar 6.9l V-8 diesel hits the governor at 3400 RPM; for that reason, I can't go faster than 75mph. Many big-truck diesels redline at more like 2000 to 2500 RPM...

I know the horse power curve peaks at 4200 RPM for the 2.2L. All 77 horses kickin at once, I was at 4500 RPM when I was at 94mph. I know that the engine's top speed is 5150 (great number, van halen friends! especially guitarists!) + or - 50rpm. So 5000 RPM it pretty much tachs out. That means it can only get to 100 or 110, and running really fast. I don't think running it at that high speed for long periods is a good thing. Right?

well, personally I'm STILL hot for the teacher ;). You've got it right Nate. I read somewhere that the real top speed for our 2.2 is 105-108 mph. I think you are right - I'd keep it at about 80-85 mph for long hauls.

The Warden- I don't believe diesels HAVE a red line. they may have speed inhibitors, but no red line e.g. you cannot "blow'em up" by overrevving.

Piotr:

Oh, yes, you can over-rev a diesel. You will poke a rod out the side of the block when the crank bearing cap doesn't make the trip down on the intake stroke intact -- the next round on the crank shoves it out the side. Terrible thing to do to a good engine.

Fortunately, this only happens if you regularly wind them up for long periods of time. I'd not run at redline for hours on end, it certainly reduces the life of the engine. Remember, MB usually provides three or four over/under sizes for everything -- Germans burn the engines up pretty fast. You can get pistons in honing sizes, for instance, believe it or not -- hone a couple hundredths of a millimeter over and put new pistons in!

Keep the rev counter under 4000 except when passing and you will be fine!

Peter