My 77 300D is a great car. I've owned it for over a year and put about 15k on, which puts me just below 200k. Runs great and looks good. I have been curious about the speed these cars were meant to go. In 1977 there were no doubt freeway capable autos, but I think I remember reading in the manual that the top speed is recommended no higher than 83 MPH. I have gone just about this fast in the car and the engine was loud.

My question is, what is the difference between driving 55 and 65? 55 seems optimum as far as fuel efficiency but most drivers don't tolerate someone going that speed. At 65, although louder, i am able to accelerate. Is the engine going to wear more at higher RPM's? As I have no tachometer i can only base my assumptions on the sound of the engine. What might the RPM be at 55, 65, 75 as a percentage of maximum and are there negative effects of travelling at higher speeds?

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The speed limit.

The only negative effect of higher speed is higher fuel consumption.

Maybe not today's speed limits. FWIW, I never drive my 240 faster than 65.

Remember, these cars were marketed and sold when the highway speed limit was 15-20 mph lower than it is now. And, it seems as if even MBZ recognized that, late in it's life, the 123 was undergeared by going from a 3.07 to the 2.88 differential in the '85 model. A 3.07 spins it pretty tight at 70 and a 3.69 (in a normally aspirated 300 or a 240) at anything over 65 is brutal.

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Never a dull moment at Berry Hill Farm.

Speed limt + 10

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" We have nothing to fear but the main stream media itself . . . ."- Adapted from Franklin D Roosevelt for the 21st century

OBK #55

1998 Lincoln Continental - Sold
Max 1984 300TD 285,000 miles - Sold
The Dee8gonator 1987 560SEC 196,000 miles - Sold
Orgasmatron - 2006 CLS500 90,000 miles
2002 C320 Wagon 122,000 miles
2016 AMG GTS 12,000 miles

Your car is geared for 55mph IIRC.

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http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42

within speed limit AND within your safe driving capabilities. (the speed limit may be too high for someone's safe drivign capabilities)

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Mike

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1975 200 - Sold (no pix);
1978 200 - Sold - http://www.pbase.com/hboy/redbaron
1979 300TD - Sold
http://www.pbase.com/hboy/greenwagon
http://www.geocities.com/hboy726/300TD.html
1985 230E - now my daily driver...

I usually go 77-80mph. Loud but who cares. And I still get 25mpg on the freeway. And, plenty of acceleration left. I can hit the 114 mark rather easily. (takes about 40-50 seconds from 75 to get there...less if downhill )

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

My 300D seems to like going about 70-75 on the freeway.

I have had it over 85 on many occasions, but as mentioned it gets a little loud.

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1979 Thistle Green 300D 272,xxx miles
1984 Orient Red 300 D 4 spd Euro Import - 189,xxx miles
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Lofat Productions
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Original owner of my 77 300d NA told me he had it up to 105mph on I-25 in Northern New Mexico. Must have been wearing ear plugs.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I run 80-90 on the open highway, however the SDL has a sweet spot at just over 100. Its way to expensive to run that fast, 90% of the time I go 40-50 or whatever traffic is doing.

W126's really like to move, out west on those big open roads I can see 110-120 being very comfortable in a 560SEL. Set the cruise and sit back.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

I've had my MB only two months, but it begs to go fast. I have trouble keeping it under 80mph. On the door jam, tire pressure says "increase by 6 psi if driving over 100mph. ZOOOOM!!!

Since my ticket, limit plus 5 for me until I'm on my own insurance.

The car hates it.

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

I'm thinking safe (for the engine) to drive full throttle if you want.. Properly tuned diesel will not over rev like a gas engine. Naturally the faster the engine turns the faster it will wear out.. but.... isn't that the way it is with everything in life?

This is just my opinion.. If it works out for you, I take full credit, if not, I take no responsibility. ;-)

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Bill

Backyard Shadetree Mechanic, 30 years running.

1987 300TD
1988 300E (sold, sniff)
1999 S320
1981 Volkswagen Rabbit Truck (Diesel)
2001 Ford Expedition
1988 Suzuki Samurai
On my list to buy: R129, for me, and a R107 for my wife.

I usually keep it around speed limit+5 unless I'm pretty sure no one's looking, then I may go a bit faster*. Of course, the 240 won't do the speed limit up some of the hills around here, so it's third gear and floored for those times. The 240 is pretty loud at anything over about 64, but it sounds good to me.

*300D tops out at 110, the 240 was down to 75 so I changed fuel filters and got it back up to 81. Tires were a little underinflated at the time, so I'll have to check again now that I've fixed that too.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

with my 2.47 I usually keep it around 75-80 at a nice leisurely 2800-3000rpm... When I want to hop on it I do though.. and it goes right up to 100ish comfortably.. I need just a little more power to get the mach numbers of 120-130mph that I want though... next up is pulling the rack limiter fork out

EDIT: to address the original poster, you could very easily swap in a 3.07 rear end and get lower rpms/higher cruising speed.. they are available for around $200 in the yards, and its a pretty easy swap.. you will sacrifice some hill climbing ability and some acceleration, but it shouldnt be too bad.

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99 E300 Turbodiesel 100k