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#1
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3.69 vs. 3.08 vs. 2.88
I'm considering swapping rear differentials in my car.
I understand that my '83 240D, has a 3.69 differential in the rear. My top speed is about 80 mph without roof racks, its about 75 mph with a 4-bike+faring Yakima roof rack on top. Switching to a lower number rear differential would reduce my top speed a bit, but also reduce the amount of fuel consumed, lower RPMs at cruise, etc. and result in increased fuel economy. What would you guys say to a 3.08? What would you say my top speed would be, and what might I see MPG-wise? Currently, top-speed of 75 mph is "nearly" WOT for me...although, I assume its not really consuming much more fuel because the engine can only spin so fast after all. Lower RPMs even at WOT = lower fuel consumption right? Thanks in advance for the advice.
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1983 Mercedes-Benz 240D Automatic, A/C, Power Sunroof, Power Right Side Mirror 231K Miles FOR SALE MAKE OFFER |
#2
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Youve got that backwards.. It will increase your top speed, thereby allowing your engine to turn at lower (possibly to low) RPMs.
Perhaps try a 3.58 3.54 or 3.46 first.
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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 |
#3
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yeah, it will increase your top speed and make it even more of a dog off the line....ever try to pedal a ten-speed bike starting in 10th gear?
I hope you're not doing much travelling on the 408 or I-4 on ramps...
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80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??) Last edited by rs899; 02-15-2008 at 11:53 AM. |
#4
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A 3.46 would make a noticeable difference but likely not hamper acceleration too badly. If you live in a flat, or mostly flat area close to sea level then a 3.08 would likely work as well BUT acceleration will definitely be affected. If you live in the hills forget it. I doubt either will make much difference in MPG since that 616 is giving all its got all the time already. It will make for a markedly quieter ride on the highway however. RT
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When all else fails, vote from the rooftops! 84' Mercedes Benz 300D Anthracite/black, 171K 03' Volkswagen Jetta TDI blue/black, 93K 93' Chevrolet C2500HD ExCab 6.5TD, Two-tone blue, 252K |
#5
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Quote:
It will take the same amount of power to do the same amount of work, but once the transmission shifts to high, it's all up to the engine--which, running at lower rpm won't be producing the same power. It's 100% all-the-time FOLLY to try to second-guess the engineers that designed the car.
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Kent Christensen Albuquerque '07 GL320CDI, '10 CL550. '01 Porsche Boxster Two BMW motorcycles |
#6
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If this is going in a 240D I doubt that a 3.08 or a 2.88 rear end will increase speed at all. It might increase MPG a little but I wouldn't consider it worth the drop in neck snapping acceleration 240Ds are know for.
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green 85 300SD 200K miles "Das Schlepper Frog" With a OM603 TBO360 turbo ( To be intercooled someday )( Kalifornistani emissons ) white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank) desert storm primer 63 T-bird "The Undead" (long term hibernation) http://ecomodder.com/forum/fe-graphs/sig692a.png |
#7
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You may be able to accomplish some (but not all of) the same thing by using tires with a taller aspect ratio than what you have. You can buy a pair of taller ones and if you don't like the effect, put them in front.
The problem with the 240D (especially automatic) is that there is no available power. You can think about what you are doing on a 300D turbo etc...
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80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??) |
#8
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I would hesitate to go any higher than a 358 gear which is only 3%. I feel you could pull that fine unless you have mountains to do on a regular basis. a 346 would be pushing it in my opinion but might work if you have pretty much all flat driving. Anything higher and you would not be able to keep it going on level ground in fourth gear, IMHO.
307 and 288 will simply not work, IMHO. Tom W
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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC] ..I also have a 427 Cobra replica with an aluminum chassis. |
#9
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I agree with Tom. The only thing you are going to get is a more comfortable cruising altitude (lower revs).
__________________
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 |
#10
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Hmm, fair enough, I won't bother then.
I'm not happy with the 22-ish MPG I'm getting currently, I know it can do better, but I'm running out of ideas on how to get there -- its had the valves adjusted, steering fixed, good new tires and alignment, only thing not done is injectors.
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1983 Mercedes-Benz 240D Automatic, A/C, Power Sunroof, Power Right Side Mirror 231K Miles FOR SALE MAKE OFFER |
#11
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What do you expect to get for MPG? My 85 300sd gets 25-26ish and seems reasonable to me.
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83 300SD Dark Silver Dark brown inside |
#12
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Quote:
Not "35+" like the Craigslist ads claim, but a bit better than I'm doing.
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1983 Mercedes-Benz 240D Automatic, A/C, Power Sunroof, Power Right Side Mirror 231K Miles FOR SALE MAKE OFFER |
#13
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Quote:
If you drove at 55, you would probably do better.
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1999 SL500 1969 280SE 2023 Ram 1500 2007 Tiara 3200 |
#14
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Quote:
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'90 300SE 298k -300K and it gets put into retirement. '80 300D 255k Purchased new by family in 1980. Had a: 1973 220 (gas) 1980 300SD 1992 400E |
#15
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Quote:
A stick conversion would probably reap your best dividends in mpg. Tom W
__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC] ..I also have a 427 Cobra replica with an aluminum chassis. |
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