3.69 vs. 3.08 vs. 2.88 - PeachParts Mercedes-Benz Forum

jkoebel · #1 02-15-2008, 10:48 AM

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.

winmutt · #2 02-15-2008, 10:58 AM

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.

rs899 · #3 02-15-2008, 11:24 AM

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...

lkchris · #4 02-15-2008, 11:43 AM

Quote:

Originally Posted by rs899

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?

Personally, I'd doubt it could ever get to the same top speed as before.

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.

ConnClark · #5 02-15-2008, 11:50 AM

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.

rs899 · #6 02-15-2008, 12:03 PM

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...

fahrgewehr2 · #7 02-15-2008, 04:55 PM

Quote:

Originally Posted by lkchris

Personally, I'd doubt it could ever get to the same top speed as before.

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.

Sure, and I have never seen an updated part#. Or different setups for NA and the rest of the world. Or different rear diff ratios throughout the years, or in different markets. Or as options...

#8 02-15-2008, 10:47 PM

Quote:

Originally Posted by lkchris

It's 100% all-the-time FOLLY to try to second-guess the engineers that designed the car.

A 3.46 would be the best option. It will not hurt acceleration as much as a 3.07 or 2.88.

GREASY_BEAST · #9 02-19-2008, 02:28 PM

Quote:

Originally Posted by lkchris

Personally, I'd doubt it could ever get to the same top speed as before.

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.

I certainly have not found this to be the case... My 300TD with intercooler, adjusted/altered injector pump, upgraded injectors, non-stock timing, 2.47 vs 3.07 rear end, upgraded front suspension components, among other things, is a far better vehicle in nearly every respect than the original car... It is faster, smoother, quieter, and more efficient, while maintaining the same level of reliability. Just because they are "engineers" doesn't mean they designed a perfect car. They designed vehicles that did well with a 55mph speed limit. For an 80-90mph practical car you need to alter the gearing. Mine does quite nicely at about 90mph.. smooth, quiet, about 3200rpm.

With the 240D you are strapped for power, so going to a taller rear is probably going to hurt your top speed and decimate your acceleration. As stated before, you might realize some microscopic gains by switching to a 3.46 (and maybe even measurable gains with a manual 4 speed or better yet finding a 5spd trans, but not from a 190D), however, if you really want to get good mileage, you should look into modifying the aerodynamics of your car. Thats why you need horsepower. Making less HP = burning less fuel = better fuel economy. If you reduce your need to make HP at speed, you save fuel.

Hatterasguy · #10 02-19-2008, 03:24 PM

If you want more speed out of a 240D your best bet is to buy a turbo charged 300D. Same car, twice the HP.

rwthomas1 · #11 02-15-2008, 11:26 AM

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

winmutt · #12 02-15-2008, 01:42 PM

I agree with Tom. The only thing you are going to get is a more comfortable cruising altitude (lower revs).

jkoebel · #13 02-15-2008, 02:06 PM

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.

t walgamuth · #14 02-15-2008, 05:28 PM

Quote:

Originally Posted by jkoebel

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.

You are losing probably at least 3 mpg to the automatic tranny. If you were getting 25 to 26 running 75 with a stick that would be ok. Run 65 and you could touch 30 with a stick.

A stick conversion would probably reap your best dividends in mpg.

Tom W

Mark DiSilvestro · #15 02-15-2008, 05:59 PM

I own an .82 automatic 240D that gets about 22 mpg city and at best, 24 highway. At least to reduce the racket at highway speeds, I would consider a 3.47 diff-swap, but nothing more radical. I may be able to try this, as I've been offered a free 80 300D with a bad tranny. Will post further if it happens.

Happy Motoring, Mark