That's what the discussion was about.

results may vary.

don't try this at home!

brian and i had a great debate about a year ago about the effect of gear ratios on fuel economy. others were involved as well.

my rear end swaps resulted in dramatically better results than yours with the exception of the 81 280e which seemed pretty much unaffected by the gears.

perhaps your results suggest that there is a limit to all things and too much gear change can yield disappointing results.

and none of my gear changes were done with turbo motors. all na.

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.

I am in Chicago and drive the same route (more or less) every week. I leave home before 6am to beat the morning traffic, and leave work around 4 to beat the rush-hour traffic.

It seemed to me, and of course this is pure subjective, that I had to give the car more 'pedal' with the 2.47 than I did with the 3.07. For example, with the 3.07, if I was going 60mph I was maybe pressing the pedal 60% of the way down. With the 2.47, if I was going 60mph it felt like I was pressing the pedal 75% of the way down. Sure, the rev's were lower with the 2.47, but I feel like the extra pedal I had to give it largely negated any benefit. Of course there was no simple way to measure it, but that was the impression I got.

I never really thought about it until know, but it seems like the 2.47 might be putting more stress on the turbo and the transmission.

All things considered though, the swap is totally reversable and is not extremely difficult, so it's not like you'll be out more than a couple quarts of gear oil and an afternoon or two from your life.

What size wheels/tires are you running?
Here's a handy formula for those thinking about the switch:

MPH X AXLE RATIO X 336 = RPM
TIRE DIAMETER

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82 300SD 202K
82 300SD 233K
83 300TD 340K
82 300TD 98k euro (parts on the hoof)
85 300TD 282K
83 300TD 197K

I think I read somewhere that a swap to 2.87 limited slip might be possible on a w126. This would be nice for the winter.

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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

Yeah, the '86+ have the 2.88:1 in the SDL but that subframe is different and the 2.88:1 for the '85 and older are '85 only.

The SD's will be for sale once I locate a 2.47:1 locally. The SDL's is going on a shelf for when I find an '88-91 300SEL.

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Naturally, this is the situation. At 60 mph, you need a specific horsepower level to maintain speed. With the 3.07, the engine is turning about 2600 rpm and has about 60 hp available for you to use. You might be using about 1/3 of it to maintain 60 mph.

With the 2.47, you still need the same horsepower, but the engine is only turning about 2100 rpm. You've only got about 40 hp available at this speed. Naturally you'll need a greater percentage of that 40 hp (nearly 1/2 of it) just to maintain speed.

At 60 mph, the 617 is way down beneath it's power band with a 2.47. It's better suited for the folks that drive 80 mph.........on flat ground.

B.C. makes good sense. However, here is why I have decided to install a 2.47:1 in my 1978 300SD.

Before I switched jobs, I was commuting 1,100 miles a week to work, all of it freeway, with roughly a 2.5 hour drive each way.

If I drove 60-65 mph religiously, I could net 30-34 mpg over several tankfuls of fuel. If I drove 70-80 mph, the car seemed 'peppier' and happier at higher rpm, but my fuel economy dropped to 22-25 mpg.

In my case, I figured the 2.47:1 would allow me to run with traffic (at 75-80 mph) and keep my rpm's where they seem happiest (2200-2500) from an mpg standpoint...

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Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

I just reread Thorsens post about commuting in Chicago. If you are in stop and go traffic I would not expect economy to increase at all. With an automatic you never reach the speed to get into fourth gear and the transmission just selects whatever gear is necessary to meet its parameters of shifting. Under the right circumstances in stop and go traffic your economy might actually fall.

Steady speed highway driving is where you can benefit from the taller gears. Besides economy it will reduce engine speed noise and presumably wear.

__________________
[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.

I agree...

My commute was all highway, 110 miles each way and no traffic lights. I would run 100-110 mph many times along that route...

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership