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.

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.

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...

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

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.

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. ;)

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...

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

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

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.

What do you expect to get for MPG? My 85 300sd gets 25-26ish and seems reasonable to me.

Given that you've got a turbocharged engine with an extra cylinder (or two?), I expect I should be able to achieve high 20s to about 30 mpg.

Not "35+" like the Craigslist ads claim, but a bit better than I'm doing.

Nope his engine isn't working as hard at say 60 as yours is, so I'm not surprised that it does better.

If you drove at 55, you would probably do better.

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...

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