1971 250

[debi]

My 1971 MB, 250 Series, needs help with compatible parts and it is SO old it's hard to find any like it. I may need to replace the differential / rear end as there is a rhymical roar I can "feel" occuring when I am driving... let off gas and it stops. Pinion gear perhaps. But, I've found a 1969 230 Series, with a blown engine, but rest looks good so far. A salvage auto yard says the rear ends are compatible. Does that mean the '69 and the '71 both have rear ends that are interchangable? And is there anyway I can change the gear ratio to be higher? Gas mileage is 11-13 on the road and with prices so high... would higher gearing help?

[bstreep]

You might try posting this on the vintage section, as there are several 250 owners active there.

[stevebfl]

It should be a straight forward swap. The 114.010/011/015 chassis all had 3.92 ratio. The biggest jump in ratio in standard US models would be to get a 3.46 differential out of 115.114 chassis 300D. All 123 differentials also would work and the 85 300D had a 2.88. The 82-84 300D had a 3.07. They all will fit in the car. All of the imput flanges are the same.

[debi]

Thanks for that info! It sounds like this is not such a way off base question: changing gear ratio so gear ratio is higher. Those numbers are impressive, but, I don't have a clue what they mean.
Is my gear ratio posted somewhere? and what is a low versus high gear ratio?
I'm glad to know that I'm not totally out in left field asking this question.

[stevebfl]

The gear ratio is stamped into the rear of the differential housing. It is viewed from the rear looking forward and is not on the rear cover. It is on the unit itself. From the rear the housing can only be seen at on corner at the bottom. The rest is the rear cover, This small corner has the numbers.


In a 3.92 ratio the driveshaft goes 3.92 times around for one turn of the wheels. High numerical numbers 4 versus 3 say, give quick acceleration but poor top end. The engine speed can be calculated by finding the circumferance of the wheel (pi)x(d). For a 25 in tire that comes to 78.5 inches.
A car traveling 60mph is going one mile per minute or 63360 inches per minute. Divide the distance by the circumferance (63360/78.5) and the number of tire rotations is 807. Multiply by the rear end ration (since all MB trannies of that era have one to one final drive) and you get 3163 rpm (revolutions per minute). If on the other hand you had a 3.46 ratio the engine would be turning 2792 rpm.

The most valuable use of change of ratio I have seen in MBs is to take the rear end out of a 280SE 4.5 and use it in a 280SL. The SL has always had poor highway caracteristics (loud, ticky motor) which are helped immensely by the lower rpms.

[Arthur Dalton]

For the 113 owners : An added note to SB's correct info on the 280se 4.5 diff change.
This rear ratio is 3:27 and I have done this change-out
and must say the improvement is excellent.
Note that I have done it to a stick shift car[4speed],
but I don't know if I would try w/auto.
This can be done on all 113's [230/250/280],with the added advantage on the earlier models [230]of up-grading to rear disc brakes. [ if swapping complete rear].

Arthur

PS- Speedo will not read correct.

[MikeTangas]

The speedo may not read correct after a swap, but it can be recalibrated. Simple process of marking out 52' 9.5" on the street, remove speedo head, attach hair pin (or other indicating device) to cable end and count the revolutions over the distance marked. It's all outlined at http://www.paspeedo.com. Found out something had been swapped on my 4.5, causing about 10 MPH difference at highway speeds, as the speedo head was stamped 1.19 for the ratio, but actual reading was 1.225.

[stevebfl]

I have done about 6 of these conversions, including my partners 71 w/auto trans. Even did one with a chevy V8 in it. It really worked good on that low reving motor.

The one we put in my partners car was a positraction unit and caused some special problems accomodating the breather hose. Its seems the limit to the number of these conversions that we do is the number of good 280SE 4.5 rear ends we can find.

[Arthur Dalton]

Steve,
I have a 280se-4.5 posi rear sitting here in the shop
and wonder if you might ask your partner what specific
brand/type of lube he uses in his.
Many Thanks
Arthur

[stevebfl]

I would use Redline synthetic GL5 with a tube of GM limited slip additive.