Rodd, regard's the 230SL rear end ratio's --- I came across the following info in Engelen's book Vom Barock zur Pagode.
There were 2 versions of 4 spd manual tranny's used on the 230SL's, the difference being primarily in 3rd gear, changing from 1:1.53 to 1:1.42 gear ratio, and in 1st gear from 1:4.42 to 1:4.05. The change occurred starting with Chassis # 13876 on Oct 25, '65 (of 19,831 total 230SL's produced ---- 4752 of which were US export models, and approximately 2650 (or just over half of the US export models (~56%)) were built after the tranny change in Oct '65. There was also a change in the gear ratio in reverse gear.
This change had the effect of increasing top speed in 1st gear from 45 km/hr to 50, and in 3rd gear from 135 km/hr to 147.
The 250SL and 280SL manual 4 spd tranny's have the same gear ratio's 1st thru 4th and reverse as the 230SL post chassis #13876.
The 230SL standard rear end ratio for the 1st version manual 4 spd was 3.75, but there was an option to use the 4.08 rear end ratio for the 2nd version tranny in non-US models, and as you stated already the 4.08 was used with the 5 spd manual tranny.
For the 230SL automatic 4 spd the specification table in Engelen's book p. 214 in hardcover book) is not entirely clear about the rear end ratio's though. Both the 3.75 and 4.08 are shown with a notation "fur USA" applying to one of these ... which ratio this notation applies to, however, is not clear.
Point is only that there were 2 versions for the 230SL manual 4 spd transmission and that for the 2nd version both the 3.75 & 4.08 rear ends are shown for the non-US models... and both type rear ends are also shown for the 230SL automatic transmission.
Whether the 4.08 ratio rear end option was ever installed in any 230SL's is another question however.
Also, according to the Service Manual (Big Blue Book, modification Oct. 67), no other passenger car used the 3.75 ratio rear end except the 300SE Sedan (standard and long versions), Coupe, & Convertable, which used this rear end ratio only on the 2nd version engine --- both the 3.75 and 3.92 were optional rear end ratio's for the 300SE with the 2nd version engine. The 1st version engine used either the 3.92, 4.10, or 4.08 rear end ratios in that model. I'm not sure that the 300SE rear-end would mate into the 230SL though.. and even if it did, then the rear-end could as easily be a 3.92, 4.10, or 4.08 as 3.75.
Not that the above will necessarily help... and probably (certainly?) even adds some confusion, but it's important to realize, I think, that the 3.75 rear end was nearly exclusivly used only in the 230SL with the option of using a 4.08 ratio rear end in either the manual or automatic transmissions.
Longtooth
67 250SL US #113-043-10-002163
'02 SL500 Sport