I swapped from 4.08 to 3.46; that's 15%. In researching for this I began to see that M-B set it up that way. That is, the diff ratios differ in 5% increments. [20% down from 4.08 = the popular 3.27.]. Correcting for this seems now a nearly lost art but simple for those who still practice it. You need to connect with a speedometer repair shop with some history, and a shop like that will have serviced taxis and trucks for years past. One of the common services is the correction of speedometers to 100% accuracy, typically by inserting a little gearbox into the drive cable from the transmission up to the speedometer. They have a myriad of choices in the form of small gears to fit in and accomplish needed adjustments.
Joe A. has had (and may have re-located) a probably museum-worthy example of one of these setups........of course. ;o)
I think the frequency of correcting for speedo accuracy by altering speedo innards is essentially zero.
M-B speedos are apparently dead accurate. [Must keep in mind that changes to tire specs figure into it also.] I verified accuracy in my car using sat-nav ("Garmin") and Interstate mile-markers to where I was confident in my speedo's performance, and then when talking to the shop didn't need for either party to get into the complexities of calculating where we needed to be with all the data: I knew that my "old" error was vanishingly near zero and that I had changed my diff ratio by 15%, so needed them to provide a 15% correction to compensate. Simple as that.
They then selected the proper little gears to go into the little gearbox to accomplish the 15% compensatory adjustment, full stop.
I use the existing stock strap of soft steel and clip to support the cable just below the gearbox, as it was placed at its location along the drive cable with that in mind -- that location is roughly adjacent to the engine/tranny joining. A certain amount of 'country windage' was involved; you could probably measure along your cable and get it quite close to optimum, but there is ample space in that area overall, so the fine detail of location is more concerned with getting that support bracket to attach just under the new, little g-box (seemed the best idea to me, at least). This can and should be done beforehand, and then the installation work done using a nice new cable assembly of good quality. I dimly remember that there are two cables of different lengths available for our application -- apologies that I can't advise on this; it was some years back. I can offer that for g-box placement the relevant distance is measured from the drive end (at the tranny) so that regardless the cable used, your bracket will fit up.
