Don'tcha love that 3.46? My '68/69 car came with a 4.08 and I couldn't get through some large intersections without shifting. There are different tastes, of course -- as in motor oil and pizza -- but that 3.46 is just about right for me.
The "recalibration" needs to be rethunk from another direction, which will eliminate the odometer issue along with any need to mess about in the exotic innards of the speedo head.
Yes, there is a little box thang that does it for you: alters the rotational speed between cable drive output and speedo head input, giving you the appropriate input for distance and rate right at the speedo fitting behind your dashboard. It's done with gear sets in a small gearbox inserted into your drive cable -- buy a nice new one to be so modified and give years of trouble-free service. Have it mounted in the middle area of the cable so that it can benefit from the factory cable retainer there on one of the bellhousing boltsand locating it pretty much out of sight as well.
They used to be much more common but some old-school speedo shops still have the kits of various gears and the small gearcases to contain them. (Joe has some of it somewhere but has a hard time locating it, and the go-to guy for that gizmo in North America passed on some years before my similar quest, leaving his son in the business of selling radioactive headlights and lacking headspace to even consider anything not using solid state electronics: total deadend there.
I got mine from Powl's Speedometer, 2340 Dairy Road, rural Lancaster PA.
www.powls-speedometer.com (717) 898-2552, a small business in southeast Pennsylvania.
You ought to be able to connect with someone closer than that, of course, and I did drive my car up to them, but it could easily be done remotely if everyone kept their heads. My relevant diff ratios were 4.08 and 3.46. This is a 15% reduction from the larger to the smaller. The M-B TDM lays all those diff drive #s out so i could see that M-B chose their ratios in 5 and 10% increments -- which is how I arrived at the happy choice of my 3.46 diff in the first place--so as a customer I knew that I wanted the gizmo to give me rotational data input at the speedo exactly 15% less than the cable drive delivers out of the transmission. So the speedo shop folks didn't have to go through all the road-testing and whatever other usual gyrations necessary to discover what needs to go into the little box. And all I had to do was convince them to take my word for it: "15%". And I relied on M-B to've given me an accurate speedo setup in the first place. The resulting setup reads out more accurately than my ability to measure; that is, real-world performance is closer to GPS than I can differentiate with my poor equipment: it is 'spot-on' for our purposes.
I thought this was here in the Forum years ago; odd it doesn't come up in searches. But then I once searched for "oil" in irritation and spite and got......no result. And it is a terrific resource the rest of the time, no argument there.
Possible web searches: mileage settings tachographs (?) Their usual work is accurizing speedos to 0 error for biz & govt.
It is, admittedly, another dying art.
All best all,
Denny
