Reviving this thread b/c numerous folks are still wrestling with getting accurate readings from their factory speedos after changing gearing and/or tyre sizes.
I had recently replaced the speedo cable on our stock ’69 SRL, so I furnished these measurements to Tom:
5/8â€x18 TPI nut at gauge end, 7/8â€x18 TPI nut that screws onto the distributor (sorry, Greg, I couldn’t resist
),er, tranny, and 3/16†dimpled Datsun barrel also at tranny endTom says he can make ratio adapters for these. Again, the best results can be achieved if you furnish him two measurements (splits the sampling error in half): (1)GPS reading when speedo reads 60 mph, and (2)speedo reading when GPS reads 60 mph.
On the LT1 swap the simple mechanical ratio adapter he sent me produces a GPS reading of 60.7 (not 67) mph when the oem 160 mph speedo shows a rock steady 60 mph.
The offset choice provided more clearance between tranny tunnel and fitting (even though the firewall, floor pans and tranny tunnel are custom), and the thought occurred to me whilst changing the cable on our stock SRL how much easier the job would be if an adapter (1:1 ratio) were fitted there as well.
Billsspeedo.com
Jon Hayden
