The calibration is derived by the ratio of the Speedo gearbox. 120mph speedos usually have a 2:1 drive, 150mph ones usually have 15:12 (or 1.25:1)
When you remove the rear wheel drive gearbox it will say on the brass ring which ratio it is.
Also to verify which Speedo you need, on the face of the Speedo it will have a number, if it says 1000 then you need the 15:12 box. I think the one for 2:1 says 1600 but I cannot be sure.
Last edited by Allan G; 04/29/21 11:49 am. Reason: 2:1 speedo gearboxes have "1600" Speedometers, not 1500
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68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)