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Speedo Recalibration for 3.9 DIff
Posted: Mon Oct 26, 2009 5:49 pm
by cheeseburgess
I am installing a 1275 engine with a 3.9 diff in a 1966 saloon with 145x14 radials.
Is it advisable to have the speedo recalibrated? If so, what is recommended TPM?
Posted: Mon Oct 26, 2009 9:17 pm
by mike.perry
I fitted a 3.9 diff and 175x13 tyres (slightly larger cicumference than 145x14s) and checked the standard 1408 tpm speedo against the sat nav. It was spot on. It just shows how optimistic it must have been on the original tyes and diff.
Check the speedo before you change then afterwards.
Posted: Mon Oct 26, 2009 9:23 pm
by bmcecosse
1966 car should have had a 4.22 final drive - but do check - I found mine had a 4.55! As mp says - with a 3.9 you won't be too far away. The speedo for a 3.7 diff is 1248 tpm - there is a Mini one that fits the bill perfectly. For a 3.9 you will ideally need something like 1315 tpm - I'm not sure if you will find a Mini one to suit. Best just try with what's in the car (1408?) - and see how it works out.