I have just been doing some checks on my S IV in which I have just
installed a Laycock D-type overdrive with 25% (nominal) ratio.
The following are givens:
Tires..........165/70/13 (fairly new) at 26 psi cold............rolling
circumference is 69.0" (verified by marking the floor and measuring one
roll) which equates to a 21.96" diameter.
Differential...........3.8889:1
Speedo gears ..........3.5:1 (4 driving teeth, 14 driven teeth)
Overdrive ratio .......nominally 25%.......actual factor is 1.25316
The following is computed:
Speedo cable factor ........1020.29 turns per mile
Speed in 4th gear at 1000 engine rpm...16.80mph
Speed in 4th and O/D at 1000 rpm.........21.06mph
Looks like the standard speedo (1020) will read the distance right at
the very least!
John Slade
Manotick, ON
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