Speedometer - Gearbox take-off

J

John Larkin

Member
Does anybody know what RPM the gearbox speedo drive should be at with the engine running at 1000 RPM in top gear? My car is a 1974 UK P6 V8 automatic.

Is the speedo cable supposed to run at propshaft speed or is it geared up or down from that speed?

John Larkin
 
Hi Bennet,

I've calculated that the speedo cable is rotating 7 times for each 12 turns of the propshaft. The speedo is marked 1248 cable turns per mile; the engine (in top gear) turns 2500 times per mile.

1248 is near enough to half of 2500 (i.e. 0.499) to indicate that the speedo drive on the gearbox should be a reduction gear of 2:1, and it seems to be 1.714:1.

I have not yet got round to getting under the car to check the speedo drive. The speedo drive was replaced with a new item (from JRW I think) a couple of years ago. I'm just wondering if I have a speedo drive for a four cylinder car, or if there is a different ratio for speedo drives for BW35 and BW65 gearboxes.
 
24 MPH at 1000 RPM engine speed in top = 60MPH at 2500 RPM engine speed = 1 mile/minute at 2500 revs/minute, so 1 mile = 2500 engine revs in top gear.
 
Calculating it I get the same figure:

1 miles = 1609 metres

Rolling radius = 1.93 metres (not sure if this is spot on)

1 mile = 834 wheel revs

Final drive ratio = 3.08 so (3.08 * 834) = 2569 prop revolutions.

I'm amazed they are so close I would have thought innacurate tacho readings and slip in the torque convertor would have put them further apart.

Maybe try counting a few more prop to speedo cable revs to get a more accurate figure for the reduction drive.
 
Using original equipment tyres of 185/80/14 the rolling radius is 315mm; this is based on information that I got from Dunlop about rolling radius above 30MPH --- centrifugal force takes up some of the the compression of the tyre sidewall that you can see when the car is stationary. This value gives a rolling circumference of 0.315m x 2 x Pi = 1.979m. 1 mile = 1609m, so 1609/1.979 = 812.97 wheel rotations. 812.97 x 3.08 = 2504 engine revolutions.

Taylor's book lists different MPH/1000 RPM for the BW35 and BW65 gearboxes in four cylinder and eight cylinder cars. The 2000SC auto was 19.2MPH, the 2200SC was 19.7MPH, and the V8 was 24MPH. I suspect that the speedo drive take-off from the gearbox is interchangeable, and that I may have an incorrect unit on my car.
 
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