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Spitfire Speedometer calibration

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N

number6

Jedi Trainee
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I am about to replace the 3.89-1 differential with a 3.63-1. I am curious to know how to calibrate the speedo
for the new ratio. After reading several manuals ,I find no information. Is it a case of getting the right gear
for the tranmission, or sending the unit out to a specialist to remagnitize . Would appreciate any comments.
 
The problem is, the speedometer also has gears to drive the odometer. Unless you want it to be off, you'll have to change gears at some point.

Another option is a gizmo called a "ratio box", which any speedo shop can make up for you. It goes in the speedo cable and adjusts the ratio as required.
 
Randall, thank you for the info. I was unaware of the internal gearing for the odometer.
Tom S.
 
The good thing, the ratio difference is only about 6% between those ratios. So at 50 MPH indicated you'd be doing about 53mph actual. After 100k miles on the odometer, the car will actually have traveled 106k miles. Most old magnetic speedos have errors about that bad anyway. That may be close enough to live with.
 
Thank you for all for the info. I know how to proceed from here.
Tom S
 
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