Fletcher gofanu at usachoice.net
Thu Feb 8 01:23:48 GMT 2007

All (I was told by a speedo tech, and true in my experience) speedos 
have a calibration number, usually on the face, in the center, toward 
the bottom or around the odometer window(s). Sometimes on the back of 
the case I hear, but I've never seen one like this. It is customary to 
mark the case with a new number if the thing has been recalibrated to 
anything other than the OE marking. It is usually a 4 digit number, 
sometimes 3 digit,  and represents the reading at mid-scale in rpm. Like 
980, 1000, 1250, etc.  Never seen one over about 1600. Midscale is 60mph 
on most speedos, so it is also the turns per mile on the odo.Some people 
claim the number to always represent 60mph instead of midscale, but the 
only ones I ever checked were 120MPH units, so I can't verify 
differences. In any event, a 3 or 4 digit number on the thing will be 
the calibration, and obviously will be the same for identical rated 
units. Speedos do vary by tire size, final drive ratio, but they also 
vary by gearbox drive, or intervening adaptors (other than 2:1 rare 
except aftermarket). How the factory accounts for these differences 
varies, but usually for identical transmissions and tires, the speedo is 
changed with the final drive. In that case, the ratio of the two final 
drives will equal the ratio of the two calibration numbers. Mechanical 
tachs are the same.

If you assume that two units are in fact in calibration, more fool you. 
Seriously, you can calculate the calibration change by these numbers. 
You can use any fixed rpm drive (electric drill) that is within the 
speedo range - rpm less than double the calibration numbers - to compare 
two units.

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