Speedometer calibration?

[twc641]

I have a 63 Austin with a Cooper S speedo
magic wand gearbox 10inch wheels
guage is off by 1.2miles +/- fast
any thoughts?

[Peter Laidler]

What part of the gauge is 'off'.....? The odometer over a distance? Or the speed reading at a known MPH. What do you mean by '... 1.2 miles+/- fast...' ?

[twc641]

I just noticed it was the odometer Example drove 5 miles Shows 6.2
I haven't checked the speed though

[Peter Laidler]

Odometer is a direct drive from the cable to the mileage recorder and gearing as opposed to an indirect drive from the cable via a magnetised void,

So the answer is to correct the gearing at the final drive AND/OR the speedo head

[timmy201]

The proper calibration requires a few things to match:
Tyre size
Speedo TPM (turns per mile)
Final drive ratio of the diff
Speedo drive gears in gearbox

There’s a calculator here that will tell you what drive gears will be needed for your particular combination of parts
http://www.guess-works.com/Tech/ratio.htm

[twc641]

easier said than done how would you go about this?

[timmy201]

The speedo TPM is written on the front of the speedo (if it’s a large centre speedo)
Tyre size is easy to find

Diff ratio can be estimated by:
Find a nice flat smooth bit of road
Put the gearbox in 4th gear
Take the spark plugs out
Get the engine to TDC
Mark the position of the tyre
Push the car forward so the engine does one full rotation
Count the number of part revolutions the wheel does
Common ratios are 3.1, 3.44, 3.76

The speedo gears can be estimated by:
Same piece of flat road
Plugs out
Pull the speedo cable off the speedo and put a piece of tape on the end a bit like a flag
Push the car forward a set distance. If you can measure 52.8 feet this is 1/100 of a mile
Count the number of turns on the speedo cable
If you get something around 10, then multiply by 100 to get the turns per mile (TPM)
Common speedos are in the range of 1000-1400 TPM

[twc641]

Thanks Tim I will give it a shot