Hi all,

I need help calibrating my Smiths telemetrix speedo...

My tyres are 205x45xR16.
my DIF is 3.62
The tyre travels 71.5 inches in one full revolution.

someone ( zetec ) said that the standard MT75 produces a calibration number of 8000, but im a bit stuck.

the website is not much help. it tells you have to caluclate the revolutions per mile, then seems to skip and goes straight on to calibrating using magnets on a prop - which mine is not - it seems ti miss out the Hall Effect speed sensor information all together!

help please! do i just stick the 8000 in that the MT75 is meant to produce?

Hall effect sensor, Number of poles on sensor x cable drive revs per mile or kilometre = calibration number
greengauges.com
far easier to use this option
Rescued attachment speedo pickup mount.jpg

The gearbox means nothing, its only the rotation of the wheels, so all the sensing is done post gearbox.
If you have hall effect the you sense rotation off passing bolt heads, if you have magnets you sense off passing magnets.

You could use the gear ratio calculator and workout what speed the car will be doing at say 20 mph (not to much if you have the car on stands and you are in first) then compare that with speedo and modify calibration to match

im using the mt75 speed senser, so there are no bolts or cable turns. its running directly from the speed sensor

If it senses off the back of the gearbox you need to know howmany pulses per revolution.

Wire a bulb in to the sensor. power to bulb wire from bulb to sensor earth other side of sensor, rotate prop 1 revolution count pulses.

My calculation for 4 pulses and your setup comes out at 12831 or 6415 for 2 etc

You need to sus out pulses per mile. You can count the number of times your prop turns over a measured distance..For example measure out a set distance, for example 1/50th of a mile, mark your prop and push the car over the set distance and measure how many times the prop turns. Get the calculator out and work out revolutions per mile...times this by number of pulses your speed sensor gives per turn of the propshaft and you have the answer. Try it 3 or 4 times to try and get a more accurate figure. Works well because it takes the amount the tyre squashes into account. Then go for a drive with a Satnav and see if you are accurate. I didn't have a satnav to check mine but was within 2% accurate and easily passed SVA.

i dont know how many pulses my speed snesor gives out.

this is all very confusing!!! :-)

Stole this off another web site hope they dont mind;

Tyre Revolutions per mile x Diff ratio x Transducer Pulses per revolution/ Speedometer drive Ratio
= Number of pulses per mile

Which in your case i think works out to;

886x3.62x7x4/22=4082

i think the speed sensor gives out 8 pulses per rev, as when i had it set to 4054 it was showing 60 mph by the top of the road - which i dont think was my true speed!

i called CAI today, and after giving them my details, he called back and said the number is 7824!

There you go, I said it was about 8000 ish.