Speedo recalibration

[SeaComms]

Picked up the Procal today and went about reprogramming the speedo. It says to measure the tyres to the nearest quarter inch and program that in.

Tyres are Duratracs 285/70 17's. Measure tyre diameter at 33 in exactly and program that in and speedo was reading abot 7k's fast at 80 on the speedo! Dropped it back 1/4 inch at a time and retested and am now back down to 32in and its reading about 2 - 3 km's fast still. Looks like it will need to be about 31.75 in to read accurately.

Is this normal?

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[Green Goblin]

I had stu programme the procal when my bigger tyres went, maybe shoot him a message

[TheoB]

Do we need to program the programming unit???

Hmmm mine hasn't arrived yet Dave, but am very keen to see what I come up with too.

[Paul-JK]

Originally Posted by SeaComms

Picked up the Procal today and went about reprogramming the speedo. It says to measure the tyres to the nearest quarter inch and program that in.

Tyres are Duratracs 285/70 17's. Measure tyre diameter at 33 in exactly and program that in and speedo was reading abot 7k's fast at 80 on the speedo! Dropped it back 1/4 inch at a time and retested and am now back down to 32in and its reading about 2 - 3 km's fast still. Looks like it will need to be about 31.75 in to read accurately.

Is this normal?

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Yes, as far as I know it's normal. A "correctly" calibrated speedo should always read faster than you're travelling. It should never read under your actual road speed. It always used to be up to +10% out, but they may have tightened that up as that was years ago.

Also, it still needs to be showing an indicated speed faster than your actual road speed even when the tyres are bald. If you have decent tyres with 14-15mm deep tread that means they can reduce in diameter by nearly 30mm by the time they're illegal. That means based on what you've said you'd need to set it to 32.75" at least with new tyres on and in reality 33" exactly isn't to far away.

[MightyMouse]

FWIW.... not one of the of programmers that I've got actually get the speed right by using the supplied settings... absolutely no idea why but.....
IIRC to get an accurate reading with 37" rubber I ended up setting the tire size to 31.5", just kept making adjustments until the actual reading matched the GPS.

And yes all the other ratio settings are correct ( they actually don't make any difference - the vehicle speed is sourced from the ABS unit which just needs to know tire size )

Weird but frankly I don't care as long as i can get it right... which it is

[Fruju]

Originally Posted by SeaComms

Picked up the Procal today and went about reprogramming the speedo. It says to measure the tyres to the nearest quarter inch and program that in.

Tyres are Duratracs 285/70 17's. Measure tyre diameter at 33 in exactly and program that in and speedo was reading abot 7k's fast at 80 on the speedo! Dropped it back 1/4 inch at a time and retested and am now back down to 32in and its reading about 2 - 3 km's fast still. Looks like it will need to be about 31.75 in to read accurately.

Is this normal?

Sent from my GT-I9300T using Tapatalk 4

Did you measure and take the average height of the front tyres or measure the spare?
I believe measurements need to be done with weight on the tyres and inflated to around 35psi.

[Rock God]

mine is spot on, did you dial in the first setting for the tyre size group.