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Old 05-10-2013
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Ok. Just found on the AEV forum a reply from AEV Dave.

You measure from the ground to the top of the tyre with normal inflation and full weight of the vehicle.

I did this and it came to 32 in which is pretty close to the measurement needed to make it work properly. I wonder why they did it this way? This is not a true representation of the tyres rolling diameter which is what the speedo is based on.

Oh well, guess it doesnt really matter as long ong as it works!

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Old 05-10-2013
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Default Speedo recalibration

Mine was the same. Just as MM said, I just kept lowering it until it matched the GPS speed. Everything else on the Procal works great.
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Old 05-10-2013
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Quote:
Originally Posted by SeaComms View Post
Ok. Just found on the AEV forum a reply from AEV Dave.

You measure from the ground to the top of the tyre with normal inflation and full weight of the vehicle.

I did this and it came to 32 in which is pretty close to the measurement needed to make it work properly. I wonder why they did it this way? This is not a true representation of the tyres rolling diameter which is what the speedo is based on.

Oh well, guess it doesnt really matter as long as it works!

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I think you will find it is ! The tyre rubber compresses a bit under the cars weight, but some of this, not all is recovered when rolling.
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  #13  
Old 05-10-2013
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Uep. It's not that the device is wrong, it's just the method they used for measuring the tyre. What they think is 32 is really 33.

All good, as long as we know how to use it! It's just a shame they dont mention how to measure the tyres in any of their documentation, especially when they use a non standard method.

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Old 05-10-2013
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fitting 35"s made my speedo read correctly.... no adjustment needed.. was reading 10k's under previously
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Old 07-10-2013
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If I recall the manual says to read physical height of the front tyres - top to ground at normal tyre pressure. Not the diameter of the tyre. Mine is spot on AND it now shifts through gears more smoothly and to a higher gear quicker, especially reaching 5th, which results in better fuel economy.
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Old 07-10-2013
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The height from the ground to centre is the true height for rolling dia..ie 2x pie x that radius. Its the same principle for drag top fuellers. The fact the tyre squats and wads up under initial torque shortens the rolling diameter and increases torque...then as the wheel speed increases centrifugal forces stands the tyre up increasing the diameter trading torque for speed.
I think AEV knew exactly what they were doing!
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Old 07-10-2013
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Still dont understand how you can stretch/shorten a the steel belt in the tyre to increase/reduce the circumference (rolling diameter)

However, further research tonight I found an ADR paper on testing speedos (the link for anyone as boring as me that wants to read it). Interestingly, the steel belts don't shrink or stretch as I would have expected, however, its the distortion in the tread on the tyres foot print that creates the difference in speedo readings with tyre pressure changes. An almost bald tyre will show almost no change, a brand new road tyre in the test showed changing from 22psi to 40psi gave only a 1.4km/hr (1.1%) difference in speedo reading at 120km/hr. However, normal tyre wear on the same road tyre (a Dunlop Monza 205/65R15) from new to ready to replace gave a 2% difference in speedo reading (nearly double the difference a change of 22 to 40 psi does).

So yes, speedo accuracy does change with tyre pressure, but tyre diameter does not. Measuring the tyre as per AEV's instructions will only provide an accurate speedo at one particular pressure. Given the tiny real difference from any pressure change, I still believe they would have been better off calibrating it using the measurement of the actual tyre diameter (across the tyre) as this does not change with tyre pressure.

Anyways, after changing the TPMS down to 27 psi yesterday, the clock on the stereo reset itself! Then after the beach run I pumped tyres back up to 32 (5psi over the new setting) and the low tyre indicator would not go off. Increased them to 34, no go. Reset the TPMS to 27 again and all good. So not sure what happened there! But all good now.
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Last edited by SeaComms; 07-10-2013 at 08:37 PM.
  #18  
Old 07-10-2013
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Now you are starting to understand it. Actually although the static diameter doesnt change, the rolling diameter does which is why tyre pressures will affect your speedo readings. it is almost easier if you think of it as radius instead of diameter. The lower half of the tyre will have a smaller radius than the upper half due to the bulge or distortion. as the lower contacts the road it affects the drive & speedo.

As I fitted my 285/70r17 tyres soon after delivery I had the dealer reset the spedo & TPMS to the best available settings. I think it was for a 255/75r17 & 26 psi. Speedo still reads approx 1 kph low at 100 k's but I can live with that. TPMS occasionally gives a false alarm as I rum 28 psi but I have learnt to ignore it.

David

P.S. Dont forget that most tyres are actually smaller than the claimed diameter. most 33" measure up at 32.5" & 35" tyres can run between 34 - 34.5". A bit disappointing but not much we can do about it.
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Last edited by wrench; 07-10-2013 at 09:24 PM.
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