Speedo Calibration

[andysteele]

I`ve got as 2009 Academy car, and finally swap the 70 profile CR322's for a set of Yoko A539 60 profile tyres (also have a set of CR500 13" 55 profile tyres on wheels to use at some point).

Having been out for a first blat on the A539's, speedo is obviously over reading now (against satnav speed). I`ve searched on here and found some threads on there being pulse settings in the electronic speedo, and being able to change the the setting to calibrate for the reduced tyre size, what i cant find is how to do this, and what settings to use....

tips/advice?

[Jonathan Kay]

What sort of speedo and dash?

Jonathan

[andysteele]

Sigma Academy 2008 build for 2009. Speedo has digital odometer, with rubber button on the front ​to change between total mileage to trip. Anything else i can tell you about it?

rubbish picture, but looks like this (actual car)

[aerobod - near CYYC]

It looks like the current Caerbont one, calibration instructions here: http://www.caigauge.com/page28.html

Before you change the setting, drive at a constant 30, 50 and 70 mph as indicated on your satnav. Note the indicated speed on the speedometer at each of those speeds. Calculate the ratio between actual and displayed speeds (for example 30 actual, 33 displayed would be 1.10; 50 actual, 54 displayed would be 1.08). Average each of the 3 ratios, then multiply the existing speedometer calibration pulses per mile number by that ratio, to give you the new calibration number.

[andysteele]

thanks, will give that a go, i was planning on a simpler approach to calculate circumference of 13" wheel + 70 profile tyres, and 13" wheel + 60 profile tyre, and use this as multiplier, will do the satnav speed trick as well and see what results i get.

[andysteele]

it was defo over reading a touch on the 70 profile tyres, on 60 profile speedo incidated 40 felt a lot slower. Doing some quick man maths, from tyre profile i think its about a 4% reduction in circumference, from what i remember of the speed difference (speedo to satnav) i calculate I`m about 14% off, and going to 55 profile will reduce another 2%.

will check what setting the speedo is on, and probably apply a 10% tweak and see where that gets me!

[SteveA]

Mine also overreads, if you calculate an accurate speed for the 13" 55 CR500s can you share it here please?

[John Vine]

The simplest method is to measure the rolling circumference of the tyre, and then work out the number of revs/mile.  Multiplying this value by 43 (the number of teeth on the reluctor ring on the RHS driveshaft) gives the calibration code (fill out to the left with zeroes to produce a 6-digit number).

To refine the calibration, check the displayed speed (or trip mileage) against your satnav, and calculate a correction factor.

You can achieve remarkable accuracy after repeating these corrections a few times.

(Edited to include the missing x43 step.)

JV

 

[John Vine]

@SteveA:  This table shows the codes for a variety of wheel and tyres sizes.  These will provide at least a starting point from which you can refine.

JV

[SteveA]

That's great, thanks very much. At Anglesey it was reading 140mph but my vbox said 104. I've just had a look at the current setting and it looks like the speedo was set up for CR322 tyres which I suppose makes sense as it was an Academy car. I have now set it for 13" CR500's. I'm not that concerned with it being 100% accurate as it is a race car and doesn't spend any time on the road. As long as it's close to what it should be it will stop passengers insisting I was doing 140 :)

Suppose I will have to work out the setting for 13" ZZS tyres now as they will be going on at the end of next month.

[John Vine]

Suppose I will have to work out the setting for 13" ZZS tyres now as they will be going on at the end of next month.

I got the code table from CC originally, so it might be worth asking them if they have an updated one that includes the ZZS (and ZZR).

JV

[SteveA]

I've fired the question off to a contact at CC, will post up any results.

[thompster]

I'll be interested in anything you get back.  According to the table my official starting point should be 039558 but my speedo is reasonably accurate and set to 008786.  It's certainly not reading 4 times over, anyway, so I wonder if there are different settings for later cars.
 

[John Vine]

Hmmm...

The 039558 value assumes that the speed sensor picks up its pulses from a 43-tooth reluctor ring on the RHS drive shaft.  What sort of speed sensor does your car have?  Does it pick up its input from the gearbox instead?  Is it a bike sensor, perhaps?

JV

[Jonathan Kay]

According to the table my official starting point should be 039558 but my speedo is reasonably accurate and set to 008786.

That ratio is intriguingly close to 4.5. But nowadays I'd always set these from speed measured by GPS rather than an estimate of circumference.

Jonathan

[andysteele]

mine was set to 38437, standard code for CR322's, as it was under reading when on these tyres, I`m going for the "trial and improvement" method, using my man maths, I`m going for 41000, currently on 60 profile Yoko A539's, and looking to get it fairly close to reality on these, so when I`m on CR500's it`ll be under read but not by much. hopefully will get chance to test it out tonight and report back!

[thompster]

Hi John (and JK),  I have a feeling you suggested I looked for the sensor last time this came up.. I had a quick look under the back and didn't spot anything so perhaps it is taken from something else?  My car is a 2006 kit (07 registered) metric chassis K series.  As suggested I should just set it according to a sat nav but I'm still curious as to why it is as it is.

[Jonathan Kay]

What John said: is your sensor near a wheel or on the gearbox?

Jonathan

[John Vine]

@thompster: Do you have one of these fitted to the tail housing of your gearbox?

JV

[thompster]

No idea but I'll check next time I'm under the car.. it says 'used 2000 onwards' which would imply I probably do.

Presumably the much vaunted calibration table only applies to earlier cars which take the speed from the driveshaft sensor so I'll just get on and calibrate off my old tom tom.

/thread highjack

[John Vine]

I think it's the later cars that have the driveshaft sensor (at least, my 2008 model does).

JV

[andysteele]

I'm a 2008 metric Sigma chassis registered in 2009, with driveshaft sensor

[andysteele]

at 41000, on A539, 13" 60 profile, I`m still over reading a touch, annoyingly its indicated 70 = 68 (3%), indicated 40 = 37 (8%). I`m going to push it to 42000, but at that i think at the top end I`ll be spot on, but over reading at touch lower down.

[John Vine]

That sounds like progress.   Given that the speedo accuracy varies across the dial, I took a slightly different approach.  I went for accurate trip mileage instead, on the basis that, if needed, I could always mark the speedo dial with dots or lines of something brightly coloured (or use JK's favoured method of Letraset) to indicate the correct speed.   I value an accurate trip mileage given that the fuel gauge under-reads hugely. Besides, if I really need to know my correct speed (in average-speed checks, for example), I just use the satnav.

JV

[SteveA]

CC contact came back today with the following info;

Avon ZZS 185/55/R13 13" 1.6022 1004 0.5100 043182

So the code for the speedo is 043182

Will change mine and test at Cadwell next month.