Speedo calibration help please

[wheelie]

Hi everyone,

My car again failed the SVA due to the speedo reading lower than my actual speed. I had it specially made by ETB Instrument using the cardboard disc test. I have done this test 6 times now and for each 100 turns of the wheel, the cable turns 82.60 times.They say that equates to 1299.90 cable revs per mile with my 225/60 15 tyres and as my speedo reads incorrectly, I must have done the test wrong as they have calibrated it correctly. It's a Jag IRS & RV8 auto.

Any ideas anyone as I'm running out of time and patience!

Thanks, Ady

[kapri]

Ady, 100 turns ?! I used Speedycables and they only ask for so may revolutions of the wheel. I gave them the results and they recalibrated my speed to include an allowable 10% overread ie 50mph is 55 on the speedo.

I'd have thought 100 turns woud have allowed any error in movement of the card disc to become very exaggerated ?

How far out is the speedo reading ?


EDIT : Speedycable calibration method

http://www.speedycables.co.uk/

[wheelie]

Hi Kev,

At an indicated 30, the car would've been doing 37, at 40 it would've been 49, at 60 it would've been 65 and at 70 it would've been 75. Can't remember what 50 was. ETB are just telling me I've done the test wrong and they've calibrated the speedo correctly and that's that.

Ady

[rem]

How far under was it? 1%, 5%, 10%?

Also, is this an electronic speedo, or analogue dial? If an ordinary dial, can you not just change the markings on it?

By the way, how do they actually check the speedo for accuracy?

EDIT: OK, you've just answered the accuracy - about 10% under.

[wheelie]

It's interesting that Speedy say don't jack the wheel up where ETB do say jack it up!

[wheelie]

Hi Rem, no idea of the %ages, its a mechanical guage. I don't know if they would accept me changing the markings if I could do it but as I've paid for a service I expect it to be correct.

They run the car on rollers and record the actual speed when the speedo shows 30, 40, 50, 60 & 70.

[rem]

Hang on a minute, that isn't a consistent error: 30 to 37 would be 23%, 40 to 49 would be 22%, 60 to 65 only 8% and 70 to 75 only 7%. I don't see how that can be?

[rem]

Right, so they know the diameter of the rollers, so can calculate the road speed.

For the gauge error to vary, it sounds like it is a function of how it works (some magnetic trickery?).

[wheelie]

Yeah, the inconsistency baffles me too but how do you query the SVA tester and his rollers? Its almost as if something was slipping somewhere.

[kapri]

It's interesting that Speedy say don't jack the wheel up where ETB do say jack it up!


If you jack both wheels up one wheel will spin in the opposite direction and NOT consistently . That's why you have it on the ground so it's 'in action' results.

[kapri]

Don't speedos work by a magnetic disc rotating and pulling on the needle ? If so incorrect clearances would cause the effect of slippage ?

[Paul B]

Fit an electronic one and you'll have no problems calculating and modifying anything. I put one in my Morris and set it with a GPS, and it was/is smack on.

I have one for my Jago too, when it is needed.

[jsf55]

Cheaper option and always there for refrence .....
http://www.nsra.org.uk/forum/topic.asp?TOPIC_ID=27273

[wheelie]

Hi Paul, how do they work / fit as I only have 2 weeks before my SVA deadline and need to get it sorted ASAP.

Thanks, Ady

[kapri]

JSF said "Cheaper option and always there for refrence .....
http://www.nsra.org.uk/forum/topic.asp?TOPIC_ID=27273 (http://www.nsra.org.uk/newforum/../forum/topic.asp?TOPIC_ID=27273) "
__________________

Just as a note, they are not acceptable for BIVA.

[jsf55]

:(

[wheelie]

Thanks guys, excellent back up plan although I'm still annoyed as to why the ETB speedo doesn't work correctly.

This'll teach me to leave things to the last minute won't it!

[stueeee]

Hi Paul, how do they work / fit as I only have 2 weeks before my SVA deadline and need to get it sorted ASAP.

Thanks, Ady

Fitted an electronic aftermarket speedos to a mate's k*tcar, it used a set of magnets on the propshaft UJ and a small pickup. As Paul says, once they're calibrated either with a GPS or on a rolling road they stay calibrated and are consistent throughout the speed range.

BTW, the fact that your mechanical speedo has a wildly fluctuating error points to problem with endfloat somewhere in the rotating magnet/disc setup that's in pretty well any speedo. When I've had this problem in the past, Speedy Cables have always fixed it at the same time as they've recalibrated the speedo, might be worth speaking to your supplier again.

HTH, Stuart

[blobby]

Just a thought,could you not use a satnav,as it has a speedo on it,or what about a bicycle speedo,the electric ones,with the magnet,that is of coarse if they are accepted in your test,i do recall there was a volkwagen rod,that had no numbers on the speedo at all,as long as you know what angle the needle was for any given speed it was ok for the mot.

[Paul B]

Hi Paul, how do they work / fit as I only have 2 weeks before my SVA deadline and need to get it sorted ASAP.

Thanks, Ady
You need some sort of electrical pulse trigger from the road going parts, wheel, axle, prop shaft etc, to give the speedo something to calculate from. You can buy these from the speedo suppliers. I used the one already in the RV8 tranny on mine, and it worked well.

Once it is in place you have a couple of ways to calculate it: on the fly, by driving over a measured distance, and feeding some numbers in, or like i did with mine, just see how it reads, compare it to the GPS, then alter the numbers in the system a bit at a time until it reads smack on.

Have a look at some of these, but there are many different types, some look like standard round gauges.

http://www.acewell-meter.co.uk/

I think mine is like this, the one I have for the Jago, but it is in the garage somewhere. $113 from Summit.

http://www.summitracing.com/parts/VDO-437053/

http://static.summitracing.com/global/images/prod/large/vdo-437053d.jpg