Speedo showing too fast

[rdl001]

I understand that many of us have the same issue where our speedo shows 10-20% more than the actual road speed.

My 96 Chim has a two wire inductive proximity sensor making/breaking on the rotating teeth of the diff input.

Is it feasible to think that If I cut off 2 or 3 of the rotating teeth, this will give less pulses and therefore show less speed on the speedo?

Your thoughts are welcome.

[Spunagain]

I had some thoughts about the speedo over-reading by about 10%.

I have done some research and my car is an early car with the 3.31 ratio diff.

I suspect the Speedo may have swapped out at some point (possibly in 1998 looking at the history!) and maybe with one from a car with the 3.77 ratio diff.
So I think the switches on the back of the speedo were wrong. I have done some calcs to see what it should be and what it might look like!

I found the settings here: https://www.caigauge.com/documents//pdfs/pil017-36...
and https://www.caigauge.com/documents//product-instru...

I think the settings should be (if the diff is 3.31) : 0000000110
If my suspicion is right it is (Wrong i.e. for 3.77 diff) 0100010110

I sent this to Str8six and they fixed mine but I am not sure they used these settings.

I hope this helps!

[phillpot]

Could this solve the problem? ......

Speedo corrector

[KevtheRev]

Interesting documents, I've been trying to calculate pulses/mile for the dip switch settings.

If I read it correctly, pulses/mile = (slots in wheel) x (Diff ratio) x (Wheel revs per mile)

which for me is 16 x 3.308 x 272 = 14396

Dip switch settings 0000000110 is a pulse/distance of 10727 which seems a lot different to my calcs.

How did you get to 0000000110 ??

[nawarne]

OP, you don't need to buy anything 'extra'.

There is an electronic pulse unit under the dash that converts the induced current pulse from the sensor to an output for the speedo.

A mate of mine got his speedo reading very accurately by altering the 'ones' and 'zeros' displayed on the unit.

Nick

[magpies]

don't forget that it is illegal for a speedo to read slow (by any margin)

[phillpot]

nawarne said:

There is an electronic pulse unit under the dash that converts the induced current pulse from the sensor to an output for the speedo.

That's interesting. Got a picture of what this little thing looks like please?

Would anyone know if the early cars with the transducer in the gearbox tailhousing have one of these or did it only arrive with the transducer that gets its signal from the diff flange?

[KevtheRev]

phillpot said:

nawarne said:

There is an electronic pulse unit under the dash that converts the induced current pulse from the sensor to an output for the speedo.

That's interesting. Got a picture of what this little thing looks like please?

Would anyone know if the early cars with the transducer in the gearbox tailhousing have one of these or did it only arrive with the transducer that gets its signal from the diff flange?

Only T5 gearbox cars have the black box under the dash. This was to provide the correct speed signal for the 14CUX ECU from the speedo transducer.
For LT77 gearbox cars, the speed signal for the 14CUX is taken straight from the gearbox.

[phillpot]

KevtheRev said:

Only T5 gearbox cars have the black box under the dash. This was to provide the correct speed signal for the 14CUX ECU from the speedo transducer.
For LT77 gearbox cars, the speed signal for the 14CUX is taken straight from the gearbox.

[Spunagain]

Here are my calcs:

Calc	early diff	later diff	long diff
wheel	225/50/16	225/50/16	225/50/16
1 mile in mm	1609344	1609344	1609344
Wheel circum/mm	1983	1983	1983
wheel revs per mile	812	812	812
diff ratio	3.31	3.77	3.08
Calc number= wheel rev per mile *diff ratio * 4	10747	12240	10000
Delta in %	0	-12.2	7.5
Speedo switches	110000000	100010110	111011010

You could also look at the current dip switch settings on your speedo, look up the correction number then multiply it by the correction factor needed and look up the dip switch for the new number.

Edited by Spunagain on Friday 17th April 19:52

Edited by Spunagain on Friday 17th April 19:56

[rdl001]

Thanks for the positive reply's.

Phillpot - I have tried the JayCar spedo corrector and it just made the speedo needle bounce around

Nawarne - I do not have any dip switches in my black box (electronic pulse unit), see photo below. In fact, I though this pcb was just to convert a signal to the ECU, not the speedo.

Spunagain - I do not have any dip switches on my speedo? or are they deep inside it?

Regards,

[Dalamar]

The original manufacturers Caerbont also supply these corrector units that I've used to correct my speedo. See the following post:

https://www.pistonheads.com/gassing/topic.asp?h=0&...

I managed to calibrate my speedo to a GPS unit.

[rdl001]

Thanks Dalamar,
Do you happen to have a model/part number for this calibration unit from Speedy cables?

[Spunagain]

Just had a look at the back of mine and there are no visible switches either.

I am guessing here but the Caerbont links I posted must be for the later Caerbont gauge cars. Mine has the ETB gauges which seem to have no switches. There does seem to be a screw on the side of the gauge under some tape which might be an adjust screw. Might be worth a call to Sr8six who managed to cal mine somehow.

[Dalamar]

Part number listed on my invoice from 2018 was RSPE and it was simply called a correction box. Price was £35 but delivery and VAT brought the total up to £57.

Worth it I thought to future proof any changes to the car and easy enough to dial it in to how close you want it to report to a ‘true’ GPS speed.

Two years on and the refurbished speedo and correction box are working fine. I checked last week when I put the car back on the road against a GPS unit - just in case.

[rdl001]

Thanks Dalamar, I will soon be purchasing one too.

Kind regards,

[SILICONEKID 357HP]

My car is a 96 changed from gkn to BTR with the 3 08 ratio .

Mine is only 1 or maybe 2mph out then after 60mph it reads to low ,probably reading around 10mph to low .

I wonder what is causing that ?

It's a bit of a worry because I dont want to be done for speeding .

[SILICONEKID 357HP]

SILICONEKID 357HP said:

My car is a 96 changed from gkn 331 to BTR with the 3 08 ratio . Guages are the later ones with silver bezels .

Mine is only 1 or maybe 2mph out then after 60mph it reads to low ,probably reading around 10mph to low .

I wonder what is causing that ?

It's a bit of a worry because I dont want to be done for speeding .

[magpies]

SILICONEKID 357HP said:

My car is a 96 changed from gkn to BTR with the 3 08 ratio .

Mine is only 1 or maybe 2mph out then after 60mph it reads to low ,probably reading around 10mph to low .

I wonder what is causing that ?

It's a bit of a worry because I dont want to be done for speeding .

do need to sort it out as it is illegal - if you have somewhere for your phone then there is a free very good speedo app called digihud with very large numbers

[Mr Supercharged]

Hi, just had my speedo back from Speedy cables and needs calibrating. Does anyone know how many teeth there are per revolution on the BTR output shaft that the sensor counts? Thanks.