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Speedo over reads, is the J type overdrive the cause.


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Hi

 

I have a 1970 CP which is now fitted with a J type overdrive, the speedo is the original and over reads to the point where I need to get something done about it.

 

Am I right in thinking the cause of the over read will be the different O/D assuming the rest of the car is pretty much stock?

 

I have a CR speedo which I was hoping to do a straight swap once its had a service and mileage adjustment.

 

As an aside just had a tacho changed from cable to electronic by JDO instruments, very impressed with the service and turn around.

 

thanks

 

Jim

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Jim,

Think about it a little! If having an overdrive put your speedo out then every other overdriven owner would have had a faulty speedo from Year Dot.

The speedo drive is always taken off the transmission after all that gearbox gubbins, so that it is reading final drive rate and wheel speed.

 

But you don't have a speedo drive! You have an electronic instrument, and have just changed to it! Could it be that the impressive service from JDO wasn't and that they got the conversion wrong? Seems to me the obvious cause.

 

Bests

John

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Jim,

Think about it a little! If having an overdrive put your speedo out then every other overdriven owner would have had a faulty speedo from Year Dot.

The speedo drive is always taken off the transmission after all that gearbox gubbins, so that it is reading final drive rate and wheel speed.

 

But you don't have a speedo drive! You have an electronic instrument, and have just changed to it! Could it be that the impressive service from JDO wasn't and that they got the conversion wrong? Seems to me the obvious cause.

 

Bests

John

Hi John,

It's a tacho I changed from mechanical to electric, and it's not in the car yet.

 

Since rebuild the only change to the drivetrain has been changing from A to J type O/D. I assumed the ratios might be different?

 

Jim

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Hello Jim

You have probably got the wrong driving gear or pinion in the overdrive(or both?)

 

http://www.overdrive-repairs.co.uk/buy_spares.php?cat=J%20Type%20Spare%20Parts%20List

 

Have a look at the list.

 

Roger

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Hello Jim

You have probably got the wrong driving gear or pinion in the overdrive(or both?)

 

http://www.overdrive-repairs.co.uk/buy_spares.php?cat=J%20Type%20Spare%20Parts%20List

 

Have a look at the list.

 

Roger

 

Mmm, I had pondered on this and hoped there was an easy answer, having just changed my angled speedo drive which required me to remove most of the inside of the car I was hoping I wouldn't have to do it again...

 

On the plus side I now have some cover plates to fit thanks to Conrad...

 

Jim

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Hello Jim

There is good news and bad news Good news is you can change the pinion if its the wrong one Bad news is it requies the overdrive to be stripped to change the gear.

 

If you remove the pinion and count the teeth perhaps you can fit a different one which may get speedo closer? ask Sheffield overdrive for advise.

 

Roger

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Just a query about your speedo choice. From memory the CR dial and needle is different to the CP so unless you've also got a CR tacho that John Ostick has converted you'll have a mismatch. Picking up on Mike's hint above, if it's not over reading by too much John can calibrate it for you for a reasonable price. Instructions on how to measure turns per mile and all that jazz are on his website.

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Thanks for the replies, at 50mph it's reading 60ish, at 30 it's about 36mph. Enough for me to want to do something about it, but not the end of the world.

 

On my trip recorder I multiply the indicated miles travelled by .82 which gives me true mileage travelled.

 

Jim

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Hi Jim

 

Mine is much the same as yours, I had the O/D uprated fromm 22% to 28% dont know if that makes a difference? I must admit I have got used to it over the year's of ownership.

 

Cheers

 

Mike B)

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Its easy enough to 'recalibrate' yourself if you check the actual speed against a sat nav and note the 'angle of difference' of the speedo needle. Then remove the speedo and front glass, pull off the needle and rotate it anti-clockwise by the estimated 'correction angle' and then refit.

You will be below zero MPH and need to depress the spring wire 'needle stop'.

May take a couple of attempts until you get it 'spot-on'

 

john

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Its easy enough to 'recalibrate' yourself if you check the actual speed against a sat nav and note the 'angle of difference' of the speedo needle. Then remove the speedo and front glass, pull off the needle and rotate it anti-clockwise by the estimated 'correction angle' and then refit.

You will be below zero MPH and need to depress the spring wire 'needle stop'.

May take a couple of attempts until you get it 'spot-on'

 

john

 

John,

 

I like your 'left field' approach unfortunately I fear my speedo error is compounded as the speed increases, plus I'm quite fastidious in my repairs, It has to be done right or I won't do it at all, as some other members on here will know.

 

Thanks

 

Jim

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Jojim

If you have a saloon box/Jtypeo/d your speedo will read the wrong speed. It can be recalibrated by a number of people (Speedograph Richfield come to mind). They will ask you to rotate the rear wheels 25 turns while counting the number of revolutions of the speedo cable. With this info they can recalibrate your speedo head - both speedo and odometer will be correct. I have a saloon box plus J type O/D in my 250 and had to go through this rigmarole!

Regards

Phil Leech

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