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speedo help.... not reading right

Posted By: brads70

speedo help.... not reading right - 11/12/12 07:20 PM

Hello all!
I replaced the speedo in my 70 Challenger with another one off e-bay after I screwed mine up trying to replace the speedo needle.
( standard gauges, not ralley) It just swung limp at the bottom of the speedo.
So I bought another cluster off e-bay. Problem is the odometer is working perfectly but the speedo is out by around 20 MPH? I have a new speedo cable already installed. Needle moves nice, not jerky.
Any idea's?
Thanks Brad
Posted By: scratchnfotraction

Re: speedo help.... not reading right - 11/12/12 07:25 PM

was it correct before swaping the speedo cluster?

maybe a speedo pinion gear in the trans could be changed to adjust the needle to the road speed.
Posted By: burdar

Re: speedo help.... not reading right - 11/12/12 07:33 PM

I think I have the same problem. I havn't verified it with GPS yet though.

My cluster was rebuild. I also went from a 2.76 gear to a 3.23. I changed out the speedo gear from a light blue/green(original) to a yellow gear. I don't remember the tooth count off hand but it's the correct one for 3.23.

Dad was following me for the cars first trip out of town since the restoration. I set my foot to 60mph but dad said I was very close to 70mph. I was turning 3000 RPM. The online RPM calculators say I should be turning less then 3000 at 60 with 3.23's and stock F70x14 tires.
Posted By: mcmopars

Re: speedo help.... not reading right - 11/12/12 08:48 PM

do you have the right gear?just a thought
Posted By: RapidRobert

Re: speedo help.... not reading right - 11/12/12 09:01 PM

Quote:

Problem is the odometer is working perfectly but the speedo is out by around 20 MPH?

If the odo is reading correctly (checked by the interstate mile markers, go past 5 of them) then the speedo assy needs to go into a speedo specialty shop to be recalibrated. If the odo is off then you would 1st need to get the correct trans speedo pinion in there so the odo reads correct (checked as mentioned) then check the speedo with a digital roadside MPH sign (or GPS) & if it's incorrect then take the assy in & have the magnetic gismo recalibrated. I picked the correct tooth pinion gear by using the MP chart in the archives which only asks for tire dia and rear gear ratio. I measured ACTUAL dia by placing a piece of flat stock across the top of one of the rear tires then measuring from the underside of it to ground. I have been advised to use "rollout" instead as in putting either rear tire valve stem at 6 o'clock then measure with a tape the rollout to get the stem back to 6 o'clock which'll be ~7 or 8 feet then dividing Pi (3.1416") into that to get actual dia but how I measured mine worked for me in this instance. #1 get the odo correct 1st with the right pinion then check the speedo
Posted By: brads70

Re: speedo help.... not reading right - 11/13/12 03:55 AM

Quote:

was it correct before swaping the speedo cluster?

maybe a speedo pinion gear in the trans could be changed to adjust the needle to the road speed.



I swapped in a Keisler A41 auto OD kit. I'm pretty sure the gear in the tranny is ok as like I mentioned the odometer is correct.

Anyone know a speedo shop were I could send it? I'm in Canada about 2 hours north of Buffalo NY
Thanks Brad
Posted By: brads70

Re: speedo help.... not reading right - 11/19/12 01:40 PM

No speedo experts/experiance on here? I went for a drive yesterday and double checked the odometer and it is correct. speedo IS out about 20 MPH
I'm wondering if it's lubrication issue? a spring somewhere wore out? or?....
Not looking forward to removing it again...
Posted By: cjskotni

Re: speedo help.... not reading right - 11/19/12 02:50 PM

Quote:

No speedo experts/experiance on here? I went for a drive yesterday and double checked the odometer and it is correct. speedo IS out about 20 MPH
I'm wondering if it's lubrication issue? a spring somewhere wore out? or?....
Not looking forward to removing it again...



From what I understand, the speedo works off of the odometer. If you are sure the odemeter is moving correctly, then there is soemthing that needs to be repaired within the speedo itself.

The fact that the odometer reads correctly, rules out any speedo cable or speedo gear issue.

I could be wrong here but that how I understand these clusters.

Maybe somebody else here will chime in but it may be time for a rebuild of the cluster itself.

Good luck
Posted By: RapidRobert

Re: speedo help.... not reading right - 11/19/12 02:58 PM

Unfortunately the speedo magnetic gismo has the delicacy of a swiss watch. No other option but to take it in or get another used one which might be cheaper depending on what's available to you
Posted By: Mopar Guy

Re: speedo help.... not reading right - 11/19/12 04:17 PM

You can recalibrate the magnet in the speedo if you can find some
one that have that eqipment !
Posted By: scratchnfotraction

Re: speedo help.... not reading right - 11/19/12 04:19 PM

if the odo is correct then the gear may be wrong size.

if it is 20 mph over road speed you need more teeth to slow the speedo needle down.

if it is 20 mph slower then you need less teeth to speed it up.

I changed from 27 tooth speedo gear with a 2.76 gear and 29" tire

to a 38 tooth speedo gear and 3.91 gear and 29" tire

it could very well be the cluster but I would look at the speedo gear to see what tooth count you have in there the see what it would call for with gear x tire size.

then retry speed/mile marker test and speed with a GPS.

thats the way i understand it.
Posted By: MoparMarq

Re: speedo help.... not reading right - 11/19/12 07:00 PM

Quote:

You can recalibrate the magnet in the speedo if you can find some
one that have that eqipment !




www.tacomaspeedometer.com

They made a nice reduction gear assembly for my Toyota when I swapped rear gears (speedo drive pinions don't interchange on the different transmissions on the Supra).

Probably someplace closer to you just like it that can properly tweak your speedo...
Posted By: brads70

Re: speedo help.... not reading right - 11/19/12 09:48 PM

Thanks for the replies guys! I have a buddy that works as a mechanic for the local police station. I'll check with him and see if he knows of a shop.

I know it's not the gears in the tranny or the cable as the odometer works perfect.
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