Calibrating a speedo?

[SuperStang83]

I just changed out the gauge cluster in my car yesterday and now my speedo is reading extremely high. The car is an 88 and it had a 140 speedo in it previously and I exchanged it for another 140 speedo that all the gauges work. With the old gauges the speedometer is the only gauge that actually worked, although it was slightly off because of the 3.73s. Now with the new gauges everything works, but the speedo reads at least double the speed that I am doing. Does the actual speedo have to be re-calibrated, and if so is this something that I can do myself?

Thank you,

Jacob

 

[jrichker]

The stock range of speedo gears work correctly with a limited range of tire sizes. The 23 tooth white gear used with 3.73 rear axle gears works good for some guys, but self destructs in a few months for others. Check out your local Chrysler/Jeep dealer for the gear or find it online.

See Gear Ratio Calc for more help

Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy within 1%-3%. The drawback is that it isn't cheap.

Please excuse the typo (theirs, not mine) in the following picture - it should read Custom ratio adapters, not Custom radio adapters.

See Speed Correction Devices (Ratio Adapters) for more info.

 

[SuperStang83]

I see what you are saying, but why would the old cluster's speedo work very well, off a few percent, and the new cluster be so far off? With the old one working correctly that would mean that I have the right gear in the tranny, right?


Jacob

 

[jrichker]

How the speedometer works:
The indicator pointer has no direct connection to the speedo cable. It uses a drum with magnets on it to couple to the pointer. The drum turns and tries to twist the circular steel disk that is mounted on the pointer spindle. The magnetic force is all that connects the drum to the circular disk.

Unfortunately I cannot find the drawing that shows how it all works.

In you case, the magnets may have been weak in the older speedometer.

 

[SuperStang83]

So does that mean that I couldn't just switch the mechanism from the old cluster to the new one? Or do you think that it could possibly work?

Jacob

 

[jrichker]

The speedo heads can be moved from one cluster to the other. It all depends on how much work you want to do. Switch the speedo head and put the spare on ebay for someone else.

 

[Mustang5L5]

Sounds to me like your original cluster was a 90-93 cluster. That's why nothing worked except the speedo (or it could have just been busted)

The new cluster sounds like an 85MPH speedo with a 140MPH face on it. If you are doing 85MPH...does the needle point to 140? If so that's what it is.

In which you can just take the actual speedo "guts" and swap it into your current cluster. You'll have to pull the needler off, unscrew the guts from the back and swap.

To reinstall the needle, remove the peg stop and place the needle so that in the correct upright position, it points at the small white dot under 0MPH. Then reinstall the peg and it should be good to go

 

[SuperStang83]

Ok, cool I think the cheapest way go is swap out the guts. On the old one all the gauges actually worked at one time, just slowly stopped working.

I took a drive today and checked my speedo against my GPS, and the speedo read right around double compared to the GPS. At 55 on the speedo I was doing 27mph and at 130 mph on the speedo the GPS read 70 mph. So it was around double the whole time.

Jacob