Certified 120mph speedo swap

[ernestmayhandsr]

I have what I'm told is the upgraded dash cluster in my 87 landau. I've seen these 120mph speedometers out of the cop cars and wonder if I can install this easily in my landau. The big difference is the speedo part of this cluster is about 3 times as wide as what I've got with the extra guages that came in mine. But this speedo looks the same as the cluster that came in some '87s except of course they only went up to '85mph. Anybody done this or know something?

[silicon212]

You'll have to replace the entire instrument cluster, and the one you have now might be wired differently than what you will need to use the certified speedo.

[ernestmayhandsr]

thanks for the info, i'm curious if the swap to this 9c1 speedo would be the same as doing a swap to a cluster without the "optional Special Instrumentation" (which I can't imagine anyone doing obviously). Would there be any difference? Is the speedo cable different?

[CD Smalley]

Cable is the same. I've replaced both in my car. They are the same part number.

[ernestmayhandsr]

Other connections i need to worry about?

[PeteA216]

W/ the certified speedometer, is it accurate, or does something need to be done with the linkage and/or tranny tail to make it read properly because obviously at identical speeds, the driveshaft will spin faster in a 9c1 caprice than in a civi caprice due to the difference in the rear end gearing. EXAMPLE: (Im gonna throw some VERY RANDOM figures out there for simplicity sake) So if a speedo is designed to read 55 mph when the driveshaft is spinning at 3000 RPM, then on a car where the driveshafts spins at 2500 RPM to make it go 55 mph, that particular speedometer will only read 45.8 mph, knocking it off by 5mph due to the difference rear end gearing.

[silicon212]

For any given speed at any given ratio, both speedos will display the same speed, while the 9C1 (U11) unit is capable of displaying more of it, at more precise measurements (the standard unit is calibrated in 5MPH chunks while the 9C1 is calibrated in 2MPH chunks).

The difference is in the heads themselves - the standard head is adjustable to a degree while the U11 unit is sealed (hence 'certified'). You have to take into consideration things such as final drive ratio, tire size etc., to actually get it to display the proper speed, as with any speedometer.

Most of the 9C1 certified units are certified with 3.08 or 3.42 ratios and P225/70R15 tire sizes - deviate from those and the speedo will no longer be 'certified', although they will display more than accurate enough for most people.

In short, if your car has a standard speedometer that is the same physical dimensions as the U11 unit, if the standard unit displays 55MPH, the U11 will also display 55MPH leaving everything else well enough alone.

[PeteA216]

Im bidding on a certified speedo right now on eBay, and I have 2:73 gearing... So the accuracy won't be off too far is what your saying. As long as I'm in the 5 mph range, I'm happy.

[GreyGoose006]

well it depends on the gearing that was used on the speedo when it was ceritified.

a 2.73 rear end is 11.4 % higher than a 3.08, so your speedometer will read 11.4 % slow. at 65 mph (real speed, as in radar gun) your speedo will read 58...

of course at lower speeds, it will be off less.


thats with the 3.08 rear end calibration.
if the speedo was calibrated for a 3.42 rear end, then the speeds will be off even more

[silicon212]

No, the speedometers both use the same RPM to measure speed with. What it displays is dependent upon the speedometer drive and driven gears in the transmission.

If a car has a standard speedometer that displays 55MPH at a given RPM, it will display more or less the same speed as the U11 regardless of the rear axle ratio. The only time you will see a difference is if the standard speedometer has been 'calibrated' for a non-standard tire size on a given car. Since the certified unit cannot be calibrated (again, it is sealed), it will display a different speed IF the original speedometer has again been recalibrated (i.e. for different tire sizes without switching gearing in the trans).

[PeteA216]

So I guess what I'm trying to figure out is will this certified speedometer out of an '87 9C1 Caprice give me an accurate MPH reading with the stock 2.73 gearing and normal sized tires without having to modify anything such as the transmission tail where the speedometer cable connects to

[silicon212]

Yes, it won't be any less accurate than the one that's on there now, but it won't be certified - unimportant for non law-enforcement use.

[GreyGoose006]

but if one is calibrated for a different rear end than the other one, it will be off.
i mean, unless i am severely misunderstanding the situation here.

[CD Smalley]

Just curious, what do those speedos go for on ebay?

[silicon212]

Originally Posted by GreyGoose006

but if one is calibrated for a different rear end than the other one, it will be off.
i mean, unless i am severely misunderstanding the situation here.

The 'calibration' of which you speak is carried out by the speedometer driven and drive gears in the transmission.

For example, my car with a 3.08 axle ratio and 225/70R15 tires requires a 40 tooth driven gear and an 18 tooth drive gear to report the speed correctly. That gear combination is used with either speedometer on a car with similar specs re: ratios and tire sizes.

The needle on the U11 speedometer is vertical at 65MPH, the needle on the standard speedometer is at the 3/4 mark at 65MPH, but both needles point to 65MPH on the scale. Do you follow me?