Speedometer types

[chojinchef]

Hello all

Been playing with speedometers lately. After the Bostig, I needed to add a tach. Bought a used later model cluster and planned on using that, but found a tach cluster from made in 85 in a NC boneyard and decided that was easier than playing with solder on the late model. Also had a busted cluster kicking around in a parts bin. Don't remember where that came from?

What I also determined after the install was that the cluster from the 85 was more accurate in the speed reading than the original cluster. On the original, I had little labels for important speeds placed in correct locations verified by a speedo app on the iPhone and my gps.

Problem with the 85, which I learned after the install was that it had the usual sliding worm gear and the odo / trip meter would stop working.

So last night I pulled apart the cluster with the intention of swapping out some bits and pieces to get myself a complete working cluster again.

The 85 spool housing (date determined by the stamp on the inside of the speedo) where the worm gear inserted through was worn and it looked as if someone had already tried to peen over the end so it wouldn't slide out.

The 86 spool housing was in great condition, so I disassembled the speedo and removed the spools from the 86 and inserted them into the drive from the 85. Lubed up the gears, rebuilt and reinstalled. Works great, but for how long?

So this morning, in a discussion with Radiorental, I have a look at my original 1984 speedo and the spool housing is made of metal not plastic and is in fantastic condition.

The earlier speedo has a face up to 85mph. All of the speedos on the bench now are 805's, which means they should register the same needle location at equivalent speed, correct? (With all other variables remaining constant) TDC for example?

The older one would register 60mph, when I was actually doing 50. Which falls in line with the European calibration spec as outlined in the Bentley for vehicles model years 84 and 85. These were calibrated so that indicated speed was actual speed + 6.2mph +5%, so my 60=50 approximation is close - actual 50 is 58.7mph. Plus, if I add in my tire size differential versus the 805, i add another .65 mph bringing total to 59.35. Problem here is, that in the case of doing 70mph and using the original speedo the needle is bouncing off the max needle stop.

After model year 85, the calculation is actual plus 3mph, which explains being a bit more accurate.

Now I pulled the broken one apart down to opening the drive itself. Please assist me in getting my head wrapped around this; assuming the tires are constant, thus the revolutions of the cable are also similar, what is it within the speedometer that makes one 805 speedo run at 50mph but indicate 58.7 and have a max of 85 printed, and another run at 50, indicate 53 and have a max of 100? Cannot only be the face printing, as the later style has a lower built in differential?

LP
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Its a mistress; an expensive, whiney, needy bitch of a mistress. She is a chunky, dirty girl with bad skin, little motivation and yet she always makes me smile. She sure has been around before shacking up with me. She has a direct line to my wallet, plays with my emotions, is consistantly jovial yet with a sarcastic and sardonic side, is consistant in her inconsistancy, and every once in a while gives me a great ride and a fantastic memory.

[J Charlton]

You could use the OBII feature of the engine to run a Scangauge - it'll give you rpm and lots of other stuff, depending on how you set it up. WRT speedometer - I use my GPS - it gives me dead accurate speed irrespective of tire sizes, gear ratios or whatever.
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[chojinchef]

Thought about that, I prefer an uncluttered dash so a fully functional stock speedo is preferable to me.

The more I think of it, with a fixed wheel size, single unmodified direct cable, and two 805 (an early and late) speedometers, the location should be the same? Which would imply that the face plate is the only difference, which cannot be as that would infer that the face plate is interchangeable. This I know is wrong, because my two speedos indicate differently.

Frankly- I will never need to know that I am going 100mph because in this vehicle - I wont.

Being accurate in the 65-75mph range - yes, very important.

Did some digging - An electromagnetic field is created within the speed cup by a permanent magnet. The amount of field determines how fast the arbor to which the needle sits upon. Calibration is performed by increasing or decreasing the magnetic field in relation to the torque necessary to raise the needle. There is also a hairspring in there that also changes the action of the needle, but I have not figured that out yet.

Ferb - I know what we are doing tonight.
_________________
Its a mistress; an expensive, whiney, needy bitch of a mistress. She is a chunky, dirty girl with bad skin, little motivation and yet she always makes me smile. She sure has been around before shacking up with me. She has a direct line to my wallet, plays with my emotions, is consistantly jovial yet with a sarcastic and sardonic side, is consistant in her inconsistancy, and every once in a while gives me a great ride and a fantastic memory.