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Speedo Gears / Calibration and NP205's

nvrenuf

nvrenuf

Holy crap, it's running!
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I've never cared about having a functional speedo or if it was correct but I want to try to get this right now that I have a nice speedo (the irony considering I have a speedo app on my phone).

I have the screw on speedo fitting for my 205 (I don't think there's a gear in it) and I'm sure there's a gear on the shaft of the tcase but beyond that I'm lost. Are the 2 gears specific to each other like a ring & pinion or is there magic voodoo involved so you only have to replace the "outer" gear?

Is there a chart somewhere for picking speedo gears vs ratio & tires?

Best source to get the parts?
 
the drive and driven gears are not a matched pair. you can replace one or the other to get the desired ratio. But there are finite combinations and that only covers so much in terms of tire size/ gear ratio.

Early 205s use a smooth bore metal gear, Later 75ish and up used a plastic splined gear.

Per ORDs tech.
Speedometer Parts

GM 205 pencil gears have varying tooth counts, those are how the speedometer is adjusted. There is a letter stamped on the side dictating how many teeth the gear has:

H 18
J 19
K 20
L 21
M 22

The speedometer gear on the output shaft can't be substituted with another, they will physically interchange but the teeth are different heights and the pencil gear won't mesh with the wrong output shaft gear. Output shaft speedometer gear tooth counts below:

Early GM metal speedometer gear - 5 teeth
Red/Yellow GM/Dodge speedometer gear - 6 teeth
Clear Ford speedometer gear - 7 teeth
Click to expand...

Here is a calculator I found


You can find external drive boxes like pictured below to add more/less ratio for cases that you cant get it correct with the available gears.

IMG_2049.jpg
 
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I'll have to figure out my percentage, a quick look at an rpm chart looks like I'm really close to a 30" tire and 4.10's.
 
Last I looked into this was 5-6 years ago for my k5 and all I found was I actually had tocalculate percentages with the overly complicated formulas then match it to the hubble telescope n have one made on another planet blah blah blah it's still like 20-25 mph slow....
 
Some smart phones have apps that let you use one as a speedometer I've read also..
 
I'd like to get mine calibrated too. I read @95-100 mph at 70 by my gps. Mostly I want the odometer to be correct.
 
Many years ago, when doing all the mods on my F250, I worked on the gears in my 205 to get the speedo close with 33in tires.
I got it within reason. Most of the time I am driving fairly close to the speed limit, or so far over that it doesn't really matter how fast I'm going.......
But, then a couple of years back, I caught a cheap heads up display GPS speedometer on sale. It sits on the dash and reflects the speed onto the windshield. The only connection was to the lighter socket.
I don't really recommend that one. It works, and is accurate, but you really have to work at it to be able to read the reflection. It came with a little piece of plastic that sticks on the glass and gives a better reflection, but I never put it on.
Been meaning to replace it with a non heads up unit, and they have come down a lot in price. Unless you buy an Autometer version........
Looking at this one, haven't bought it yet. It does have a permanent odometer function. Of course there are going to be brief times when it can't see the birds, but should not be a problem.
https://www.amazon.com/dp/B074JBYM5K?tag=reviewedme-20
Of course, if you like a needle, they are out there too.
https://www.amazon.com/dp/B074J9MFQV?tag=reviewedme-20
Its nice to have your original speedo working correctly, but I have to wonder if its worth the trouble these days.
Especially if you have to redo it all when you change tires or gears.
When I first modded my truck, I had the factory wheels and tires for summer fishing and general driving use, and put on the 16.5 12.50/33s during hunting season.
Which meant that during the summer, I was driving slower than I thought I was since I had done the calibrating for the 33s.
If I had had these options back in 1989, no way I would have fooled around with the gears on the 205.
 
Fordum said:
Looking at this one, haven't bought it yet. It does have a permanent odometer function. Of course there are going to be brief times when it can't see the birds, but should not be a problem.
https://www.amazon.com/dp/B074JBYM5K?tag=reviewedme-20
Click to expand...
I'd buy that just to be able to tell people how many knots I was driving! Funny how those folks can get all that technology crammed into that little disk, but they can't hire someone to write the description in a recognized language.
 
Fordum said:
Looking at this one, haven't bought it yet. It does have a permanent odometer function. Of course there are going to be brief times when it can't see the birds, but should not be a problem.
https://www.amazon.com/dp/B074JBYM5K?tag=reviewedme-20
Of course, if you like a needle, they are out there too.
https://www.amazon.com/dp/B074J9MFQV?tag=reviewedme-20
Click to expand...

I too have been looking at a GPS speedo for the exact reasons you mention. My speedo has never worked well in the last 20yrs!
The ELING units have reviews all over the place...not sure if they are worth the gamble?
Pricier, but Speedhut gauges have a lifetime warranty on the electronics and seem to have good reviews on most car forums. Good customer support when needed.
I may opt for a Speedhut unit n the near future.
www.speedhut.com
Gauges, Custom Gauges, Tachometers, Speedometers, Shift Light, Pod (speedhut.com)
 
I have Speedhut gps in my dune buggy, love it. Speed hut make a gps unit that will dive a speed cable, kinda spendy @$300

you can customize the gauge, I tried to come close to the Porsche 914 tach

IMG_20180520_183011.jpg
 
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