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Speedometer calibration rev per mile

2K views 6 replies 4 participants last post by  SSJohn 
#1 ·
hi all,

I encounter a (strange) problem related to my speedometer which i need to calibrate when i swap my rear end ratio. 191 camino

Recently i replaced my speedocable, and since then the speedo gives a reading.
1. Not very stable
2. It makes a soft noise from behind the dash(yes i lubed the new speedocable)

I did some runs with gps to check current actual speed with gps.

Dash speed 35, gps 31
Dash speed 50, gps 47
Dash speed 60, gps 57
Dash speed 65, gps 62
So lets say the reading is 3mph high

So far so good.
I am running 2.56 rearend (checked).
Running 255/60 15" tyres (27.05inch diameter in theory).
If i match the tyre size with my rpm gauge at certain speed and then I recalculate to tire size it returns 26.75" so it will be around this size in reality.


My th700r4 currently runs a 19tooth and 40tooth wheels for the speedo.

If i do all the math and compensate for the 3mph offset as mentionee above, it returns in a rev per mile of 871. Even without the 3mph offset it returns in 917rev per mile.

On every website, I found 1000 or 1001 rev per mile for a 1971 el camino.

My question:
Is it possible something is wrong in my speedometer? That is should be 1001 rev per mile but something messed up inside and now return in a weird number.


Btw, if i do the math for my tyre size and take 3.55 rear end with 1001 mile per rev. The gear selection is 15tooth with 40tooth and this is almost the higest gearing possible (t15/t45 is max).

I hope my question is clear. I am glad to give more info if needed.
 
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#2 ·
GM traditionally adhered to a standard of 1000 cable RPM at 60 MPH across all divisions. You are not far off of that. If I were you I'd strongly consider just leaving it. Well, unless you don't have anything to do with your time and money. :)

Did you replace just the cable or the housing also?
 
#3 ·
Try a 20 tooth gear. It's only money, and not even that much.
 
#4 ·
I replaced inner and outer cable.

The speedo itself is not changed.

I am not really interested in the 3mph offset, however i need to determine new gears when the rear end jumps from 2.56 to 3.55.

If i put all my knowns in a online speedogear calculator, with 19tooth it returns a 36tooth.

In practice i am running 19-40tooth and it is almost correct (3mph off).

This means my speedo is not 1000rev per mile. Is this normal to happen in time, that a speedo starts deviating?
 
#5 ·
The factory calibration is usually a little fast, i.e. your speedo reads 65 and actual speed is 62. You’re pretty darn close to that. After your gear change swap out the speedo gear with the closest tooth count and call it good.

If you need absolute precision go with a gps speedo.
 
#6 ·
The factory calibration is usually a little fast, i.e. your speedo reads 65 and actual speed is 62. You’️re pretty darn close to that. After your gear change swap out the speedo gear with the closest tooth count and call it good.

If you need absolute precision go with a gps speedo.
True i am happy with the current offset/deviation. But how i need to change the speedogear with when i swap from 2.56 to 3.55 rearend.
 
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