Speedometer calibration rev per mile - Chevelle Tech
Chevelle Tech General Tech Area
Please try and post in specific forums

 
LinkBack Thread Tools Search this Thread Display Modes
post #1 of 7 (permalink) Old Jul 2nd, 20, 4:15 PM Thread Starter
Tech Team
 
Join Date: Mar 2020
Posts: 119
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.
Bart Fransen is offline  
Sponsored Links
Advertisement
 
post #2 of 7 (permalink) Old Jul 2nd, 20, 5:49 PM
Boldly procrastrinating
Tom Terrific II
 
Join Date: Oct 1998
Location: Glendale, Az
Posts: 28,872
Garage
Re: Speedometer calibration rev per mile

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?

Tom Terrific or Terrible Tom, depending on the phase of the moon, passing cosmic rays or other factors not fully understood except by my wife.
Tom Mobley is online now  
post #3 of 7 (permalink) Old Jul 2nd, 20, 6:52 PM
Senior Tech Team
Jon N.
 
Join Date: Jan 2003
Location: back in the "Hew-Hess-Hay"
Posts: 4,359
Re: Speedometer calibration rev per mile

Try a 20 tooth gear. It's only money, and not even that much.

Jon N. ~ What doesn't kill you makes you stronger.

The only reason someone would want insurance is if they want to pay someone elses bills or want someone else to pay their bills.

BIGOT is a word used by people that can't use logical reasoning to prove that you are wrong when you don't agree with them.
dyno jonn is offline  
 
post #4 of 7 (permalink) Old Jul 3rd, 20, 1:30 AM Thread Starter
Tech Team
 
Join Date: Mar 2020
Posts: 119
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?
Bart Fransen is offline  
post #5 of 7 (permalink) Old Jul 6th, 20, 5:22 PM
Tech Team
John
 
Join Date: Dec 2012
Location: 55.916, 10.733
Posts: 641
Re: Speedometer calibration rev per mile

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.
SSJohn is online now  
post #6 of 7 (permalink) Old Jul 7th, 20, 12:52 PM Thread Starter
Tech Team
 
Join Date: Mar 2020
Posts: 119
Quote:
Originally Posted by SSJohn View Post
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.
Bart Fransen is offline  
post #7 of 7 (permalink) Old Jul 7th, 20, 5:59 PM
Tech Team
John
 
Join Date: Dec 2012
Location: 55.916, 10.733
Posts: 641
Re: Speedometer calibration rev per mile

Quote:
Originally Posted by Bart Fransen View Post
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.
What color gear, and how many teeth, is in it now?
*edit*
Just reread your original post, you’re running 19/40. Looks like you’ve done the math, put the 15 in when you change gears. Speedo gear goes in the transmission at the speedo cable hookup. Take out the small bolt, remove the housing and it will be self evident from there. Good luck!
SSJohn is online now  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Chevelle Tech forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address. Note, you will be sent a confirmation request to this address.

Email Address:
OR

Log-in










Thread Tools Search this Thread
Show Printable Version Show Printable Version
Email this Page Email this Page
Search this Thread:

Advanced Search
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome