This will take you to the main site where there is history, technical information and other information on these cars.
This takes you back to the main page of the forums.
This is the control panel to change your password, information and preferences on this message board.
Click here if your lost your password or need to register on this message board. You must be a registered user to post. Registration is free.
Search this board for information you need.
Click here to buy cool Squarebirds mechandise.
Click here to support For $20 annually receive 20mBytes webspace, a Squarebirds e-mail address and member's icon on the message board.
Old 10-23-2018, 07:51 PM
vernz vernz is offline
Join Date: Jul 12 2008
Posts: 215
vernz is on a distinguished road
Default 57 speedometer calibration

I keep learning new things about my new '57. I took it out a couple of days ago with a gps unit to see how close the speedometer was to reality. The indicated speed needed to hit actual speeds were: 32 to hit 30, 37 to hit 35, 42 to hit 40, 48 to hit 45, 58 to hit 55, then things got interesting. It bounced around a bit above 60 and the speedometer seemed to be reading about 64 to hit 60. Then it stayed in the 62 to 64 range through actual 65 and 70. I really had not intended to try 70 but my wife who was watching the gps called out 70 while the speedometer was still indicating about 64. The indicated vs actual speeds below 60 stayed consistent. To me that sounds like a speedometer problem rather than a cable problem. Thoughts?

Reply With Quote
Old 10-23-2018, 08:46 PM
jopizz's Avatar
jopizz jopizz is offline
Excellent Auto Mechanic for over 40 years.
Join Date: Nov 23 2009
Posts: 5,689
jopizz is on a distinguished road

Are you using the same size tires as came from the factory.

John Pizzi - Squarebirds Administrator
1959 Convertible

Thunderbird Registry #36223 856-779-9695
Reply With Quote
Old 10-24-2018, 08:14 AM
simplyconnected's Avatar
simplyconnected simplyconnected is offline
Slow Typist
Join Date: May 26 2009
Posts: 7,642
simplyconnected is on a distinguished road

Originally Posted by vernz View Post
...then things got interesting. It bounced around a bit above 60 and the speedometer seemed to be reading about ...
Think about what's happening inside the cable assembly. You have this spiral of brass that is not straight. In the bends, it rides in the same spot pretty much all the time as long as the resistance remains the same. By your description, you normally run at speeds around town.

Lubricity and speed have much to do with resistance and flexibility. After decades of riding in the same 'groove' at city speeds, now you bring it up to highway speed. Resistance increases, flexibility decreases and the spiral starts to bind just enough to take the cable out of the groove it formed inside the cable assembly. Then, it jumps in and out, in and out of the groove. This change in resistance causes the cable's speed to undulate.

This is why most mechanics will change the entire cable assembly. Everything starts from 'new' and lubricity is ensured before installation. No groove, no binding, no speedometer needle bounce because the speed of the cable doesn't undulate as resistance increases. The speedometer is simply reporting the speed of the cable.

I have seen speedometer cables break while others actually wear a hole in the outer shield. Is it common? Yep. So I suggest you invest in a new cable assembly. They are inexpensive because all models of all classic cars and trucks use them.

As far as ratios, you may run any height tire or any rear end gear as long as you match the speedometer's transmission gear. Just about any transmission shop should have any gear you need. The EXACT ratio may not be available but most cars show slightly higher speed than they are actually going. (This helps with speed limits and tickets.) Speedometer gears in the trans take about ten minutes to change.

Expect your true speed to vary from factors like, highway bends, tire pressure, etc. The change isn't much but it's there. Going around tight bends at a good clip, the outside rear wheel goes faster and centrifugal force makes the tire squat which makes it slightly smaller in diameter (especially if tire pressure is low). - Dave
My latest project:
CLICK HERE to see my custom hydraulic roller 390 FE build.

"We've got to pause and ask ourselves: How much clean air do we need?"
--Lee Iacocca
Reply With Quote
Old 10-27-2018, 11:32 PM
paul274854's Avatar
paul274854 paul274854 is offline
Join Date: Mar 10 2011
Posts: 51
paul274854 is on a distinguished road

Could very well be a speedo problem. Probably needs some lube or cleaning but I would start by cleaning and lubing the cable first to see if that is the solution. If not, then the speedo needs servicing
Midland Park, NJ
56 TBird, 54 Ford, 48 Ford,
Reply With Quote
Old 11-01-2018, 12:29 PM
vernz vernz is offline
Join Date: Jul 12 2008
Posts: 215
vernz is on a distinguished road

Thanks for the input. That was interesting about the speedo cable wearing in over time. I'll go ahead and replace the cable first. If that doesn't help I will go ahead and work on the speedometer itself.

Reply With Quote
Old 11-01-2018, 12:32 PM
vernz vernz is offline
Join Date: Jul 12 2008
Posts: 215
vernz is on a distinguished road

Forgot to mention. I do have radials rather than the original tire size. I'm not as worried about the actual vs indicated speed being off a bit. I was more concerned about the significant inconsistency above 60.

Reply With Quote

Thread Tools
Display Modes

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

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

All times are GMT -4. The time now is 06:32 AM.

Driving, racing or working on cars can be hazardous. The procedures and advice on this website including the message board are opinion only. and its webmasters and contributors do not guarantee the correctness of the advice and procedures. The and its webmasters assume no liability for any damage, fines, punishment, injury or death resulting from following these procedures or advice. If you do not have the skills or tools to repair your car, please consult a professional. By using this site you agree to hold harmless the, its authors and its webmasters from any resulting claim and costs that may occur from using the information found on this site.

Powered by vBulletin® Version 3.6.8
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.
Any submissions to this site and any post on this site becomes property of . The webmasters reserve the right to edit and modify any submissions to this site. All material on this is site is copyrighted by the Reproduction by any means other than for personal use is strictly prohibited. Permission to use material on this site can be obtained by contacting the webmasters. Copyright 2002-2016 by