Speedometer Diagnosis
Thread Starter
Melting Slicks
Joined: Feb 2001
Posts: 2,608
Likes: 3
From: TEXAS - you mean there are other states?
Speedometer Diagnosis
What tests can I use to diagnose my speedo gauge unit. I want to ensure the mileage and odometer work properly before replacing in the dash. 
Melting Slicks
Joined: Apr 1999
Posts: 2,675
Likes: 13
From: Sudbury MA
I tested my 68 with a variable speed electric drill and a short 6 inch piece of speedometer cable.
Chuck one end into the drill and the other end fits into the speedometer and slowly start the drill to be sure youare going in the right direction.
Let it run a little until the odometer clicks over and then install it in the dash.
If it is digital, I don't know how you would test that.
Kurt
Chuck one end into the drill and the other end fits into the speedometer and slowly start the drill to be sure youare going in the right direction.
Let it run a little until the odometer clicks over and then install it in the dash.
If it is digital, I don't know how you would test that.
Kurt
Thread Starter
Melting Slicks
Joined: Feb 2001
Posts: 2,608
Likes: 3
From: TEXAS - you mean there are other states?
I see a Wilcox posting here - am reading now.... it looks informative.. will follow up:
corvetteforum.com/c3-tech-performance/2227754-speedodometer-problems
corvetteforum.com/c3-tech-performance/2227754-speedodometer-problems
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,852
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Well I hope my spelling on this post is better.
Use the drill method on the speedometer cable at the transmission.
If you use an electric drill and know the RPM's of that drill you can also verify the accuracy of the calibration. The speedometers are calibrated linear but they also have a margin for error of about 3/5 +/- at any given point.
On original speedometers it is rare to find this large of a discrepancy but it is possible.
So, how do you check it with a drill for calibration.
Easy.
500 rpms = 30 mph
1000 rpms = 60 mph
1500 rpms = 90 mph
Willcox Inc.
18243
Use the drill method on the speedometer cable at the transmission.
If you use an electric drill and know the RPM's of that drill you can also verify the accuracy of the calibration. The speedometers are calibrated linear but they also have a margin for error of about 3/5 +/- at any given point.
On original speedometers it is rare to find this large of a discrepancy but it is possible.
So, how do you check it with a drill for calibration.
Easy.
500 rpms = 30 mph
1000 rpms = 60 mph
1500 rpms = 90 mph
Willcox Inc.
18243
Last edited by Willcox Corvette; Oct 21, 2009 at 12:22 AM.
