I have a 1980 L48 Vette with the "great" 90 mph speedometer. one of my friend just gave me a almost brand new 140 mph speedometer. I have been told that I have two choices to re-calibrate my 140 mph speedometer to work properly. The first alternative I have is to seed the 140 speedometer to a place like Redline and have it recalibrate. Total cost of this operation for me is +/- 150 $. Second alternative is to replace the actual driven gear in the transmission that is set for a 90 mph speedometer and get the right one with the right number of teeth have been trying to figure out which one would be the right one and I used speedometer program like the one at http://www.bgsoflex.com. After entered all the parameters (i.e. Speedometer Revolutions/mile, Rear Tire Diameter (Inches), Differential Gear Ratio and rive Gear Teeth Number) the program computed that I need a 32 teeth Driven Gear.

Input parameters are the following:

* Speedometer Revolutions/mile = 1000
* Rear Tire Diameter (Inches) = 27.10
* Differential Gear Ratio = 3.07
* Drive Gear Teeth Number = 14

Computation Results:
# computed Driven Gear is 32
# Ratio of driven over Drive Gear is 2.3

I am not sure this is right because nowhere it asked me to enter the actual speedometer( maximum speed indicated). Can someone please indicate to me what I don't get right?

 

Not sure why the max speed would matter.Have you tried just sitting it in place and trying it?

 

Should not require any recalibration at all.

 

I'd do this before I spent $150. That's just common sense.

 

Bigbuffer,

We can calibrate the speedo for you if you want, but it’s not necessary and to be honest 150.00 to calibrate a speedometer is way steep (IMHO)! We've been doing them in house for over thirty years and 150.00 is. . . Well again way too steep!

We check them for free, and if calibration is needed it’s only 35.00. It only takes about 15 minutes to check and calibrate a speedometer that is defect free! If there is problem in the speedo we will contact you before we do anything.

Now if the original speedometer is off there is no reason to re-calibrate the 140 speedometer. The 140 speedo will already be calibrated to the input of the speedo cable.

Willcox!

 

Good day,

Thank you all for your responses. The first thing I did when I got the 140 speedo was to installed and try it. The reading I got was erratic i.e. the needle was "crazy". The 140 speedo come a 1977 Vette and it has only 43,000 km on it. I think that the speedo cable is fine sine it gives me the right reading with the 90 MPH speedo. I did multiples searches in the various forums and I came to the conclusion ( maybe the wrong ones :-)) that it was something with the the number of teeths on the driven gear in the transmission.

 

The 1977 Corvette odometer was not calibrated in km. 1977 was the last year for miles here in Canada. If you have assured yourself that the cable is OK, the fact that the needle is going crazy may indicate problems in the speedo head itself, possibly some foreign material in the drum mechanism.