I have an 81 auto. Speedo was fine until I sent it off and had it upgraded from 85MPH to 140MPH. Now I am reading 10 MPH slower than I'm acutually traveling. When I contacted the vendor who did the upgrade they are willing to recalibrate it again, but I was thinking is sure would be easier to just change the gear in the tranny.
Any downside to this thinking?
I'm thinking I need to drop down 2 teeth on the gear I have in there now (1 tooth for each 5 MPH)?????
I'm sure I have the stock gear in there now, any guess on what size any came with?

Any help would be greatly appreciated
Thanks,
YBnormal...drive a vette

 

Send it back. Changing the gear will change the odometer reading.

 

Are you sure it would mess up the OD reading? I think the Speedo and the OD are linked, so right now his OD reading is off. I recalibrated my tranny using a GPS to get the readings and I think I have to go 90 before I see 1 mph difference, so you might try that. Anyways, my 2 cents is that it would work.

 

When the face is changed the magnetic calibration on the speedometer has to be changed to reflect the different spacing on the speedo face. Sometimes this can be achieved by simply changing the spring tension arm on the speedometer and sometimes you have added magnetism to the first worm magnet. Basically you have to do what ever it takes to achieve the proper calibration across the scale when changing the face.

Changing the speedo gear +/- one tooth per 5 mph might solve your problem, it may not. In defining this “may or may not” scenario one would have to assume the calibration is off over the entire spectrum of calibration to make the speedo accurate with a gear change.

Most original speedometers are close in calibration between 0 and 70 mph but tend to fall off scale at higher speeds. The GM margin for error would be +/- 3mph at any given point while setting the calibration (I’ll have to look this up on the chart for your speedometer to verify this but I should be close).

In layman’s terms, if the speedo is calibrated wrong across the scale changing a tooth will fix the reading problem. If it is not magnetized properly and is off at one speed it may be accurate at another, changing the tooth count would only make the same problem at a different speed. As Wombvette has stated it will also affect your odometer output depending on the year of the car. If the odometer is driven by the speedometer cable, speeding it up or down will change the speed in which the odometer turns.

Personally I would pull the speedo back out of the car and either check it with the scale below, or return it for the correct calibration.

I ‘m not sure why the speedometer is off, could be a calibration issue, could be the tension arm got moved during shipment, but as a rule of thumb you can check them if you have a known source for RPM’s. These numbers are taken right off the face of my Stewart Calibration Machine.

RPM MPH
250 15
500 30
750 45
1000 60

Willcox Inc.

 

Hey Willcox, if I have a problem I will just drop my stuff off to you, but I am curiors as to what the heck a guy could use that would know the RPM's from 500-3000?

 

Yeah bring them by. If you have the small guages out bring them I can check them too!

It's rare, but I'm at the shop right now if you want to swing by. Oh a Drill, if you know the RPMS are correct will work. The problem is most are not!

I think I have the right customer, Norm said to tell you we had the t/m bracket too!

W

 

Good info. Learn something new most every day on the CF.


TSW

 

Thanks everyone for the replies. First I need to double check across the entire scale to verify the 10 MPH error. However I was thinking the time I checked it at 20 (reading 10), at 30 (reading 20) and 60 (reading 50). That was last summer and from memory so I will have to recheck to verify. I was so pissed after paying good money to have it upgraded and calibrated and for it to be off that much I called the vendor and raised living HELL! He did agreee to recalibrate it but I was just trying to keep from pulling it out again.

If I do decide to change the gear I go down two teeth correct?

YBnormal...drive a vette