speedometer reading

[dab67]

Sometime back I posted a question regarding the difference in speedometer reading and the actual speed for my 67 Camaro  At 30mph on the speedometer I was actually going 39, at 40 it was 52 and at 50 it was actually 65. A 33% difference. This was based on three readings for each speed shot with a radargun from my neighbor who is a State Trooper. I was just told how to determine if the speedometer would need recalibration or if the gear in the tranny needs to be replaced.  This was told to me by a shop that rebuilds trannies and has run inot this problem on numerous occassions. The rear gear in my car is a 2:73.
Here is what I was told, find a mile marker on the highway. Look at the odometer reading when you pass the first mile mark, when you pass the second mile mark  the odometer should have move 10/10ths or 1 mile. If this is true, then the speedometer needs to be replaced or recalibrated. If the odometer move more let's say 13/10ths or 1.3 miles than the gear in the tranny has to be changed to more teeth than what is in the car now. If the odometer moves 8/10ths .8 than the gear in the tranny has to be changed to less teeth.
How does that sound? Does it make sense?

Dave

[JohnZ]

If the speedometer reads faster than you're actually going, you need more teeth on the driven gear (so it rotates slower); if it reads slower than you're actually going, you need fewer teeth on the driven gear (so it rotates faster). Each tooth change is about 5%.

Speedometers hardly ever need to be "re-calibrated" (that's a process to re-gauss the magnets in the speed cup); 99% of the time they just need the driven gear changed to match the diameter of the rear tire and axle ratio.

[dab67]

OK JonhZ:

Let me see if I have this correct:
 I have a 2:73 rear gear. To match the correct "drive gear" with the rear end and speedo gear (which is a blue 20 tooth gear) I have to have less teeth on the drive gear because my speedometer reads less than what I am actually traveling correct? How many different drive gears are there? I would imagine I will have to pull the drive gear out of the tranny and count how many teeth are on it. I'm not confusing this with the speedo gear correct? Because if that is the case the blue 20 tooth gear is as low as I can go!!!!!!!!!!!!

Dave

[JohnZ]

Assuming you have a Muncie, there's only one "drive gear" (the steel one pressed on the output shaft inside the tailhousing); it has 8 teeth, although there are two different diameters depending on the original axle ratio, as each diameter works with a different diameter "family" of plastic driven gears that drive the speedo cable. I only have the Corvette combinations - maybe Ed has the Camaro combinations, as the axle ratios and tire diameters are different.

[dab67]

JohnZ:

I have a powerglide.

[JohnZ]

JohnZ:

I have a powerglide.

Powerglide uses the same driven gears as a Muncie.

[Mr12771]

Dave
 I can help you out with what you need. I do this all summer long for street rod and muscle cars. I stock most of the gears and if I don't have it I can get it next day.

 What is your tire size with rim size? I all ready know you have 2:73 rear gears. I just need your tire size and I can tell you what should be the right gears. Sometimes the speedometers are off , but I can get it a lot closer then you have right now.

Mike

[dab67]

Mike:

Here is the size of my rear tires and rims P235/60R15

Dave

[Mr12771]

Dave

 You need a 9.8 tooth gear  but since they don't make them you need a 10 tooth drive gear inside the trans. With that 20 tooth driven gear you have. That should put you right on with your rear gear and tires.  I have them if you need one. It's very easy to change.

 Mike

[rich69rs]

http://www.camaros.org/forum/index.php?topic=945.msg5085#msg5085

Above discussion is probably worth revisiting.  Be sure and look at Ed's post referencing the chart at this link:  http://www.chevelles.com/shop/speedo.html

[dab67]

Mike:

How about rebuilding my tranny? I'll give you a call before this Friday and we can talk.

Dave