Speedometer correction

[Joe473]

So my speedo appears slow on the top end and wanted to check calibration. I clocked 10 miles on a highway and only clocked 9.3 on the odometer. How many teeth should I go down on the driven gear attached to cable?

 

[blubuckaroo]

Try one tooth less, but I think you're still going to be reading a bit slow. Two teeth less and you're going to read a little too fast.

 

[Viperwolf1]

So my speedo appears slow on the top end and wanted to check calibration. I clocked 10 miles on a highway and only clocked 9.3 on the odometer. How many teeth should I go down on the driven gear attached to cable?

You have a difference of about 7.5% now. A one tooth change will give you a 5-6% difference. You'll be within the factory tolerance which I think was 2.5%.

 

[Joe473]

Thanks, what I found wierd was I remember looking over the winter and it was a 18 or 19 tooth in a truck that originally came with 3.50's. Didnt check ratio but based on driving still feels like original 3.50's just has 30x 9.50 tires now.

Did some other research tonight and looks like 17 tooth would be correct if it is still 3.50 gears.

 

[Viperwolf1]

Thanks, what I found wierd was I remember looking over the winter and it was a 18 or 19 tooth in a truck that originally came with 3.50's. Didnt check ratio but based on driving still feels like original 3.50's just has 30x 9.50 tires now.

Did some other research tonight and looks like 17 tooth would be correct if it is still 3.50 gears.

You were on a better approach in the first post. There is a hidden factor that you don't know at this point. The gear that drives the speedo gear came in 2 flavors, 6 tooth or 7 tooth. Therefore you can't base your tooth count on someone else's experience. By measuring and determining the error like you did above you don't need to know that hidden factor.

 

[Whoaa]

I don't know what my gear count is. Mine came originally w/ 3:50 gears in the diff's, C4 auto, Dana 20 T-case, and the stock tires were probably 28" diameter.

Now I'm running tires that are stamped 37" tall, but are actually about 36 3/4" tall, C4, dana 20, and 4:88 gears. I have not used a GPS or other device to try to determine actual speed...but "assumed" its fairly close because of taking into consideration the new tire diameter and lower gears are "about" the same percentage of change based on engine RPM at indicated 60 mph speed?

However, now I want a correct speedo. What steps do I take?

 

[Viperwolf1]

I don't know what my gear count is. Mine came originally w/ 3:50 gears in the diff's, C4 auto, Dana 20 T-case, and the stock tires were probably 28" diameter.

Now I'm running tires that are stamped 37" tall, but are actually about 36 3/4" tall, C4, dana 20, and 4:88 gears. I have not used a GPS or other device to try to determine actual speed...but "assumed" its fairly close because of taking into consideration the new tire diameter and lower gears are "about" the same percentage of change based on engine RPM at indicated 60 mph speed?

However, now I want a correct speedo. What steps do I take?

Get a reading of the indicated speed and true speed. Divide indicated speed by true speed (MPHi/MPHt) then multiply that number by the number of teeth on the driven speedo gear. That will give you the number of teeth needed on the new gear. Keep in mind that they range from 16 to 21 teeth. If you get a number outside that range you may be able to change the drive gear inside the D20 to get a better result.

 

[Steve83]

The caption of this diagram contains the Ford TSB about speedo calibration:

 

[jimschulz]

This may Help. Make sure you know what your drive gear count is.

 

[blubuckaroo]

I've found it easier to have a handful of the little gears on hand and just change them and recheck the odometer.
Remember though, even with the odometer correct, your speedometer may still be incorrect if the drum is out of calibration.