How To Calibrate A Speedometer

[Gary Lewis]

Let me say right up front that I've not put this speedo in my truck and tested it, but I'm pretty sure it'll work perfectly.

 

Ok, with that disclaimer having been said, the following is how I think you can calibrate a speedometer on these trucks. From what I've read, the speedometer cable speed to show 60 MPH is 1000 RPM. So to do this you'll either need to have something with a known RPM or do some serious trial and error.

 

My lathe has a number of speeds and since it is gear-driven they should be pretty accurate. Unfortunately none of those are exactly 1000 RPM, but one is close at 1150. By my calculation that should make the speedo read 69 MPH. So in the first video at the bottom of this post is a test of a speedo I have and it read 67 MPH when driven at 1150 RPM. And in the second video you'll see that after tweaking the clock spring it does read 69 MPH.

 

And, just to check it out I shifted gears to a set that gives 890 RPM, which should show 53 MPH and it did. Then I went to 700, which should give 42 MPH and the speedo read 41. Finally I tried 300 RPM, which should be 18 MPH but the speedo read 12. So, while you can calibrate the speedo at a given speed, that doesn't mean it'll be right everywhere.

 

But, how do you calibrate it? You rotate the fixed end of the clock spring a bit. Where's the clock spring? It is in the circle here:

 

 

And here's a closer view of it:

 

 

Finally, here's a view showing which way you rotate it to make it read faster or slower. BUT, you do NOT have to pull the needle and faceplate off to do the adjustment. I just did that so you could see how to rotate the arm. You can reach in with small screwdriver and slide the arm up or down, but try not touch the spring itself as it needs to be nicely coiled like this one is - now.

 

 

And, here are the videos. First, this one shows the initial test where the speed came up to 67 MPH instead of the 69 MPH target:

 

 

 

And this video shows the test after I tweaked the clock spring slightly and it comes right up to 69 MPH:

 

Maybe tomorrow I'll be pulling Big Blue's speedo in order to install the new speedo cable that's to be here tonight. And that speedo is off ~5 MPH at 30 MPH and up, so I'll be calibrating it and we'll then know how well the process works.

[Machspeed]

Well look at you! Good way to put your engineering skills to work, my friend. Nice job, Gary.

Curious, what is your lathe?

[Gary Lewis]

Well look at you! Good way to put your engineering skills to work, my friend. Nice job, Gary.

Curious, what is your lathe?

Thanks! It was fun, and something I've been wanting to try for a while. But since I need to pull the instrument cluster out of Big Blue to put the new speedo cable in I thought now was the time.

So I used a spare speedo I have just in case it didn't work.

The lathe is an Enco. Don't remember the size, but I can go look if you are interested.

[Chris Guthro]

Man that’s cool Gary!!! Love the how to! Got a few buddies with lathes so may have to check this out

[Gary Lewis]

Man that’s cool Gary!!! Love the how to! Got a few buddies with lathes so may have to check this out

Thanks, Chris. I'll post another video tomorrow of what I find with Big Blue's speedo. I expect it to read ~64 MPH instead of 69.

And, before someone says "change out the driven gear", I'm currently running a 16-toothed gear and don't see a 15 advertised. Besides, the odometer is pretty accurate with the 16. It is just the speedo that's off. So I plan to pull it tomorrow, test it, and hopefully calibrate it.

[Machspeed]

The lathe is an Enco. Don't remember the size, but I can go look if you are interested.

No need on the lathe, just curious.

While we're on the subject of speedometer accuracy, do you have over sized tires on your truck? If so, how do you go about ensuring speedometer accuracy in those types of changes?

 

[Gary Lewis]

The lathe is an Enco. Don't remember the size, but I can go look if you are interested.

No need on the lathe, just curious.

While we're on the subject of speedometer accuracy, do you have over sized tires on your truck? If so, how do you go about ensuring speedometer accuracy in those types of changes?

I do have oversized tires on the truck. They are 33" tires and certainly change the speedo reading.

So your point is valid - I probably should have said that this approach will/should work if you have stock tires and gearing. But since my odometer is accurate then this is the only way to fix the speedometer error. As said, I expect to see ~64 instead of 69 MPH when I put the speedo on the lathe. Then I'll tweak it up the 5 mph and hope that it will read correctly when I put it back.

But, I won't button everything up until I've taken a drive to prove the theory as I may have to come back and make further adjustments.

[grumpin]

I do have oversized tires on the truck. They are 33" tires and certainly change the speedo reading.

So your point is valid - I probably should have said that this approach will/should work if you have stock tires and gearing. But since my odometer is accurate then this is the only way to fix the speedometer error. As said, I expect to see ~64 instead of 69 MPH when I put the speedo on the lathe. Then I'll tweak it up the 5 mph and hope that it will read correctly when I put it back.

But, I won't button everything up until I've taken a drive to prove the theory as I may have to come back and make further adjustments.

Very cool indeed!

[kramttocs]

I do have oversized tires on the truck. They are 33" tires and certainly change the speedo reading.

So your point is valid - I probably should have said that this approach will/should work if you have stock tires and gearing. But since my odometer is accurate then this is the only way to fix the speedometer error. As said, I expect to see ~64 instead of 69 MPH when I put the speedo on the lathe. Then I'll tweak it up the 5 mph and hope that it will read correctly when I put it back.

But, I won't button everything up until I've taken a drive to prove the theory as I may have to come back and make further adjustments.

Agreed! Well done and well documented

[Gary Lewis]

Agreed! Well done and well documented

Thanks, guys.

I plan to put this info onto the page at Documenation/Driveline/Speedometer Gears, Cables, & Sensors. And in the process of figuring out how to do that I took a peek at the calculators on that page. I'll have to confirm what my drive gear is in the t-case today, but if I put the 33" tire size into the Setup Gearing calculator I see that my speedo cable RPM should be 949 at 60 MPH. So instead of dividing my lathe's speed by 1000 to get the multiplier for the speedo reading, I need to divide by 949.

That's a step I missed in the calibration procedure - finding out what your speedo cable RPM should be before adjusting the speedo. However, if you have stock gearing and tire sizes then you can probably assume 1000 RPM.

And, I need to point out that this does not replace getting the right driven gear. Instead, this is for use when:

• You have the right drive and driven gears, but the speedo just reads incorrectly. For instance, on Big Blue the odometer is very accurate, so I have "the right" gears. But the speedo is off and I want to correct it.

• There is no driven gear to correct the speedometer. For example, if you have a 16-toothed driven gear and the calculator says you need a 15. Sorry Charlie, those are hard to find if they exist at all.

But, are there other scenarios where calibrating the speedo is appropriate? Thoughts?