speedometer calibration

[gmc 2500 4x4]

i have a 69 gmc k2500 with t-221 and sm 465. i swapped out the 4:56 gear and put in 3:54, and has 33'' tires. anyone know the reducer adapter i may need ? or where to get help?

[Verbal]

I like the chart at this link. https://speedometercablesusa.com/gear_box_adapters.html

I have no personal experience correcting speedometer though (yet)

[gmc 2500 4x4]

thanx for the post, i just found that link u posted, good info. i might have to wait till truck is on the road to do a test then go from there. i found lots of info out there on the subject

[AJC]

From hatzie PDF of Square Schematics & GM Training Manuals thread.

Originally Posted by hatzie

Useful Docs and Spreadsheets
What follows is an Excel 97 spreadsheet I put together several years ago to calculate the Speedo gears and engine RPM at various highway speeds. I was doing a "What-If" on transmissions, tires, and axle gear-sets and wanted to get transmission/transfer-case speedo gears as close to correct as possible. My 76 reads 1-2mph higher than actual speed. If my speedo reads 35 the local police dept's "Your speed is this" sign will read 33 or 34.

For Speedo gears you only need to edit the Tire Size & Axle ratio. The speedo gear table will automagically calculate the ideal Driven and Drive gears.
You then get to find which ones you can actually buy. You can edit the ratio adapter section to get you closer... If the gears you can actually buy are not close enough.

For the RPM tables you'll need to edit the transmission and Transfer case gear ratios as well. There are tables of ratios found in common trannys and transfer cases in the RH pane.

• RPM_Gear_Axle_Transmission_TransferCase.zip archive with Excel 97 XLS format and Libre Office / Open Office ODS format.

A Rockwell T221 should have a drive gear with 4 teeth and a driven gear option of 11, 12 or 13 teeth.

If you want I can do the math for the ratio adapter. I just will need the speedometer gear tooth count. Looks speedometer gear like the picture below.

[gmc 2500 4x4]

im assuming my rockwell has the 4tooth inside and i have what is pictured with 11 teeth, and some very useful info you shared, let me know what you come up with thank you!

[AJC]

You will need a correcting adapter with a ratio of 1.281779661 or as close as possible. Now if your 33" tires run +/-0.5 of an inch, you could still see up to a 3% error. Which would be +/- 2 MPH at 65 MPH.



Here is a GoogleDoc I made that you can play around with tire size and gearing. The formula is from Speedometer Cables USA under "Need a correcting adapter".

CORRECTING_ADAPTER_CALCULATOR

[gmc 2500 4x4]

Originally Posted by Verbal

I like the chart at this link. https://speedometercablesusa.com/gear_box_adapters.html

I have no personal experience correcting speedometer though (yet)

thank you!

[gmc 2500 4x4]

Originally Posted by AJC

You will need a correcting adapter with a ratio of 1.281779661 or as close as possible. Now if your 33" tires run +/-0.5 of an inch, you could still see up to a 3% error. Which would be +/- 2 MPH at 65 MPH.

Attachment 2150855

Here is a GoogleDoc I made that you can play around with tire size and gearing. The formula is from Speedometer Cables USA under "Need a correcting adapter".

CORRECTING_ADAPTER_CALCULATOR

thank you!

[gmc 2500 4x4]

speedometer cables hooked me up, gave them my info 1.2844. is the closest i can get