My speedometer is now accurate

[DeadheadNM]

I’ve had problems with my speedometer head since post-build mile number one. The initial one had a busted odometer. This was swapped for a nice one supplied by member Tbone1964 (Tom). Given the rear end gear change and tire diameter increase the speedometer was off. Today I was able to correct the error with the addition of a gear ratio adapter from Deluxe Speedometer and Cable Service of Denver. They created the adapter in the ratio needed for my error based on comparison readings with a GPS receiver. I’m super pleased. Good service and price. USA made Eaton parts.

https://speedometercablesusa.com/

[WorkinLonghorn]

Thanks DeadheadNM, I didn't know they still made these adapters. My old +10% original needs replacement.

[truckster]

This is what I'm looking for on as much stuff as I can find:

[SeventyOne]

Super helpful piece of the puzzle. thanks!

[wesmigletz]

Thanks for sharing. I have a few vehicles I would like to correct.

[DeadheadNM]

Originally Posted by truckster

This is what I'm looking for on as much stuff as I can find:

Absolutely!!

I needed about a 13% bump in speed at the speedo. This box has a ratio of about 1.13. Cost $108 with shipping and I got a pen from the business too. Most of you probably are aware but these K5s had a ratio adapter as standard from GM. I had a stock 1.3 or so adapter lying around and that turned out to be exactly what I needed for my k20.

[SeventyOne]

Originally Posted by DeadheadNM

Absolutely!!

I needed about a 13% bump in speed at the speedo. This box has a ratio of about 1.13. Cost $108 with shipping and I got a pen from the business too. Most of you probably are aware but these K5s had a ratio adapter as standard from GM. I had a stock 1.3 or so adapter lying around and that turned out to be exactly what I needed for my k20.

On that note do you know if all the stock K5 adapters were the same ratio?

[DeadheadNM]

Originally Posted by SeventyOne

On that note do you know if all the stock K5 adapters were the same ratio?

These were not. I believe the gear ratio in the rear axle was a big discriminator but I'll need to research that in the parts manual. I suspect there are two ratios one near 0.97 the other near 1.3 but these are a guess. I'll try to firm up the knowledge and repost. There are gear drives in the transmission that come into play as well and I suspect it's because of these that there are only two ratio adapters.

EDIT: this chart from the parts manual lists two adapters for a 3.73 rear end and several for 3.07. The chart is for four speed manual transmission equipped trucks. There are other charts for auto and three speed manual.

[SeventyOne]

Originally Posted by DeadheadNM

These were not. I believe the gear ratio in the rear axle was a big discriminator but I'll need to research that in the parts manual. I suspect there are two ratios one near 0.97 the other near 1.3 but these are a guess. I'll try to firm up the knowledge and repost. There are gear drives in the transmission that come into play as well and I suspect it's because of these that there are only two ratio adapters.

EDIT: this chart from the parts manual lists two adapters for a 3.73 rear end and several for 3.07. The chart is for four speed manual transmission equipped trucks. There are other charts for auto and three speed manual.

Interesting. I just emailed speedocableUSA to get a correct adaptor as I think I'm running about 22% too fast. But I have 4.56 gears and 37'" tires so

[jjzepplin]

Man I got like six of those things. I guess I best get them out and take a look see.

[SeventyOne]

Originally Posted by jjzepplin

Man I got like six of those things. I guess I best get them out and take a look see.

Ha yeah i got one. It's just wrong

[DeadheadNM]

Let us know what shakes out.

[Ol Blue K20]

This is great to hear. I'm needing one too.

[SeventyOne]

Snapped a pics of my existing adapter:

"666F 01"
"1.2020"

Funny thing is my speedo seems to be a be about 20% too fast

[JMac11]

Need to check mine, it's 10mph slow

[MJN]

Great to see USA made. I had the same issue with my '74 K/20 and had a gearbox made up by the guys at Transmission Center, www.transmissioncenter.net If I recall mine was about 17% off when I took my speed measurements compared to GPS

[2006gmcdmax]

Thanks for sharing the information! This was much needed!

[jjzepplin]

Originally Posted by SeventyOne

Snapped a pics of my existing adapter:

"666F 01"
"1.2020"

Funny thing is my speedo seems to be a be about 20% too fast

Attachment 2026783

If you take the adapter out of the equation, what happens?

[SeventyOne]

Originally Posted by jjzepplin

If you take the adapter out of the equation, what happens?

I haven't gotten that far
Not sure if the speedo cable will thread onto the Tcase, or If I need a 0% adapter.

[jjzepplin]

It should fit right in if it is original. The adapter just fits in between the cable and the t case to change the ratio.

[SeventyOne]

Originally Posted by jjzepplin

It should fit right in if it is original. The adapter just fits in between the cable and the t case to change the ratio.

I'll give it a shot!

[69Tom]

Good info. Thanks!

[K5Fan]

I have the exact adapter that SeventyOne has, and my speedo is 10 miles an hour low, but gets worse the faster I go! I also have the 4 speed with the granny gear.

DeadHead, when you got your adapter did you have to tell them what size tires you were running? I looked at the website, and the picture of the chart you posted, but I couldn't see where it related to the tire size. Maybe I'll take a fresh look at it tomorrow. Thanks Pete!

Chris.

[K5Fan]

Duh, I see the tire size now. Thanks again.

[Ironhorse]

Several years ago I had the same issue with my speedometer being off. 71 4x4 after a swap from manual to auto transmission. I run 31x10.50x15s on 8'' rally wheels. I noted my speedometer reading against a GPS phone app and figured the difference. Contacted www.transmissioncenter.net and gave them the information. They built me an adapter that after an easy 15 minute installation was dead on with the speedometer and the phone app GPS speed. Just attached to the side of the transmission where the speedometer cable attaches.
Think my cost then was around $65 plus shipping but I'm sure costs are higher now. Very easy to do without going in to the transmission and changing the plastic gears.