Speedometer correction help

[93M6Formula]

So having an issue on my 82 GMC K1500, Somehow my speedometer was pretty accurate (GPS Verified) with stock 3.08 gears and the 32s I put on. I now just did a 3.73 gear swap and naturally the speedometer was 10 MPH fast. So I did the math and ordered a 42 tooth green gear and pulled out my 40 tooth black gear and the speedometer didn't change... still read 10 MPH fast exactly.

What can possibly cause that with a mechanical speedometer? Note also, this has cruise control so the cables come from there first before the gauges.

[Palf70Step]

With my last truck when I went from 3.73 to 3.08s, I never could get it accurate with just changing the speedometer gear. The more research I did, the more I was going to need to change the gear on the output shaft also to get it right. I was too lazy to doi that, so I just drove with the tach and had a calculated cheat sheet on the RPM to speed I needed. Sometimes I even used the phone GPS.

Saw another post talking about this.

https://67-72chevytrucks.com/vboard/...d.php?t=656949

I was thinking out loud about yours is a K and bet transfer case has some play in that.

[MJN]

You need a ratio adapter gear box to correct the speedometer. The guys at https://transmissioncenter.net/ made one for me and its dead on.

[kwmech]

3.08 to 3.73 is a 17.5% difference. 40 to 42 teeth is a 4.8% difference. Doing a speedo is percentages. You can be off 10mph but it will be at a specific speed it will not be constant throughout the range

[MJN]

I found notes from when I had the ratio adapter made for my truck- this was 12 years ago so just going off my notes here:
Drive 10 miles according to your GPS
Divide odometer reading by 10 = % the odometer is off
Count the # of teeth on gear in the transfer case
Multiply the # of teeth by the % the odometer is off = # of teeth needed

When I called the transmissioncenter.net they explained this over the phone on how to do this. I ended up with a 16% increase ratio adapter and it matches the GPS.

[93M6Formula]

Thanks for the tips guys, apparently the site I used to do the math wasn't right I guess.

[S10Fan]

Originally Posted by MJN

You need a ratio adapter gear box to correct the speedometer. The guys at https://transmissioncenter.net/ made one for me and its dead on.

I've used them for all my speedometer calculation issues, they are top notch folks to deal with!

[Hart_Rod]

I used this place: https://speedometercablesusa.com/gear_box_adapters.html

[hatzie]

You don't necessarily need a ratio box.
It depends on the transfer case. The NP205 and NP203 need ratio boxes.
The NP208 uses the same drive and driven gears as the TH700R4.

The parts books have tables for selecting speedometer drive and driven gears based on axle ratio and tire sizes. You can download the parts manuals from the manuals thread.

[93M6Formula]

Originally Posted by hatzie

You don't necessarily need a ratio box.
It depends on the transfer case. The NP205 and NP203 need ratio boxes.
The NP208 uses the same drive and driven gears as the TH700R4.

The parts books have tables for selecting speedometer drive and driven gears based on axle ratio and tire sizes. You can download the parts manuals from the manuals thread.

See this is why i'm confused, or maybe I don't understand how these speedometers work... My 208 case takes the same gear as a 700R4 so that's what I bought, it works fine but reads exactly the same as my 40 tooth original gear. I used the calculator on this website and it came up with a 42 tooth gear for my setup. https://transmissioncenter.net/shop/700r4-driven-gear/

[hatzie]

There's a drive and a driven gear.
The drive gear is on the end of the transmission/transfer-case mainshaft.
The driven gear is on the end of the speedometer cable.

The 3.08 axle ratio requires an 18 tooth drive gear. The 3.73 needs a 15 tooth drive gear so you're going to have to change it. You'll have to pull the output shaft housing off the transfer case to access the driven gear.

The 42 tooth driven gear paired with the 15 tooth drive gear is for 28" diameter 215/75R15 tires.
The 40 tooth gear paired with the 15 tooth gear is for the L78-15 40" diameter tires.

Pay attention to the markings on the aluminum driven gear carrier. It needs to be clocked to match the driven gear size used.

Download the 1979-1984 parts and illustration books from the manuals thread linked in my signature.
The speedometer gear tables are in the middle of the parts book in section 4S.
The 1982 speedometer gear tables start on PDF page 1023.
You'll need your tire size and axle ratio. The TH700 is the same speedometer gear setup as the NP208.

There are 50-80 pages of tables in the parts book showing what combination of drive/driven gears you need for a given axle ratio, tire size, and transmission along with the GM part numbers and the color plastic GM originally molded them out of.

If you refuse to download your own copies of the parts books this link is to Ian Hardings' GM Parts Wiki in roughly the right place to start looking. Ian has copies of my scans as well as other parts manuals other folks have scanned and Ian has scanned. The ones you're downloading from the manuals thread are the exact same thing as GM Parts Wiki.
http://www.gmpartswiki.com/getpage?pageid=59796