Speedometer gear help/advice?

[Prospect62]

Guys I have an 84 M1009 CUCV with 35" tires and 3.08's (NP208 t-case). In full disclosure I have already asked this question over at Steel Soldiers but it was recommended I come here as there aren't many guys running 35's over there. I'd also ask this in the 73-91 K5 sub-forum but apparently you have to pay to do that now.

Can anyone tell me what speedometer gear I need to correct the speedometer? Right now I think it reads too fast, meaning it says I'm doing 50 when I'm really doing like 30 but I haven't taken exact speed measurements. Some say it "should" be reading too slow, not too fast. TBH I can't recall as I've only driven the truck about 1/2 mile since I've owned it but the speedo is definitely off. The truck is up on jackstands now anyway getting the brakes and such rebuilt.

Basically I'm wondering if somebody who did 35's with 3.08's could tell me which gear they used and how they went about it. I did some research and got so confused between drive gears, driven gears, etc etc etc my head hurts.

Also apparently there's some sort of gear reduction box on the Blazer CUCV's that I might be able to replace instead of doing the speedo gears.

Just lookin for some direct advice from someone who has been down this road. Thanks.

PS - Yes I've been a lurker for 14 years. This M1009 is my third K5, and this is the first time I've needed to ask a question.

 

[6872xtc]

I haven't ever use 35s with those gears, but I do know that you will need to know what how far off it is at a given speed. It is probably off a percentage, but I usually find out what it reads at 50 mph actual speed. It pretty easy nowadays with GPS or an app on a phone.
Once you have that percentage number, you can check into a ratio adapter box to install onto the transfer case speedometer output, or you can pull the speedometer gear out ot the transfer case to see what color, or tooth count you have. Then you will need to dive into the research to find out what gears you need. There is a range of gears that work together, so you will need to know what one is on the output shaft in the transfer case. Otherwise you would have to find a matched pair. Then you will need to pull the tail housing to install them.
Finding custom ratio boxes used to be simple, but may be a challenge now.

I would also watch to see if the speedometer works smoothly when you get it driving, as the speedometer head itself can be off with these old trucks. Often it will be inconsistent if it has internal problems. As in, it will change speed while you are going a consistent speed.

I am probably not telling you anything that you don't already know, but I personally wouldn't get too concerned about fixing it until you know more about what it is actually doing. Drive it some more..

 

[Prospect62]

I haven't ever use 35s with those gears, but I do know that you will need to know what how far off it is at a given speed. It is probably off a percentage, but I usually find out what it reads at 50 mph actual speed. It pretty easy nowadays with GPS or an app on a phone.
Once you have that percentage number, you can check into a ratio adapter box to install onto the transfer case speedometer output, or you can pull the speedometer gear out ot the transfer case to see what color, or tooth count you have. Then you will need to dive into the research to find out what gears you need. There is a range of gears that work together, so you will need to know what one is on the output shaft in the transfer case. Otherwise you would have to find a matched pair. Then you will need to pull the tail housing to install them.
Finding custom ratio boxes used to be simple, but may be a challenge now.

I would also watch to see if the speedometer works smoothly when you get it driving, as the speedometer head itself can be off with these old trucks. Often it will be inconsistent if it has internal problems. As in, it will change speed while you are going a consistent speed.

I am probably not telling you anything that you don't already know, but I personally wouldn't get too concerned about fixing it until you know more about what it is actually doing. Drive it some more..

I agree with ya. It's not a huge concern, just something that irked me when I did take it for a shakedown run. I assumed that there would be a ton of guys who have run 35's with 3.08's who could just tell me "here you need to do this" or at least get me close. Seems like a pretty popular tire and axle combo, as crappy a combo as it is.

I can do my own investigating, I was just trying to save myself the leg work assuming it's been dealt with a million times by now.

 

[6872xtc]

I agree with ya. It's not a huge concern, just something that irked me when I did take it for a shakedown run. I assumed that there would be a ton of guys who have run 35's with 3.08's who could just tell me "here you need to do this" or at least get me close. Seems like a pretty popular tire and axle combo, as crappy a combo as it is.

I can do my own investigating, I was just trying to save myself the leg work assuming it's been dealt with a million times by now.

I understand, and I get your point of view. But I don't know very many who have run that combination. I have 4:56 with my 35s. My son installed 4:10 to go with 33s.
Hopefully someone will be able to help you.

 

[ZooMad75]

So there will need to be some research and math involved to dial it in.

Gm offered two different ratio adapter boxes, one was 0.8319 or 1.4571.

There were two speedo gears used within the t-case. 15 tooth Grey and 18 tooth blue.

There were 13 different speedo driven gears used.

44t dk gray
42t green
40t black
39t brown
38t blue
37t red
36t white
35t orange
34t Lt green

21,20,19,18 did not show any color description.

So since gm didn't ever offer 35" tires with 3.08 axles you are not going to get a dead nuts combo of drive and driven gears to make the speedo accurate. You can get close and that's where the ratio adapter box on the cable would provide the fine tuning.

Starting point is as others have pointed out. Drive it with a gps/app on phone and drive at a certain speed. I like 60 mph since it's a good backup to measure with a stopwatch (1 mile at 60mph will take 1 minute)

Driving at 60 indicated on the gps see what the speedo reads. Use the the numbers to figure out the percentage of error. Divide actual speed (gps) by the speedo reading. Use that number to figure out what direction to move to change the value.

You'll probably need to at least pull the driven speedo gear out from the t-case to see what you have. Look down the hole in the housing to see what drive gear is inside.

The ratio adapter boxes are available in the aftermarket still and will have a wider option of correction factors.

Looking at it another way, comparing what a factory setup cruise rpm would be right at 2,004 rpm at 60 mph in direct drive 1:1 gear with 31" tires. Changing only the tires to 35" tall would slow the rpm down to 1,775 at the same speed. Or said another way would be that the speedo is 88% slower going the same actual ground speed (measured with gps or watch on a measured mile).

Obviously driving a 6.2 truck with 35" tires and crappy 3.08 gearing would be a dog. Option B is to gear the axles correctly for the tires you are running. 3.42 would get the speedo within a couple percent or less as the cruise rpm at 60mph would be 1,941. 3.73 would give you back more torque but push the cruise rpm up to 2,150 at 60 so the speedo would be fast and still require some correction. It's the most expensive option but would make it more of a pleasure to drive not being so weak down low.

Option C: get a gps app for your phone and a mount to hold it. Turn it on when you drive and don't worry about it. Cheap, quick and accurate.

 

[Prospect62]

Thank you, ZooMad75. That was quite informative. While I have the truck up in the air for the next few weeks I'll pull the speedo gear and see what I have currently. Then I'll come back and see what will get me close.

Any idea on a source for those conversion boxes?

 

[ZooMad75]

Google my friend. Search speedometer ratio adapters or correction box.