How to calibrate speedo?

[Lgarretto]

Hey guys. Probably a bit of a silly question. But I can't find good info for it.
So I have looked around and found some stuff on calibrating a speedometer for a truck with an auto. But my 89 has a stick. The hm290 I believe. The truck had 3.08 gears but I changed them to 3.73 gears because all of my bearings were shot anyway. I also added a posi unit but that is beside the point.
I'd like to get my speedo a little closer, but if it's gunna be a pain or too costly I'll just stick an aftermarket gauge in there. I read something about rewiring some contact points in a box behind the glove box, but I didn't get very specific info on that. Is there a speedo gear on the trans I can change? How do I do it?
I appreciate any info you guys can give me on the subject. Thanks

 

[studigggs]

Looks like a pain to me...http://67-72chevytrucks.com/vboard/showthread.php?t=313815

 

[Steven Petersen]

Hey guys. Probably a bit of a silly question. But I can't find good info for it.
So I have looked around and found some stuff on calibrating a speedometer for a truck with an auto. But my 89 has a stick. The hm290 I believe. The truck had 3.08 gears but I changed them to 3.73 gears because all of my bearings were shot anyway. I also added a posi unit but that is beside the point.
I'd like to get my speedo a little closer, but if it's gunna be a pain or too costly I'll just stick an aftermarket gauge in there. I read something about rewiring some contact points in a box behind the glove box, but I didn't get very specific info on that. Is there a speedo gear on the trans I can change? How do I do it?
I appreciate any info you guys can give me on the subject. Thanks

I’m pretty sure 89 is the same as 88. You have to pull the cluster. On the backside there’s a place for a clip that has 8 pins. You break off pins according to your tire size and gear ratio. They came from the factory without the clip. There are resistors that they popped according to tire and gear ratio it came from the factory with. GM had a harness you could buy to pop all the resistors and then install the clip. I couldn’t find the harness, so I popped the resistors by giving them 12 volts. I found a clip on EBay and everything works great. My speedometer is dead on.

 

[Steven Petersen]

I’m pretty sure 89 is the same as 88. You have to pull the cluster. On the backside there’s a place for a clip that has 8 pins. You break off pins according to your tire size and gear ratio. They came from the factory without the clip. There are resistors that they popped according to tire and gear ratio it came from the factory with. GM had a harness you could buy to pop all the resistors and then install the clip. I couldn’t find the harness, so I popped the resistors by giving them 12 volts. I found a clip on EBay and everything works great. My speedometer is dead on.

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

 

[Lgarretto]

Oh OK. I guess I overlooked that. Cool. I'll look into doing that. I've had the cluster out before. But only unplugged from the little plug in back.
Am I right in understanding it as actually physically breaking off the pins that need removed? Like clip them off with some snips maybe?

 

[Steven Petersen]

Oh OK. I guess I overlooked that. Cool. I'll look into doing that. I've had the cluster out before. But only unplugged from the little plug in back.
Am I right in understanding it as actually physically breaking off the pins that need removed? Like clip them off with some snips maybe?

Exactly. But if yours has never been recalibrated it won’t have the clip.

 

[Lgarretto]

OK.
Thanks guys. I will read through and make a decision. A speedo that goes higher than 80 may be nice... But thanks. That aughta do it

 

[Steven Petersen]

OK.
Thanks guys. I will read through and make a decision. A speedo that goes higher than 80 may be nice... But thanks. That aughta do it

The circle gauges will actually go all the way around and start counting again. My friends and I use to have the conversion figured out. Like 25=105.

 

[Lgarretto]

Mine is a ribbon gauge. I think that's what it's called. Just a white bar that goes from one side to the other. But it isn't like any of my other gauges work anyway so I am reluctant to put too much time and money into the speedo.

 

[Steven Petersen]

Mine is a ribbon gauge. I think that's what it's called. Just a white bar that goes from one side to the other. But it isn't like any of my other gauges work anyway so I am reluctant to put too much time and money into the speedo.

You must be registered for see images attach

88 gauges. 89-91 are the same minus the hash marks. Any they have an orange line on the end of the indicator. These gauges will go all the way around.