The Funny Gauge Cluster Problem

[TeKdo]

Just did a gauge cluster swap on my '86 Comanche to an '87 - '89 Cherokee gauge cluster (long story) and I've been having a little bit of speedometer trouble. Got a new cable (ATP Y914) and a new drive gear (37 tooth for 4.10 and 29" tires according to the Novak Chart), but the speedometer isn't accurate at highway speeds.

At speeds below it's dead accurate...or at least withing 1 -2 miles of GPS indicated depending on how fast the GPS refreshes, but after about 45, it reads 5mph slow. Weird part is that the trip meter is accurate within 0.5 miles and odometer is spot on.

There is a little bit of needle bounce at lower speeds which I'd expect at an old cable driven system and it's really smooth at about 60mph indicated (65mph GPS).

Cable is fully seated onto the cluster (snapped on and can't be pulled off without moving the locking collar), speedometer drive housing is clocked properly, cable is fully screwed into the drive housing, and the cable is secured well to the chassis and doesn't have any sharp bends or kinks.

Is the speedo something I'm just gonna have to live with or is my cheap cable what's causing it to read slow?

 

I've heard of people slowly grinding down the speedo cable at the drive gear end to help reduce the needle wiggle, but I dunno if that would help with the mystery 5mph drift at highway speeds. I do know, or at least kind of remember, that the speedo cable at the drive gear housing doesn't actually have the plastic sheathing touching the actual housing when I push the cable all the way in...maybe this could be a bind?

 

I've also heard of people pulling the speedometer cable about a 1/2" out of the sleeve at the gauge side for more engagement, but I'm hesitant to do that because the engagement felt good and the odometer is pretty damn accurate.

 

Starting to think that this might just be a curse of Wheezey because the old gauge cluster did a similar thing but I know that the cable and drive gear on that old setup was hashed...but this stuff is all new.

[pizzaman09]

Are sure the difference is a step function and not a percentage difference from zero to highway speed?  The fast you go the larger the error will be if the speedo gear is wrong for the diff ratio and tire size.  Are your tires actually 29inches or are they bigger than claimed?

[Eagle_SX4]

You are probably off 1 tooth on your speedometer gear. I bought what the chart said was correct but it was not right for my gear/tire combo.

 

If the speed is reading slow try going down 1 tooth from what you have. Should spin the speedometer just a little faster and correct the issue.

[TeKdo]

2 hours ago, pizzaman09 said:

Are sure the difference is a step function and not a percentage difference from zero to highway speed?  The fast you go the larger the error will be if the speedo gear is wrong for the diff ratio and tire size.  Are your tires actually 29inches or are they bigger than claimed?

They're actually about 28.88 inches tall according to the Spicer calculator. I'm running a 235/75/15 right now.

[TeKdo]

54 minutes ago, Eagle_SX4 said:

You are probably off 1 tooth on your speedometer gear. I bought what the chart said was correct but it was not right for my gear/tire combo.

 

If the speed is reading slow try going down 1 tooth from what you have. Should spin the speedometer just a little faster and correct the issue.

That's what I'm leaning towards right now, but it is dead accurate at anything below 45. I can sit at 35mph indicated on the cluster and it's 35 - 36 mph GPS speed and the odometer matches up with how far I've driven in GPS tracked distance (I did 80 miles yesterday and it matches to my 80 miles logged on gps). Would an incorrect speedo gear read low only at highway speeds or everywhere?

[Eagle_SX4]

The wrong speedo gear will be off an exponential amount. Meaning the faster you go the farther off it is. Slower speeds will be off by much less than faster speeds.

[89 MJ]

It seems weird to me that the speedo is off, but the odo is close to correct. 
 

Like Eagle_SX4 said, the mile per hour shown will be off by a percentage. (Speed shown) = (actual speed) * (percentage inaccurate), if your speedo read 110% of your actual speed, and you were actually doing 50, your speed shown would be 55, 60 would show 66, etc. 

[TeKdo]

15 minutes ago, Eagle_SX4 said:

The wrong speedo gear will be off an exponential amount. Meaning the faster you go the farther off it is. Slower speeds will be off by much less than faster speeds.

 

5 minutes ago, 89 MJ said:

It seems weird to me that the speedo is off, but the odo is close to correct. 
 

Like Eagle_SX4 said, the mile per hour shown will be off by a percentage. (Speed shown) = (actual speed) * (percentage inaccurate), if your speedo read 110% of your actual speed, and you were actually doing 50, your speed shown would be 55, 60 would show 66, etc. 

Gotcha, thanks for explaining it to me because it was driving me crazy. Eagle_SX4's explanation makes sense too since I was driving around town all night last night (my Comanche is currently my winter delivery vehicle). Gonna try a 36 tooth gear to see if that fixes it and I might look into trying to trim the cable to see if I can clean up that little needle bounce.