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Speedometer Reads Very Slow


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I took my truck for a short drive yesterday with a chase car to tell me my actual speed. My speedometer was reading 20 mph while I was doing 40. It was reading very slow with the other rear end ratio in my truck too. The needle moves very smooth, with no bouncing or getting caught anywhere. It also appears the odometer is reading ok, but I have not verified that. Anyone have a guess as to why? I wouldn't think it is the cable with how smooth the needle moves. 

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Joe, can we collaborate on this? Mine is similar. It holds steady but seems to have three locatons that it likes: "0"; 20; 45. Using a traffic mph sign at 20 mph my true speed is 30 mph. I do not know what 40+ means.

I agree, that if the problem was the cable, then things should be irratic or not work at all. So I'm thinking the speedometer is dry, needs to be lubed. So that is next. Not sure how, but there is a small adj screw on the shaft housing, so will start there. I see no need to fix the ratio until the speedo works steady.

Keven suggested I loosen the cable. I did, maybe not enough.

Edited by pflaming
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Yes there is a wick for oiling. Oiling may help some. There is also a spring that gets weak with age. You may find its just out of adjustment. Things like tire size and rear end ratio changes also affect it. If you have a given error like it always reads 20 low I'd think thats more a ratio tire thing and if its off more of a % I'd say speedometer out of adjustment.

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Yeah there is a wick on the back of the speedometer. I thought I did put a couple drops of 3 in 1 oil in it but now I'm not sure. Paul, that adjustment  screw is just an oil plug for the wick. Just remove it and add a few drops of oil and put the screw back in. If your cable is too tight it can bind, I think the general rule is just finger tight on those. They don't need to be tight but rather held in enough so it doesn't back its way out. When I get home I will pull the cable out and inspect it to double check it, then maybe add a few drops of oil to the wick. I'll report back...I did download a speedometer app on my phone so I won't need a chase car. I tried it on my way to work and it is accurate. 

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Yes there is a wick for oiling. Oiling may help some. There is also a spring that gets weak with age. You may find its just out of adjustment. Things like tire size and rear end ratio changes also affect it. If you have a given error like it always reads 20 low I'd think thats more a ratio tire thing and if its off more of a % I'd say speedometer out of adjustment.

 

I just used a MPH calculator, and with the 4.11 at 1600 it is 35 mph...with the 3.54 at 1600 it is 40 mph. It should be 5 mph off at the speed I was traveling. Tire size are stock 6.50-16s. It seems to be a percent thing as far as I can tell, so that leads to the speedometer. Hopefully it is just the cable! 

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. . . There is also a spring that gets weak with age. You may find its just out of adjustment. . .

 

If the spring gets weak then, all other things being equal, the speedometer will read high. The spring works to return the needle to zero, so if it is weak it won't be pushing the needle to zero as hard.

 

If the needle is sticking at some specific points, I would guess as others have, that there is some dirt or binding in the mechanism. A careful cleaning could help that.

 

If the needle is bouncing around, it is most likely the cable binding up.

 

If the odometer is correct and the speedometer is consistently fast or slow, then the relationship in forces between the magnet inside the aluminum cup and the spring are off. On the speedometers I've looked at you can adjust the spring tension to adjust the speedometer. If the magnet has gotten too weak with age either a professional repair or replacement is probably in order.

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My speedometer is pretty accurate. It shows about 45 all the time. So I use a GPS on all my trips.

Thats no fun. You can't explain to the speed enforcement officer that you just assumed your speedometer was correct and get off with an equipment fix-it ticket instead of a speeding ticket. :)

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Thats no fun. You can't explain to the speed enforcement officer that you just assumed your speedometer was correct and get off with an equipment fix-it ticket instead of a speeding ticket. :)

Good point. And the tattle tale MAX SPEED recorder on the GPS always tells the truth. Good thing is I can reset it to zero with the push of a button.

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I got home (after an interesting day at work) and I oiled the wick in the speedometer, looks like I never did oil it in the first place. So I oiled it and I took off the speedometer cable. The cable feels fine and I oiled it a little to be safe. I put it back together and it is still reading slow. Exactly half the speed I am traveling. The odometer is reading correctly. Looks like a magnet/adjustment problem...darn. Thanks for info. 

Edited by 52b3b Joe
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It's a funny argument where one says that their speedo was showing less than they were travelling at when one is pulled over by 'the law'. Here in Oz a vehicle can be 'defective' if its "speedo cannot be reasonably relied upon to be producing accurate results". I guess then it all boils down to which of the defective vehicle ticket or the speeding ticket is the lesser amount - and you just hope that you don't get issued with the both tickets!

 

Desotodav

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Gotta use GPS for Bonneville. When you're speeding down the salt, your tires break free at different speeds. Your tach might be showing 5000 rpms, but the previously calculated speed to that RPM is not reliable. So with GPS, easing off the throttle, its possible to see your speed climb as the tires get traction back.   

 

 

48D

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Gotta use GPS for Bonneville. When you're speeding down the salt, your tires break free at different speeds. Your tach might be showing 5000 rpms, but the previously calculated speed to that RPM is not reliable. So with GPS, easing off the throttle, its possible to see your speed climb as the tires get traction back.   

 

 

48D

I did not give that a thought. Great idea.

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I just went through something similar.  I pulled both ends of the cable and pulled the cable out of the shroud-it was covered in gritty grease and the cable had frayed strands.  Luckily I had a spare inner.  I shot some brake cleaner through the cable shroud to clean out anything that was lurking in there, blew air through it and then pumped some light oil down through it.  I slowly fed the inner cable through it putting a light coat of vaseline on it as I went.  After I got it back together I realized that the speedo was still reading slightly slow (due to the two speed adapter being removed), but my odometer was clocking miles like crazy.  Took an old one I had completely apart and saw how it worked.  On mine, the mile and 1/10th drums were stuck together.   Took them off the shaft and carefully cleaned the shaft and put it back together.  Did a test run with a drill and saw that the odometer was working properly.   Never been inside a speedo before, but there isn't much to them.  Now I'm gonna dig into the adapter and see wuzzup wid dat.  Mike

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