The Stovebolt Site Forums The Shop Area General Truck Talk Speedometer isn't right, after all

After I replaced the temp gauge in my '51 3100, I was pretty happy that the speedo was "working". By working, I mean not jumping all over the map. I followed another members advice and sprayed WD40 down the speedo cable tube when I had it off for the install, and it started working smoothly. Well, yesterday, I took my Garmin Forerunner with me in the truck, and it seems I'm driving a lot slower than I thought since the 'fix'. At 55 on the gauge, I'm doing 48mph; at 50, it's about 44; and at 60 it's running about 53-54. Anybody have any suggestions on exactly what can be done about this? Like a new speedo gear, or has anybody had any luck with a new cable? Anything? Thanks for any thoughts on this problem. (Thought I had it licked.)

could be a change in drive gear size is called for, especially if you've made a rear-gear change, but you could also just use the Garmin to "calibrate" it and live with it ..... might have to do with your wheel and tire size compared to what was original equipment ..... I'd also recommend you pull the speedo cable and clean the inner and outer well with a none-oily solvent like BrakeKleen, then use a dry graphite on it, the WD40 [which didn't clean the housing out] will gum it up over time mixed with dust, as will any oil

Bill

Thanks, Bill, will do. Didn't even think about all the crud that would just end up down at the bottom of the casing. I just bought this truck a few months ago, and the back of the speedo housing is stamped 2009, so I think it may be new (or rebuilt). But I also just read another post saying that someone changed out to a new cable and it worked. Thanks for the reply. Lee

showme, after cleaning the cable, I've always used liquid graphite, with no problems.
http://www.oreillyauto.com/site/c/d...1_1155&keyword=speedometer+lubricant

Brian

On the next test drive, watch the odometer, it is gear driven right off the cable. The speedometer is magnetic driven and it's accuracy is determined by the magnets strength. If the odometer is correct, then the speedometer head is dirty or weak and its something you will have to clean or live with. Now if the odometer is off, then you can adjust it with gears in the transmission.
Drive at least ten miles to get a good measurement and compare it to your electronic devise or mile markers on the road. Figure the percentage its off and change the gear ratio the same amount. Slow the driven gear down to reduce the milage, or speed it up to increase the miles. The speedometer will speed up or slow down the same percentage.

Joe

Thanks for the replies! So now I have to ask, how do I adjust the gear ratio? Do you mean change the gear out in the tranny, or is it possible to adjust it without changing that gear? This is a new one for me. Up to this date on any of my other vehicles, it was either a new cable, or the tranny's speedo gear at the input was bad. Thanks for the help, everybody. Lee

What gears are you running in the back? What size tires are in the back? If any of these are different than factory stock, you will need to get a speedo ratio adapter. In your case it looks like the speedo is reading about 13% faster than what the truck is traveling at, assuming your garmin is correct. If so, you need a 13% speed up adapter. Call Jim at PATC Transmissions 888-877-1008.

I was running about 21% off. They sold me a 20% speed up adapter for $70. Now it reads close enough to dead on.

BTW-If you have a smart phone, there are a couple real nice speedometer app's. Much easier to use with bigger readouts, etc. I use Speed Box.

cmayna,
Thanks for the info. With the original 4-speed transmission, my '59 Apache was geared too low even for around town. I had 3:38 gears installed in the rear end rear end, knowing the speedo was going to be off. After reading your post this am, I got the Speed Box app or my iPhone. Called Jim at PATC Transmissions...he wanted speedo readings at true speeds of 40, 50 and 60. Didn't really enjoy going 73 mph on I-680, but got numbers and called them back in to Jim. I have have an adapter enroute.
Again, thanks or the post.
Bob

Per the question of tire sizes, the wheels and tires can't be original. The PO had put 15" wheels with 15-70-15 tires on them. Normally, I'd go looking for the right wheels and tires just to see if that would help, but the tires had only 1k on them when I bought them, so I'm keeping them on at least until I need new tires, anyway. (Now I hate to check under the after market hub caps! Although he painted the rims to match the pinstripe, I'm guessing the clips aren't there for the original hub caps that he gave me with the truck). Oh, well. Anyway, that's probably a big part of it. Would smaller rims/tires put the speedo faster, or slower? (Novice alert!) I will still remove and clean the speedo cable, but think i'll buy a new one anyway.

Joe H - What does making the speedo magnets right involve? Is that for a shop only or can anybody with tools and patience do it?

cmayna- thanks for that info! When I'm done with the cable clean up and/or new cable purchase and speed/odo check up, that will be my next step

Bob, I assume you are kidding about going 73 mph. That was the speed your speedometer was reading for Jim asked you for the reading at a true 60...yes?

No, I was serious. Jim wanted true speeds when the speedometer read 40, 50 and 60. Initially, I didn't get the 60 because it was going to make me go too fast for comfort. But Jim said he needed the reading to accurately calculate the adapter gearing. At 60 mph on the speedo, my true speed was 73.

Bob

Bob,
That is not correct. Jim wants speedometer readings when you are traveling at a true 40, 50 & 60 via a GPS or similar device. He just confirmed this with me moments ago.

Craig

The magnets are not something you can repair unless you know and understand them. I would assume one would have to re-magnitize them to strengthen them back to where they started, and to do that you would need to know the starting point. A good repair shop could fix it pretty easy if you have the funds for it.

joe

Quick question how much of the liquid graphite should I apply into the new speedo cable, and do you just put it in at the top and it will work itself to the bottom or do you have to put it in at both ends?

just a light spray will work. I loosened the cable at both sides and sprayed at the "top", behind the gauge, and turned the inner cable for a minute.

Thanks for the quick response, Hugh

I pull the cable out from the speedometer end of the cable housing, and clean it. When I reinstall the cable into the housing I lube the cable with the liquid graphite.

Brian

What "liquid" is liquid graphite made with? Is it oil based or something that dissolves once it has penetrated, such as brake or electrical cleaner, or what? I've always used dry graphite. Seems like if it was oil based it would coagulate with the graphite being a solid.