NiftyFifty Posted August 3, 2010 Report Share Posted August 3, 2010 So I finally have my truck to the point where it's driveable and tonight I took it for it's true maiden spin around town...BUT...my speedo seems to be way slower then what I think I'm doing....and I tried my other speedo from my parts truck and seems to be doing the same...maxed out motor screaming is like 30 mph...now I have a 50 1 ton with the 3 speed and a jeep wrangler rear diff with what I think are 3.54 or in that neighbourhood... just so you know the situation I also have a brand new cable so that shouldn't be an issue and the trans gear seemed good when I took it out... Now the only thing left is...what is that shaft/screw on the back of the speedo on the output for where the cable screw on?? does that do anything? Anyone have any clues??? Supposed to be goinf in for a safety inspection in the morning...and now that's a no go unless I get this figured out. Thanks Quote Link to comment Share on other sites More sharing options...
Merle Coggins Posted August 3, 2010 Report Share Posted August 3, 2010 The "shaft/screw" on the back of the speedo is a lube wick. If you remove it you'll see a cotton wick inside. Drip some light oil into the hole and into the wick. It will help keep the workings lubed up. If your truck originally had a 4.3, or even worse a 4.89, diff and now you have a 3.54, your speedo will be off by a bunch. With the lower ratio diff your driveshaft speed is much lower at a given speed, which means the speedo cable is spinning slower and the needle will register a slower speed. To correct that you need to switch out the speedo gear in your trans with one that matches your gear ratio. Some of the other guys may have the gear number you need and where to find one. I can't help you with that. Merle Quote Link to comment Share on other sites More sharing options...
NiftyFifty Posted August 3, 2010 Author Report Share Posted August 3, 2010 So can anyone tell me what gear to use? Oh...and I messed up...I have the 4 speed in my truck...and 3.54 gearing I also heard from a buddy about some sort of external reducer or adjuster...anything exsist? Quote Link to comment Share on other sites More sharing options...
NiftyFifty Posted August 4, 2010 Author Report Share Posted August 4, 2010 Ok...so I see these Stewart Warned in line reducers...but can anyone tell me what gearing to ask for....has to be someone out there who updated their rear diff that has some answers.... Quote Link to comment Share on other sites More sharing options...
Merle Coggins Posted August 4, 2010 Report Share Posted August 4, 2010 (edited) Well, I upgraded my diff but I don't have the answer you need. I switched from a 4.10 to a 3.73 and did nothing to my speedo gear. My speedo currently reads about 3 MPH low at 65. (62 indicated = 65 on my GPS) I can live with that. I wonder how far off it was before? How are the adapters rated? If you can deternine the percentage that your speedo is off you can determine how much the adapter needs to change. If you have a road nearby with mile markers you can figure it out. At a mile marker note your odometer reading. Drive 10 miles exactly according to the mile markers and note you odo reading again. Lets say, for example, that your odometer registered 9 miles on the 10 mile road. This means that your speedo is 10% off. The adapter would need to increase the cable speed by 10%. Or if you have a GPS you can do the same thing by comparing the mileage. You can also compare the speed readings, but mileage readings will give you a more accurate indication of how much to adjust the speedo input. Merle Edited August 4, 2010 by Merle Coggins Quote Link to comment Share on other sites More sharing options...
Reg Evans Posted August 4, 2010 Report Share Posted August 4, 2010 I have a 3.0 rear end in my truck so I had a speedo shop make a reduction box that screwed right into the transmission where the cable went. Only trouble was the box was a little too big to clear the E brake drum so it wouldn't fit. I've lived with mine being off by about 25% ever since. I can't remember what shop made it for me because I found them on line but one of these days I'm going to find a shop that'll make one that installs on the back of the speedo rather than the trans. Quote Link to comment Share on other sites More sharing options...
NiftyFifty Posted August 5, 2010 Author Report Share Posted August 5, 2010 Well, I upgraded my diff but I don't have the answer you need. I switched from a 4.10 to a 3.73 and did nothing to my speedo gear. My speedo currently reads about 3 MPH low at 65. (62 indicated = 65 on my GPS) I can live with that. I wonder how far off it was before?How are the adapters rated? If you can deternine the percentage that your speedo is off you can determine how much the adapter needs to change. If you have a road nearby with mile markers you can figure it out. At a mile marker note your odometer reading. Drive 10 miles exactly according to the mile markers and note you odo reading again. Lets say, for example, that your odometer registered 9 miles on the 10 mile road. This means that your speedo is 10% off. The adapter would need to increase the cable speed by 10%. Or if you have a GPS you can do the same thing by comparing the mileage. You can also compare the speed readings, but mileage readings will give you a more accurate indication of how much to adjust the speedo input. Merle MERLE!!!! YOUR A GENIOUS!!!! I have a GPS that lays in the cupboard and never moves...I'm just going to mount it in the truck and use it....might as well get some use for something!! I'll still keep searching for one of those "reducer/increaser" blocks...and I see a lot you can just add a small piece of cable...ever consider that Reg? I would honestly say mine is off by 50%...I was FLYING and I was only reading 30 MPH...bet I was doing at least 50-60...so it's really non-useable this way. Quote Link to comment Share on other sites More sharing options...
Jim Gaspard Posted August 21, 2010 Report Share Posted August 21, 2010 Came across this in September's Goodguys mag. The gear adapter may be the answer for changing our rear differential gears and keeping your speedometer true. Quote Link to comment Share on other sites More sharing options...
NiftyFifty Posted August 21, 2010 Author Report Share Posted August 21, 2010 Came across this in September's Goodguys mag. The gear adapter may be the answer for changing our rear differential gears and keeping your speedometer true. Good info on where to find...I saw one on ebay but no idea if it had the correct gears in it...someday I would like to go without relying on my GPS. Quote Link to comment Share on other sites More sharing options...
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