Blue85

I️ changed tires on my 85 from 31s to 32s a few days ago, and for giggles decided to pull out the gps just to see how much my speedometer is off. Speedometer is off pretty much 5-6 MPH anything over 50MPH. So I️ used this website (http://www.therangerstation.com/tech...eedogear.shtml) for some figuring for new speedo gear so I️ do not have to worry about using the gps whenever I️ drive the truck. Through their formula, I️ used, (actual speed-observed speed)/actual speed. So that would be (60-55)/60=.083 or the speedometer is about 8% slow. Just taking a guess my stock gear is a 19 tooth, the next formula I️ need is, old drive gear x (1-error). So that would be 19x(1-.083)= 17.423. It makes sense to me that I️ need a 17 tooth speedo gear to get my speedometer back in check. Anyone else do this swap and seems like I️ am doing it the right way?

Blue85

Got under the truck today and figured out that, the current gear is a 20 tooth. So after a corrected and better calculation of current teeth x observed MPH / actual MPH = (20x55)/60= 18.33. Thus to be correct, I️ need an 18.33 gear (18 tooth) for my speedo to be correct again. That comes up with a part number for a MANUAL transmission to be C0DD-17271-B (yellow), sound correct?

265258

I don't know if it sounds correct, but since you seem to have this down well, what would I need for 33" tires..... LOL.

Blue85

I don't care to do the math for ya, but you could use my same formula if you went under your truck and pulled your speedo gear and counted the teeth. Then get a GPS or something (I downloaded an app for my iPhone plainly called "Speedometer") to figure out how fast you really are going. Then look at your observed speed on your speedometer. So lets say for instance, you had a 20 tooth gear like I had stock. Indicated speed on speedo reads 60, but using an App or GPS you're really going 67MPH. Thus using my formula from my second post - (20 tooth gear X 60MPH indicated speed) / (67MPH actual speed from GPS or App) = 17.9.I would round up and say you need an 18 tooth speedo gear to correct your indicated speed on your speedometer.

On a side note, my calculation did work just fine for me. New speedo gear came in the mail Wednesday night, and I installed it Thursday afternoon and went on several drives down the highway and through town. Still using my "Speedometer" App to see if I had done things correctly, my indicated speed and actual speed were spot on.

265258

Do the speedo gears just o right on to the cable or is there anything I need to change in the transfer case? I'm a ways off from being ready for it cause my speedo bounces, but once I install the lift and tires, I will want to change it. I never thought of using an app, usually I just call the police and see where they have a speed trap set up.

Blue85

Yes, speedo gear is held in with a retainer on the back of the transfer case with a 7/16 bolt if I remember correctly. It’ll be a little stiff to get the cable and old gear pulled out of there, but it’ll come out just fine. Be cautious on how the gear is held in the cable. Should be a metal retainer that clips around the gear onto the cable. When you install the new gear, make sure the retainer goes back on the same way it came off.

Does the speedo bounce like all the time, or just when first starting to get going? Mine always bounces until I get to about 20MPH, then it flattens out, especially at a sustained speed.

265258

Sometimes it bounces non stop at any speed. Sometimes it starts working great. Can't nail it down. The speedo cable is good. I've pulled it out from both ends and checked it all. The gear is on the right way. Lubed it real well, then I found a nicer gauge cluster with the Tachometer option and trip meter at the wrecking yard, put it in and it bounces. The ols speedometer I replaced was just plain broken. Made a hell of a noise. It actually ruined the end of the old inner cable so I bought an inner cable kit and just replaced the end that goes into the speedometer and used my same cable due to a slight difference in thickness with the new one. I would like to try and find a new speedometer, maybe just the main mechanism.

Blue85

I’m not going to say this is your problem, but when I bought my truck, the speedo and odometer worked fine for about a week. Then all of the sudden the odometer stopped working completely. Tried to fix it and get the gears back in alignment, but it was toast. At the same time the odometer went out, I did notice my speedo was quite jumpy. So it might be your actual speedo giving you such a fit. Or possibly the inner gear in the transmission might be getting some wear too? Really don’t know.

saemetric

My speedo was reading 67-68 mph when true speed was 60. Pulled the driven gear and it was the grey 18 tooth gear. Put in an orange 20 tooth and now reads true. An easy fix but not sure how the wrong gear was in there. Bought the truck used , but has correct rims and tires. Not sure if rear axle has been previously switched or not. PO used it to tow a trailer so maybe he put lower gears in it. Don’t really care now as long as Speedo is correct.

Blue85

Yeah, hard to say what’s actually been done to your truck. I know mine is stock gearing due to the door code, and I jacked up the rear of the truck and counted how many times the driveshaft turned to make a full revolution of the rear tires.