which speedo gear???

[rvaedge]

I recently put 31x10.5 on replacing my 235 75's. My speedometer is obviously off because of the change (speedometer reading 45, ACTUAL speed about 48) and I am TRYING to figure out which speedo gear to use. I currently have the pink 19 tooth gear. Using this formula "New Teeth = (Current Teeth x Speedo Reading)/Actual Speed" which I found by searching, I plugged in the numbers (19 x 45)/48= 17.8 which would mean to correct it I would need the 18 tooth gear. HOWEVER, I called a local dealer and told the parts guy my situation and he said to go up a tooth to 20 teeth. Now I'm all confused. Does anyone know which one I would need. I assume the formula is right and the parts guy at the dealership is either clueless or didnt understand me correctly. Like I said, I currently have the 19 tooth gear, I went from 235 75's to 31x10.5, and my speedometer reads 45 when I am ACTUALLY going around 48. PLEASE HELP ME!!!

 

[ExplorerDMB]

Who did you call? Universal, Richmond, Sheehy? Anyway, that really doesn't matter -- but I would do some more searching to make sure that formula is right. Stuff like this can be VERY confusing when talking to someone whos never dealt with it before. I would agree more with the formula, but it's sometimes hard to say and is trail-and-error.

-Drew

 

[aldive]

Does a '97 have gears? My '99 is computer controlled.

Good luck in solving your problem.

 

[rvaedge]

I talked to a guy at sheehy. I think he may not have understood me correctly. I could just buy a 20 and a 18, they are only like 10 bucks each. Aldive, I checked to make sure, and I do have a gear. I think they went to computer controlled after, I think 98? Thanks for the quick replies. Keep them coming!

 

[Arnie897]

Robert posted this up a few years ago...

The part number I needed for my '97 5-speed auto that originally came with 4.10 axle and 255 70R16 tires was C0DZ-B. It is the 19 tooth pink one. My original gear was the 20 tooth black one. One less tooth was all I needed. I used this formula: New Teeth = (Current Teeth x Speedo Reading)/Actual Speed. Be aware though that the table came from a Ford Ranger site. Unfortunately I don't have the link anymore so I don't know whom to give credit. The gear I used says that it goes on a manual transmission. When the parts lady first brought out the 19 tooth gear for the automatic, it was too big. Fortunately I took the gear out and brought it with me to compare (boy did it shift funny without the gear!). Here is the table that I used:

Manual Transmission:

16 tooth wine(C0DZ-A)
17 tooth white(C3DZ-C)
18 tooth yellow(C0DD-B)
19 tooth pink(C0DZ-B)
20 tooth black(C1DZ-A)
21 tooth red(C4OZ-A)

Automatic Transmission:

16 tooth blue(D0AZ-A)
17 tooth green(C7SZ-A)
18 tooth gray(C7SZ-B)
19 tooth tan(C7VY-A)
20 tooth orange(C8SZ-B)
21 tooth purple(D0OZ-B)

 

[Bronco638]

Use the Ford Motorsport formula:

Driven Gear Teeth = (Drive Gear Teeth x Axle Ratio x Tire Rev. Per Mile) / 1000

There are 7 teeth on the gear in my BW1354 (and since the speedo worked fine with my 4405, I'll assume there's 7 teeth on that gear too). You can find the tire's revolutions per mile under the "Specs" link on TireRack.com. From there it's simple math:
? = ( 7 * your axle ratio * 677 ) / 1000

I have a ScanGauge and the speedo and SG readout are almost identical.

 

[Truck39Trash]

Just to update, I put the 18 tooth speedo gear in (from ford dealership $9) and I definitely notice a difference in shifting. The hesitation that seemed to be there is now gone. I recommend the 9 bucks and 10 minutes of work to have a better driving car!

He had 235/75/r15 before he moved up to 31x10.5 so I hope that helps

 

[rvaedge]

yeah guys, I'll update here also. I swapped the 19 tooth pink speedo gear that I had, for a yellow 18 tooth one, and it smoothed out my shifting quite a bit, and I'm pretty sure it put my speedometer much closer to where it should be.

 

[Green98XLT]

I'm about to put 31x10.50's on my 99 SOHC (replacing the original 235-75's) will it screw up my shifting as well? I expect it to make the speedo off a bit but I can deal with that until I get my micro tuner to calibrate it.

 

[rvaedge]

From what I've gathered from everyone, it sounds like putting bigger or smaller tires on ANY car will change the shifting due to the fact that the computer is getting information that isnt entirely correct (ie: rpm's). This throws the shift parameters off, so yes it will change your shifting a bit. Its not AWFUL by any means, but noticeable.

 

[TheFox88]

How bout on a '92? I know mine has a gear but i don't know what gear is in there right now or how many teeth it has. I will also be going up to 31's and would like to keep my speedo close to true. I have the open 3.27's and an electric shift transfer case. What gear would i need to use? Also is the install easy?

Sorry for sounding like a noob here, but this is a subject that i have not looked at yet.

 

[rvaedge]

To tell you the truth, I dont know anything about 1st gens, only 2nd gens. I would imagine its somewhat similar though. Take a look at your transfer case tailhousing, which is where mine was, or I think they can sometimes be on the transmission tailhousing, or rear diffy (that might be completely wrong though), but look around and try to find the speed sensor. Take it out and count the teeth on the gear, then use this formula to find out how many teeth you need on the new gear : New Teeth = (Current Teeth x Speedo Reading)/Actual Speed. When you complete the formula, you may get a number like I got, 17.8, just round it out. It wont be perfect, but it will be closer. If you have 235 75 r15's on right now, and you're moving up to 31's, use 45 mph as the speedo reading, and 48 as your actual speed. I used this site to find out the diameter in inches of the radial tire http://www.onlineconversion.com/tire_size_conversion.htm and this site to find out how far off the speedo would be http://www.onlineconversion.com/bigger_tires.htm. Hope this helps