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I intend to put up a web page to address these questions, but until then:
1. Yes on the driven gear. The parts catalog shows the same style and part numbers for all transfer cases Ford used. And I know the same gear works in a C6, NP435, NP208, and BW1356, so that fits with what the catalog is showing. As for the drive gear, the one in the t-case, see below.
2. The chart needs to be part of the web page as there is a different chart for each axle ratio and you didn't specify what you have. Then, within each chart you get to pick the tire size you have, or something close. But, if you aren't changing axle ratios or tire size your original driven gear doesn't need to change.
3. I doubt you will need to change the drive gear. The catalog shows the drive gears vary by application, meaning a pickup vs a Bronco, and by rear axle ratio. So, if you aren't changing out the transfer case the drive gear won't change, and all you have to do is use the right driven gear.
Boy this makes me glad I put the 1992-1997 dash and instruments in. The PSOM only has two variables, axle size and tire size. You have a calibration number you enter to get the correct readings.
Actually, I simplified it in the above. On top of there being driven gears with 16 to 21 teeth, there are drive gears with 6 - 8 teeth. And, there are oddities in the tables for the drive gears. For instance, in 1983 a pickup with 3.55 gears would have gotten an 8-toothed drive gear. But a Bronco with 3.55 gears might have gotten either a 7 or an 8-toothed drive gear.
Ok, I've put up three new web pages to cover speedo gears, both drive and driven. The main page is here (Speedometer Gears - Gary's Garagemahal), and it says the following, including links to the daughter pages:
To me the speedometer gear situation is somewhat complex, so I'm hoping to lay it out here in a way that simplifies it somewhat.
There are two gears involved in the gearing to the speedometer:
Drive gear: This is the gear that is on the output shaft of the transmission or transfer case, and it is named "drive gear" since it drives the gear on the speedometer cable.
Driven gear: This is the gear on the speedometer cable.
So, what gears need to be used to get the speedo to read correctly? "It depends" is the best answer as there are two variables involved - the rear axle ratio and the tire size, or more correctly the tire's rolling diameter. Given the number of tables involved I've put up two separate pages for the two gears:
Drive Gears: This page lists the drive gears used for each year, transmission or transfer case, and axle ratio. But, the Driven Gear page also lists the drive gear in the tables, so you can look the drive gear up there as well.
Driven Gears: This page lists the driven gears used for each axle ratio, specific transmission, and tire size. But it also includes the drive gear part numbers, so you can get both part numbers in one table if you prefer.
Please note that I didn't realize that the Driven Gear tables also list the Drive Gear until I had the cutting and pasting done to create the pages. And, since there are 27 separate screen grabs on the Drive Gear page I'm loathe to delete it. However, if y'all think it is confusing please tell me as I could meld the two pages, Drive and Driven, into one longer page and leave out the year-by-year Drive Gear tables.
Was hoping to use the chart to help me with My Frankenstein 84 F150 /89 F250,('84 F150 engine & trans. in '89 F250 chassis) but I am thoroughly confused.
I used the transfer case from the '84 (NP208) because the speedo. drive gear in the original '89 BW 13-45 was slipping. But I used the driven gear/cable from the '89 F250. Gear ratios in the rear end were the same for each truck but F250 has 16" wheels and taller tires.
I know that the gear in the BW 13-45 was yellow.
Boy do I need help!
In my case, Im planning on buying 5.13's and buying 35-36" tires. Just so you know. I cant look at your pages in depth right this second, too many irons in the fire, but wow, what you did is impressive!!
Whisler - If you give me all the spec's I can do the math to tell you what driven gear I think you need. Here's what I think I need:
Tire sizes: I need both the original and to-be tire size
Axle ratio: You've said that they are the same, so maybe I don't need it, but it might help.
Drive gear: You said the BW1345's gear is yellow, which looks to be an 8-toothed gear. But, what color is the one on the NP208?
Driven gear: What color and how many teeth does the original driven gear have? (I'm asking for both color and tooth count to add to my understanding of these gears.)
Thanks for the offer Gary!
Tire sizes: original '84 LT235/75/15, present '89 LT235/85/16.
Axle ratio: 3.55
Drive gear: NP208 is installed and my be hard to see into but I can try. According to your chart it "should be" blue
Driven gear: original '84 driven gear is black and has 20 teeth. When I try to look into the NP208, I'll check the color and teeth count on the '89 driven gear while it is out.
And, as I understand it, you are going with the NP208 and will be running the LT235/85/16 tires. So the question becomes what drive and driven gear combo to use to get the speedo to read correctly. Yes?
All correct except:
1989 drive gear was yellow.
1989 driven gear TBD.
Tires are 235/85/16 and 1989 driven gear/cable is currently in the truck. Yes the question is which combination will give the correct speed. Of course the easiest thing to change is the driven gear.
Thanks again for your time.
How big of a hurry are you in? I ask because I can solve the problem pretty quickly one time, but what I'd prefer to do is to create a spreadsheet that will be on my web site where you can solve the problem. And, you can do what-if games. For instance, you could try any drive & driven gear combo and see what % error you would have.
All you would do is put in the "old" and "new" info and it'll give you the answers. And there will be a link to Tirerack's page where you can get the average diameter of each tire size. And, for the morbidly curious, I might even include how it calculates things.
I remember dealing with that several times. Item for you Gary, and anyone else. On the applications where the drive gear is built into the shaft. In swapping transmissions, if you pull one from a car, a C6 say, the built in drive gear will in all probability have one less tooth than a truck transmission. When get into the 80s truck that were built with rear gears like 2.75, you will probably have the same situation.
As for a spread sheet, sounds like a good idea as the Ford catalogue can even drive a good parts man bats. I had a devil of a time getting the speedometer in my GT-350 even close due to the limited number of teeth available on the gears. Be aware, there are right hand and left hand gears which is determined by the placement against the drive gear and they will chew up in a hurry if you used the wrong direction.
Funny story on those gears, my daughter knew how to change to one on her Pinto. She got a speeding ticket on I-81 near Roanoke VA. The judge there was know to only accept certifications from one location in that area. I put a driven gear in that would make her speedometer read low enough to get her out of the ticket and taught her how to change it back to the correct one. That and a receipt for the wrong gear (to show she replaced it since all it had was the Ford PN) got her charge reduced to improper equipment. She just had to be extra careful on her speeds (correction list stuck on the dash) until the proper gear was back in the car.
Bill - Excellent point about the built-in gears. I would have missed that completely.
The catalog shows that only the C4's and C6's have a drive gear rolled into the output shaft. And it shows there were 6, 7, 8, and 9-toothed output shafts. Further, I'd have to go to an application chart to find out which vehicle needs which parts list, and then peruse the parts list to find the output shaft.
I'm just going to tell the reader that a C4 or C6 has a permanent drive gear, so all they can do is to swap the driven gear.
How big of a hurry are you in? I ask because I can solve the problem pretty quickly one time, but what I'd prefer to do is to create a spreadsheet that will be on my web site where you can solve the problem. And, you can do what-if games. For instance, you could try any drive & driven gear combo and see what % error you would have.
All you would do is put in the "old" and "new" info and it'll give you the answers. And there will be a link to Tirerack's page where you can get the average diameter of each tire size. And, for the morbidly curious, I might even include how it calculates things.
Game? Or quick?
Any way you want to do it is fine with me. I'm not in a big hurry (about anything in life these days) and the spreadsheet seems like it would help the most people. Thanks for all the effort.
04-28-2016, 09:42 PM
