Correct speedometer gear

[fomocoguy]

Ok, so yesterday I'm cruising along in the the crew with a new motorcycle project in tow, and I realize that I'm passing everybody, but I'm only doing 60 in a 60mph zone. Then I slow to 55 and everyones doing about the same. After thinking about it, I'm pretty sure the 33's have my speedometer off a bit, so the question is does anybody know where I can find speedometer gearing info so I can buy the correct gear to match my tires? I don't like getting tickets...

[fordman]

lmc has them i think. probably so do the other vendors. i have no idea which one to get though. you will have to get a tooth count on yours and figure in the tire size to get the correct tooth amount.

[craftsman]

Here something that you can use to help correct it
http://www.4lo.com/calc/gearratio.htm

http://www.wallaceracing.com/Calculators.htm

[Dragon]

Any speedometer shop. I have 35" tall tires and my speed became almost right so my rear end has been re-geared.

[fomocoguy]

Thanks for the replies everyone! I was just hoping there would be a ford chart somewhere that would give me guidelines and part numbers. I've seen those before; just not for this application. Do you guys know if the manual trans uses the same gear as a C6?

[Dragon]

supposed to be the same for the same years. It simplifies inventory.

[craftsman]

"Do you guys know if the manual trans uses the same gear as a C6?"


The c6 will have more drive gear teeth, so i think that will change the driven gear teeth count(the one on the cable).

[Dragon]

The diameter of the drive gear changes the ratio because it is a right angle worm drive not the drive teeth count. If we hit it at an angle or parallel to the Drive shaft then the teeth count would matter. But we hit it in all cars straight on the end so the diameter matters. Larger the diameter the slower it appears to spin.

[Hawkrod]

The diameter of the drive gear changes the ratio because it is a right angle worm drive not the drive teeth count. If we hit it at an angle or parallel to the Drive shaft then the teeth count would matter. But we hit it in all cars straight on the end so the diameter matters. Larger the diameter the slower it appears to spin.

I am sorry but that is absolutely false. This is a basic automotive principal and any competent mechanic will explain to you that the number of teeth is all that changes the speed and it has nothing to do with diameter. The fact that it is a worm gear does not change anything, X number of teeth divided by Y number of teeth determines the speed. If you had two driven gears with different diameters but the same tooth count they would still spin the same speed provided they both were engaging the drive gear, you can't speed up or slow down the tooth to tooth ratio by changing diameter. In the case of Ford transmissions there are different drive and driven gears for different types of transmissions. The driven gears come in left and right spirals depending on which side of the transmission the gear comes out of. Ford charts only list original applications and assume stock tire size so they are useless for determining what to change to. The trick in this case is to determine how far off you are now and figure out what percentage that is. If you are showing 66 when you are doing 55 then you are 20% off. If you driven gear has 10 teeth then you need one with 8 teeth etc... Hawkrod

[fordman]

i would think that the number of teeth would determine size of the gear. the more teeth the possible size of the gear. but that is just a guess.

[Hawkrod]

i would think that the number of teeth would determine size of the gear. the more teeth the possible size of the gear. but that is just a guess.

You would think that but it isn't really so beyond a minor degree. That is to say that Ford only used a couple of different drive gears but dozens of different driven gears so in order for the drive and driven gears to actually mesh, the diameter has to be within a specific thresh hold or they driven gear would not engage. For example, lets use a Ford 3.03 speed as used on an F series pickup, the drive gear can be one of several different ones, with a number of different teeth with the most common being 8. There are actually more than one 8 tooth gears because some ratios do cause an issue with diameter of driven gears but lets choose a common one such as C8OZ-17285-B. We know we have 8 teeth on the drive gear but depending on axle ratio or tire size the driven gear can change. For example, an F100 with 3.25 ratio with H78-15 tires uses a C0DD-17271-C driven gear with 19 teeth, but if the same truck has 7.00x15 tires it uses a C0DD-17271-B gear with 18 teeth. Take that same truck and make it an F250 with 8.00X16.5 tires and it uses a C0DD-17271-A gear with 20 teeth. These all use the same drive gear. For this specific drive gear there are 15, 16, 17, 18, 19 ,20 and 21 tooth driven gears. They are all the same diameter so that they will properly mesh. This actually only works because of the tooth angle as these don't direct mesh like some other gear applications like a trans or rear end where there is a load. The speedometer has almost no load so there can be lots of back lash so the drive gear has a narrow tooth pattern so that it can mesh with a variety of driven gears. Hawkrod

[Dragon]

I have a degree in physics and if I wanted to spend the time to draw the diagram I would show what I mean. I don't have the time so I will verbalize it here as best as I can

With the driven gear hitting dead center on line with the drive gear center at 90 degrees diameter is the fastest way to change the ratio and keep the same pitch of the drive gear. When you reduce or raise the diameter of the driven gear then the ratio changes. If you lock the pitch of the drive gear then when you reduce the pitch of the driven gear the pitch mesh angle will be changed into higher angle of contact causing wear of the weakest gears teeth.

The Drive gear diameter of the unit does not need to be changed because pitch angle does not change. The Pitch depth of the Drive gear is deep, the pitch depth of the driven gear is not. So the diameter of the driven gear can be changed with tooth count allowing one drive gear to mesh with several driven gears. You have to change the diameter of the driven gear as the number of teeth change, Adding a tooth is a 1/16" diameter change

Put a top loader 4 speed in a truck and if you use the driven gear from the truck the teeth get torn up. The diameter of the top loader's drive gear is almost a 1/4 inch in diameter smaller than the NP435 or 303 tranny gear. Use the car gear and every thing is okay of you have car tires. But go up to the 33 or 35 inch tires and the largest top loader gear is required to fit to get close. To make a match at 35 inch tires you have to change to the next smaller drive gear and then the middle of the 5 driven gears.

Let's agree to disagree.

[Hawkrod]

I have a degree in physics and if I wanted to spend the time to draw the diagram I would show what I mean. I don't have the time so I will verbalize it here as best as I can

With the driven gear hitting dead center on line with the drive gear center at 90 degrees diameter is the fastest way to change the ratio and keep the same pitch of the drive gear. When you reduce or raise the diameter of the driven gear then the ratio changes. If you lock the pitch of the drive gear then when you reduce the pitch of the driven gear the pitch mesh angle will be changed into higher angle of contact causing wear of the weakest gears teeth.

The Drive gear diameter of the unit does not need to be changed because pitch angle does not change. The Pitch depth of the Drive gear is deep, the pitch depth of the driven gear is not. So the diameter of the driven gear can be changed with tooth count allowing one drive gear to mesh with several driven gears. You have to change the diameter of the driven gear as the number of teeth change, Adding a tooth is a 1/16" diameter change

Put a top loader 4 speed in a truck and if you use the driven gear from the truck the teeth get torn up. The diameter of the top loader's drive gear is almost a 1/4 inch in diameter smaller than the NP435 or 303 tranny gear. Use the car gear and every thing is okay of you have car tires. But go up to the 33 or 35 inch tires and the largest top loader gear is required to fit to get close. To make a match at 35 inch tires you have to change to the next smaller drive gear and then the middle of the 5 driven gears.

Let's agree to disagree.

Sorry, no way to agree to disagree because you are so badly mistaken. You will have to draw a diagram. I am an ME specializing in automotive technology and you are wrong no matter how good you make it sound. The 18 and 19 tooth speedo gears are the exact same diameter but the speed is different. The difference in size only relates to spped when the two turning objects are not meshed as in a gear application. When you are dealing with a gear aplication and a single size driven gear the only way to change speed is by changing tooth count and making the driven gear smaller will not have an effect if it has the same tooth count and still engages. It is an imposibility in both physics and mechanical engineering to make a gear spin faster or slower than that of the ratio of gear teeth. If you think otherwise then you clearly did not do well in your field. Hawkrod

[fomocoguy]

Ford charts only list original applications and assume stock tire size so they are useless for determining what to change to. The trick in this case is to determine how far off you are now and figure out what percentage that is. If you are showing 66 when you are doing 55 then you are 20% off. If you driven gear has 10 teeth then you need one with 8 teeth etc... Hawkrod

This is the tid bit I was looking for. I will clock my truck with my Gtech and go from there. Thanks!

Thanks to all for your help!

[cdherman]

Hawkrod is correct on this one. I won't bother to re-interate all he has said.

The "Drive" gear in the tranny is a spiral gear. You count the number of teeth on it by looking at it from the end. It looks kind of like a flower, where each "petal" is the beginning of a new tooth. These are available in varying tooth counts, but usually do NOT need to be changed for a given vehicle. If you have radiacally departed from stock setups, then sometime you can run out of range with the standard setup.

You find tune your speedo with the "Driven" gear, the one on the end of the speedo cable. More teeth on the "driven" gear means that the speedo gear turns more slowly (more of the teeth on the spiral gear" have to go by to rotate the driven gear).

Finally, if you are sly and understand ONE BASIC CONCEPT, you can calculate all the ratios down to decimal points. NAMELY: Odometers record one mile per 1000 revolutions of the speedo cable. If you know the circumfrence of your tires (there are tables from manufactureres and elesewhere), you can calculate all the various rotations PER MILE, working back from the tires, till you get to the drive shaft. Most trucks have 7 petal (teeth) on the Drive gear. Drive shaft x7 / teeth on Driven gear will give you the rotations of the cable per mile. Get it close to 1000 and you will have the speedo corrected.