Hey guys, I've done my research, reading old posts and checking out links to places like TCI and Fred Aldrich's site, and I have learned a lot, but I still have some questions. Here's my situation: The speedo in my 69 is 18 mph too fast. According to everything I've read, each tooth added to the plastic driven gear reduces the speed reading by 5%. My old driven gear has 19 teeth. By my calculations, I need a driven gear with 25 teeth. I've got one. Just before I went to install it, I read more old posts and learned that I'll also have to change my drive gear in the transmission tail. Here are my questions: Am I correct in thinking I need a 1.76 dia drive gear? If so, how many teeth should it have, and can anyone tell me the part #?

If you need a 25 tooth its
Part #3860348
25
Orange

Available at any chevy dealer for about $5

But!! I wouldn't use the 5% rule for things, 5% rule only works with a specific DRIVE gear. You are best to remove your speedo cable and with a flashlight look in and find the color of the drive gear to find the # of teeth, then take your axle ratio and your tire diameter (use tirerack to find true tire diameter) to compute the real gear you need. I had to change both to get dead on +/- 1% accuracy.

TCI Gear Selector (http://www.tciauto.com/tech_info/speedo_gears.htm)

Another really good computing url is
Bowling and Grippo Speedo Gear Selector (http://www.bgsoflex.com/speedo1.html)

Speedo Gear selection program (http://www.tricity.wsu.edu/~jroal/speed.exe)
This tool will ask you how many seconds it took to cruise a mile (using milestone markers on highway), what MPH your speedo said you were doing, and the driven gear you currently have and will tell you the best driven gear to use to make it accurate.

BEST way to compute speedo gears (http://www.mustangii.org/tech/calculators/speedo.shtml)
Now this is the BEST place, it will give you the percentile +/- that the gear selection gives you, this way you can find that configuration that gives you 0-1% inaccuracy instead of 3-5% inaccuracy...

To find the right ratio all you should have to do is establish a known speed either by following another vehicle, using GPS, or counting seconds in a mile. Then with the actual speed, displayed speed and speedo gear ratio...

(Displayed Speed / current ratio) = (Desired Speed / new ratio)

Solving for the new ratio the formula is

new ratio = current ratio * (desired / displayed)

There are some rules governing which pinion gears will fit with each worm. Sounds like you are aware of that. The dealership should be able to help you find what you need with the information you have.

Brian and dnult, thanks for those quick responses. I already bought the 25 tooth orange driven gear, and I already know that my speedometer is 18 mph too fast. Other posts said that the drive gear and driven gear come in "sets." So what I'm trying to find out for sure is exactly what drive gear I need and the part # for the drive gear. I'm thinking it would be the 1.76 dia, but I don't know how many teeth that would have, what color or what part #. I called Rick's and they have nothing smaller than a 1.82 in drive gears. Brian, were you saying that I might be able to use a driven gear with less than 25 teeth and still be able to achieve an 18 mph reduction?

Buy a Nordskog Speedo unit and it takes care of mph. Does it all by GPS. Analog unit is good, speedo & odometer, but, the digital unit also does coolant temp, oil press, mph, speed, odo, volts, and turn direction, plus other stuff.

What I am saying is you need to provide us the exact tire diameter and rear axle, with that info, I can tell you which DRIVE gear you will need to match that DRIVEN gear you have. I can give you the part # that you can have the local chevy dealer get for you for about $5 out the door. They don't have to come in sets, you can buy any drive or driven gear, and it might be optimal to get another new DRIVEN gear if you want the most precise speedometer possible.
So
What is the brand and size of tires you are running? What is your axle ratio? I'll then tell you exactly what is best to do.

Brian, the tires are "Turbo Action Lee Radials." When I measured, I got 26 1/4". I don't know the rearend gear ratio, but I do know it has been changed. I think the whole thing came out of a Nova--probably why the speedo is off by so much. The former owner said he thinks it's a 2:73, but I think it's lower just from driving it. I'll do a test to find out the ratio later this week and post what I find so you can help me make the right choice. Thanks,
Paul

on the Turbo Action Lee Radials, are they 255/60R15 or 235/60r15 or what size?
As for ratio, easiest is to jack up rear end just to get wheels off, place tranny in neutral, place a piece of masking tape on the drive shaft, and a piece of tape at the top of one of the rear wheels, now have someone turn the rear wheel while you count the exact # of turns the driveshaft makes compared to the rear wheels making a full turn, you then have your ratio. With this we can fine tune those speedo gears to be very accurate. We also might take into consideration if you are considering changing the rear tires soon to a larger size, because that affects it as well.

Just go to the dealership with your gears and give them the information you have. They should be able to give you what you need. Transmission shops may also be helpful. You don't need rear-end ratios and tire sizes to figure this out. I'm guessing Brian has a program or table that lists the gears by those details. But knowing what you have and where you need to be is all that is required to get the right gears. If the dealership can't help you, god help us all.

Dave, what you aren't taking into consideration is sure he will get kinda close, but it won't be precise, a simple calculation of ratios will allow him to have a speedometer DEAD ON within 1%, rather than having to deal with a 3-5% error rate which results in up to about 4 mph off at 80mph!

The formula I stated will be as precise as the number you feed into it. Probabably within the running circumfrence of a wheel. So if the number going in are 100% so will the result.

Thanks again guys. Sorry I haven't chimed in for a while. I didn't expect any more posts until I got back on with the rearend info. I'm going to try to jack it up this evenning and count drive shaft and tire revolutions. Luckily there are dozens of past posts on how to do that, and I've read up on almost all of them. I don't remember any of them saying to remove the tires though...That's a good idea to improve visibility. I've been waiting to round up a partner to help, but with that technique, I might be able to count both shaft and wheel myself. I was thinking, like dnult, that it might be possible to calculate what's needed without finding the rearend gear ratio, but hey, it's something I ought to know anyway, right? We can calculate the speedo gears both ways and see if we arrive at the same point. Plus, I'm hoping that with Brian's way it might be possible to avoid changing the drive gear. By the way, my tires are 15's, and I don't anticipate changing them anytimes soon ($).

Ha, I'm a dumby :D I just realized, Brian, you meant get the wheels off the GROUND, not take them off! Oh well, my username is accurate you know. I'll post what I find this evenning.

I took the car to a mechanic in town since the inspection sticker was out, thinking I'd kill two birds at once. I asked him if I could use his lift to check the ratio in the rearend. I put the car in neutral, marked the driveshaft, and marked the tire. I turned the back right tire. The drive shaft would turn about a quarter of a turn, maybe a little less, and then it would stop. The other tire didn't spin backwards. It too rolled forward, but only a little every now and then, not continually like the wheel I was turning. :confused: What gives? Does this mean it has a limited slip rearend? The mechanic was stumped and said the only way for me to find out would be to take the cover off and look at the ring gear. When I came home, a neighbor of mine who works at Carquest said that if we had held the other tire in place while we turned, the drive shaft would have turned without stopping. Is this right? I plan on trying it again tomorrow evening with just a jack, leaving one tire on the ground. Maybe I'll have more luck. I don't want to have to open it up if I can help it.

If it was limited slip, both wheels would turn forward, if not, one wheel will turn the opposite direction. Yes you might need to have one tire on the floor so just jack ip one side just enough to allow your wheel to turn.
Issues you might run into:
Emergency Brake on, make sure its off
Rear drum brakes are too tight, thus they lock up when you turn the wheel. I am wondering if you are running into this situation.
You know you could also if you have a flat driveway or in your garage, have your friend push the car while you watch the drive shaft! Obviously you would be careful as you watched the shaft turn from the side.

Dude do this, if you have someone cruise at 60mph on a road, follow behind them and tell me your RPMs of your engine!! We can compute off that to find out what your gear ratio is!

Brian, no luck there either: the car has no tack. It had an after market one stuck on the column, but I took it out a long time ago, probably wasn't accurate anyway. I did the test again tonight with one tire off the ground. Here's what I found: If I turned the wheel exactly one turn, the drive shaft turned just a tiny bit past two turns. To use a clock as an analogy, it turned two turns and about two minutes. When I turned the wheel exactly two turns, the shaft turned two turns and "four minutes." I marked both tire and shaft carefully with two pieces of tape. So what does this tell me? Do I have some crazy high speed ratio or did I do something wrong? :confused:

Yea that doesn't sound right, 2.04:1 ratio?

well I would think you would have a 2.41 or 2.56 which are the lowest a nova ever came in.

Problem is neither of those ever compute to use a 25 for the driven gear:

# Drive Gear Teeth = 13
Gear Ratio = 2.56
Tire Diameter = 26.1
Closest driven gear = 26 teeth
Approximate Error = -1%
__
Drive Gear Teeth = 14
Gear Ratio = 2.56
Tire Diameter = 26.1
Closest driven gear = 28 teeth
Approximate Error = -1%
___
# Drive Gear Teeth = 14
Gear Ratio = 2.41
Tire Diameter = 26.1
Closest driven gear = 26 teeth
Approximate Error = 0%
__
# Drive Gear Teeth = 13
Gear Ratio = 2.41
Tire Diameter = 26.1
Closest driven gear = 24 teeth
Approximate Error = 1%


As you can see, none of those 4 configurations want a 25 tooth driven gear to be exact.


I used a bunch of math and came to the conclusion that you probably have a 2.56 rear end with a 12 tooth drive gear and 19th tooth driven gear, this with 26.1" tires would figure in the 78mph figure on the dash instead of 60mph. That said, the best gear is a 24, not a 25 for you
# Drive Gear Teeth = 12
Gear Ratio = 2.56
Tire Diameter = 26.1
Closest driven gear = 24 teeth
Approximate Error = -1%


But, you know what, a 25 isn't going to hurt, go ahead and put it in, at the same time take a quick look at the drive gear with a flashlight and see if you can get a color from it.

Worst case the changing of the tail gear isn't that big of a deal. I did it and it was pretty straight forward.

Brian, I appreciate all that math on your part. I guess I'll see what happens with the 25 tooth gear I already have; altough, I read an old post saying if the driven and drive gears weren't a match, the driven gear would be chewed up. I'll also see if I can make out what color my drive gear is. You know, I'm wondering if their might be something to what you said about the emergency brake and brake drums being too tight. Both recently saw repair from a mechanic, and the wheel was really hard to turn when I did the test. Thanks again for your info and your patience!

You can test the rear brake theory but doing this, drive about 10mph on the road and at a certain marker like a tree or light post slam on the brakes, now get out of the car and see what tires locked up first, front or rear, if the front, good, all is well, if the rear is what locked up, thats a problem, you are locking your rear brakes up too fast. Adjustment requires you to remove the wheels, and using a small screwdriver (possibly two might be needed) you can lift away the lock and turn the star dial to adjust the tension of the rear brakes until they are smooth turning.

Went to put in the 25 tooth gear yesterday, and guess what...It won't even fit into the transmission! :( The plastic shaft of the driven gear fits into the sleeve just fine, but the head of the gear won't even start to go inside the hole in the tail housing. I plan to call some of the camaro parts suppliers and see if I can find a 24 or 25 tooth gear of a smaller diamater. If that doesn't get me anywhere, I don't know anything else to do other than try to buy some kind of conversion box to slow my speedo down.

Paul most any good tranny shop will have those gears or access to them as well, doesnt have to be specialty house. I had to replace one on the interstate on a trip to Seattle and in Fargo the Chevy dealer sent a runner to a tranny store that had all the parts needed. Call around locally.
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Thanks, Click. I'll give that a shot tomorrow. I just hope I can get whoever places the order to realize that I need one that's SMALLER. I'll take the part # and orange gear with me, and hopefully this will keep them from ordering the same thing I already have. Like I said, if that doesn't pan out, I'm going to check into the conversion option.