Speedometer

[Dave Marshall]

Finally got the '65 on the road for the first time in 11+ years. It has what appears to be a 3.36 rear end and has the appropriate 20-tooth blue speedo gear correct for a 3.36. My problem is at highway speeds it appears to be reading 15 mph low. Going 55 mph it reads 40. Since it has what appears to be the correct gear does anyone have a suggestion what might be my problem? As usual, thanks for any and all help.

Dave Marshall

[Joe L.]

Re: Speedometer

Dave------

The first thing that you need to do is to be SURE that everything on the "lower end" is right. You say that it APPEARS to have a 3.36:1 rear gear. You need to be certain of that. If you're basing your understanding on the stamped code, that only tells you what was ORIGINALLY inside the case and not necessarily what's in there now.

Second, is the transmission original to the car and do you know that the speedometer DRIVE gear is the correct one? For example, if the transmission originally resided in a car with a 3.70:1 or higher numerical ratio, then the installed transmission DRIVE gear would be the "small" OD gear. This gear will not mesh properly with the "small" series of DRIVEN gears (of which the 20 tooth is an example). It's unlikely that this is the source of your problem, but it is a possibility.

If everything on the "lower end" is confirmed to be ok, then your problem has to be in the speedometer head. If that's the case, you'll need to remove it and send it to a professional for repair. But, as I say, there's no point in doing this until you KNOW that everything else is right.

[Dave Marshall]

Re: Speedometer

Thanks Joe,

I determined the 3.36 ratio by the wheel and drive shaft rotation method. If I did that correctly, it seemed dead on 3.36. As far as the transmission, I am not certain it is original to the car so will need to check the drive gear as well. You have put me on the trail of the things to check and I thank you for that.

Dave Marshall

[Mark #28455]

transmission drive gear

There were 2 drive gears, I think they had 7 and 8 teeth. If you have the 7 tooth gear, it will read about 15% too slow. They are different colors, but with the trans installed, it will be a nuisance to check the color. Fortunately, you can get an adapter box from e-bay or a good speedo shop. On the other hand, you can swap for the 18 tooth driven gear, which will leave you off by about 5%, but will have no externally visible parts.
Mark

[Joe L.]

Re: Speedometer

Dave------

The method that you used to determine the axle ratio should provide for positive ID of the ratio. So, that part of the equation has been solved.

Now, do you know if the transmission is a close ratio or wide ratio? Most, but not all, close ratio transmissions were originally used for applications with 3.70:1 or numerically higher rear ratios. Most, but not all, wide ratio transmissions were originally used for applications with 3.55:1 or numerically lower rear ratios. So, knowing if the transmission is a close or wide ratio may IMPLY what speedometer DRIVE gear is installed.

More important and an easier way to get at this, did you remove a speedometer DRIVEN gear from the transmission and replace it with the 20 tooth blue gear or did it always have a 20 tooth blue gear in it AND PREVIOUSLY WORK OK WITH THAT GEAR? If you removed and replaced a gear, what color or tooth count was the removed gear?

[Joe L.]

Re: transmission drive gear

Mark-----

The DRIVE gear tooth count was the same for both common drive gears----8 teeth. However, the diameter of the 2 gears was different---a smaller OD DRIVE gear was necessary to mate with larger OD DRIVEN gear (neccesitated by the greater tooth count).

All of the 63-67 Muncies used steel DRIVE gears---there is no way to tell them apart by color. The only way to tell them apart is to measure the OD. 68-70 were the only Muncies that used plastic DRIVE gears. They are the same, basic configuration as the steel with respect to OD. The "small" gear was BLUE; the "large" gear was LIGHT GREEN.

1971+ Muncies use only one DRIVE gear. It's steel with the "large" diameter, so it can only be used with 22 (silver) and fewer tooth count DRIVEN gears. For 71+, rear ratios of 3.70 and numerically higher were accomodated by the use of an adapter.

[Dave Marshall]

Re: Speedometer

Joe,

The 20 tooth blue gear was the one in the car when I bought it back in 1995. When I drove the car home the speedometer didn't work at all. It has been so long since I took it all apart that I can't remember if it was even hooked up at the trans or the back of the speedometer. Also, I don't know if it is a close or wide ratio trans. I'll do some checking and see if I can determine anything. Thanks for all the leads.

Dave Marshall

[Joe L.]

Re: Speedometer

Dave-----

There are several ways to check for close or wide ratio. However, there are some things easily checked that will strongly infer what the transmission is.

First, do you know what engine that was originally installed in the car? If it was base engine or L-75, then the transmission would have originally been a wide ratio M-20. If it was any other engine, then it would have been a close ratio M-20 (later called M-21).

Second, is the transmission installed in the car now the same one that was originally installed? This can be easily confirmed by noting the VIN derivative stamped on the transmission. If it matches the last digits of the car's VIN, then it's likely the original transmission. Of course, the internal gearset could have been changed, but that's quite unlikely.

With the above 2 pieces of information, you can infer the transmission first gear ratio.

[Duke W.]

Re: Speedometer

Bring the revs to exactly 3000 in third gear. Note the speedometer reading as accurately as you can.

Shift to fourth gear and bring the car to the EXACT speed as above.

Read the tach. If it's reading close 2000 you have a wide ratio transmission; if about 2300-2400 you have a close ratio.

Duke

[Dave Marshall]

Re: Speedometer

Joe and Duke,

Thanks for the info. I'll have to get busy and see what I actually do have. It'll probably get interesting as to what has been done to the car over the last 40 or so years. Thanks again.

Dave Marshall