C2 Speedometer Drive Gear Issue

[Donald T.]

Well, I'm still having issues with the speedometer in the 65. The transmission was recently rebuilt with all new gears, and I suspect the rebuilder installed the incorrect drive gear.

It has a 3.55 rear which I double checked and confirmed the ratio by comparing drive shaft and tire revolutions. The speedometer only works intermittently and is reading about 8 MPH slower than actual speed. I pulled the driven gear and it is the correct 21 tooth red gear, but it is showing significant wear already. The driven gear appears to be slipping on the drive gear causing intermittent failure of the speedometer.

I do not relish the idea of pulling the transmission to replace the drive gear. Assuming the drive gear is incorrect, how much error in speedometer reading will I have if I install the 22 tooth green driven gear for a 3.70? Any other suggestions?

Thanks!

Don

[Joe L.]

Re: C2 Speedometer Drive Gear Issue

Well, I'm still having issues with the speedometer in the 65. The transmission was recently rebuilt with all new gears, and I suspect the rebuilder installed the incorrect drive gear.

It has a 3.55 rear which I double checked and confirmed the ratio by comparing drive shaft and tire revolutions. The speedometer only works intermittently and is reading about 8 MPH slower than actual speed. I pulled the driven gear and it is the correct 21 tooth red gear, but it is showing significant wear already. The driven gear appears to be slipping on the drive gear causing intermittent failure of the speedometer.

I do not relish the idea of pulling the transmission to replace the drive gear. Assuming the drive gear is incorrect, how much error in speedometer reading will I have if I install the 22 tooth green driven gear for a 3.70? Any other suggestions?

Thanks!

Don

Don------


I expect that your analysis of the situation is correct. If a "small" OD DRIVE gear was installed, the teeth would not engage correctly with a "small" OD DRIVEN gear like the red 21 tooth. I would expect a problem exactly like the one you're having.

As far as error with a 22 tooth GREEN DRIVEN gear, you could expect about 5% error. However, it could be more or less. That's because even with the "correct" gear, there could be some degree of inaccuracy. So, you will be adding to or subtracting from that error. Speedometers are rarely perfectly accurate due to the fact that there are only so many gears available.

If the error proves to be too great for you to live with, you have 2 options:

1) Replace the DRIVE gear;

2) Have a speedometer shop install an adapter. This can be set up to provide an extremely accurate speedometer reading. In fact, this is how police cars used to be prepared with a "certified" speedometer.

You can use a GPS unit to VERY accurately determine how far off you are after installation of the 22 tooth GREEN DRIVEN gear.

[Donald T.]

Re: C2 Speedometer Drive Gear Issue

Thanks Joe. I'll try a 3.70 driven gear and then see how far off I am. If it's close then I'll just run with it for the time being.

Thanks!

Don

[Wayne M.]

Re: C2 Speedometer Drive Gear Issue

..... I'll try a 3.70 driven gear and then see how far off I am. If it's close then I'll just run with it for the time being.

Don -- in addition to the info. Joe has provided (theoretically a 5% slow reading with green 22-tooth plastic gear), your road test may be closer (or further off) than this figure, depending on what tire circumference you have. I believe that Duke W. said that the speedo's and speedo driven gears were calibrated and selected to give accurate readings with 6.70 (ie. '63 & 64) tires. Repro 7.75's or replacement radials may reduce or accentuate the error. So the road test is the first step.

[Donald T.]

Re: C2 Speedometer Drive Gear Issue

Just a follow up. I installed the green driven gear today. The good news is that the speedometer is working again. So it does appear that the rebuilder installed the incorrect drive gear. The bad news is that the speedometer is reading much lower than actual. I need to increase the reading by about 15%. So looks like I'll be ordering a ratio adapter to correct the error and avoid pulling the transmission.

[Joe L.]

Re: C2 Speedometer Drive Gear Issue

Just a follow up. I installed the green driven gear today. The good news is that the speedometer is working again. So it does appear that the rebuilder installed the incorrect drive gear. The bad news is that the speedometer is reading much lower than actual. I need to increase the reading by about 15%. So looks like I'll be ordering a ratio adapter to correct the error and avoid pulling the transmission.

Donald-----

The adapter is the only way I know of to correct the speedometer without changing the DRIVE gear.

[Donald T.]

Re: C2 Speedometer Drive Gear Issue

Just an update to my speedometer error problem. I just received a customer ratio adapter from PATC. They custom built and shipped it in less than 2 weeks. Just installed it and went for a spin, and the speedometer is right on the money. All is right with the world again.

http://www.partshp.com/speedometer_gears.htm