72 rear spring re-installl + speedo gear question

[Gary S.]

According the instructions in the 72 Service Manual, once the rear spring is up in the proper position and the cover plate installed, you should torque the bolts to spec. Then once the entire spring is connected (the ends are now attached to the long bolts), I lower the car and re-torque the spring cover plate. I guess I don't see the benefit of doing this. Is there some easy way of getting to the cover plate with weight on the wheels? Finally, what is the danger of not re-torquing the cover plate? Is there an engineering reason to do this?

Is there a numerical ratio on the speedometer gears that would allow one to know what gear to install in the transmission to raise or lower the speedometer indicated speed? That is, if my 72 with 4.11 gears and a red 21 tooth driven gear reads too high or low by, say X mph, how many teeth would the proper gear have to have? Is a there a chart for this or is it trial and error?

Thanks

Gary

[Wayne M.]

Re: 72 speedo gear question

Gary --- Plastic colored gears are available in 1-tooth increments, giving 5 % increment to the speedo readings. If your red 21 tooth gear gives you a reading slower than actual (or less miles than actual on the odo) then you could install the 20 tooth blue gear # 3987920 (old # 3860343) for a 5% correction.

I assume you have the ratio adapter and not a straight connection of speedo cable to the trans tailshaft. However, if you are reading too fast, you should have a 22 tooth gear, which is the green one, but a larger diameter diameter than your red gear --- it will not fit. So you will need the proper mainshaft drive gear# 3708145 (for the group of 22 - 25 teeth [larger dia.] sender gears) which is smaller than the drive gear you now have (for small sender gear family of 18 - 21 teeth).

Joe Lucia posted a way back that he had found a 22 tooth small dia sender gear, and maybe he will post the part #. Another possibility is playing around with various gear sets in your adapter, but I don't know if they are commonly available.

[Wayne M.]

Re: 72 speedo gear question

Gary --- Plastic colored gears are available in 1-tooth increments, giving 5 % increment to the speedo readings. If your red 21 tooth gear gives you a reading slower than actual (or less miles than actual on the odo) then you could install the 20 tooth blue gear # 3987920 (old # 3860343) for a 5% correction.

I assume you have the ratio adapter and not a straight connection of speedo cable to the trans tailshaft. However, if you are reading too fast, you should have a 22 tooth gear, which is the green one, but a larger diameter diameter than your red gear --- it will not fit. So you will need the proper mainshaft drive gear# 3708145 (for the group of 22 - 25 teeth [larger dia.] sender gears) which is smaller than the drive gear you now have (for small sender gear family of 18 - 21 teeth).

Joe Lucia posted a way back that he had found a 22 tooth small dia sender gear, and maybe he will post the part #. Another possibility is playing around with various gear sets in your adapter, but I don't know if they are commonly available.

[Gary S.]

Re: 72 speedo gear question

This is what I was looking for! Thank you. I have the 90 adapter and the red gear but I wasn't aware of the 5% change per tooth. I had posted questions earlier about this comination and got many great responses. My local Chevy dealer gave me a printout of all the gears, part numbers and colors. In addition, I have found a local, non-GM source who keeps a stock of all the gears.
Gary

[Gary S.]

Re: 72 speedo gear question

This is what I was looking for! Thank you. I have the 90 adapter and the red gear but I wasn't aware of the 5% change per tooth. I had posted questions earlier about this comination and got many great responses. My local Chevy dealer gave me a printout of all the gears, part numbers and colors. In addition, I have found a local, non-GM source who keeps a stock of all the gears.
Gary

[Joe L.]

Re: 72 speedo gear question

Gary and Wayne-----

For 71+ Corvettes, only 1 speedometer DRIVE gear was used. This is a steel gear of GM #3978758. This is a "large diameter" drive gear which is equivalent to the GM #3708144 (discontinued) steel gear used from 1958 to 1967 and the GM #6261794 nylon drive gear used for 68-70. The GM #3978758 DRIVE gear is designed to fit the larger mainshaft used for 71+ and will NOT fit earlier mainshafts. All of these DRIVE gears work only with the "small diameter" driven gears of 18-22 tooth count. The 22 tooth gear of this series is a SILVER gear of GM #3987922. It is the highest tooth count gear that can be used with the above-referenced DRIVE gears. For 71+ with manual transmission, if more than 22 teeth on the driven gear is required for accurate speedometer readings, then an adapter has to be used in conjunction with one of the 18-22 tooth gears. For earlier Corvettes, the DRIVE gear can be changed to "small diameter" (GM #3708145 for 58-67 and GM #6261795 (discontinued) for 68-70). With these gears, the "large diameter" series of driven gears can be used which have 22 to 25 tooth counts.

[Joe L.]

Re: 72 speedo gear question

Gary and Wayne-----

For 71+ Corvettes, only 1 speedometer DRIVE gear was used. This is a steel gear of GM #3978758. This is a "large diameter" drive gear which is equivalent to the GM #3708144 (discontinued) steel gear used from 1958 to 1967 and the GM #6261794 nylon drive gear used for 68-70. The GM #3978758 DRIVE gear is designed to fit the larger mainshaft used for 71+ and will NOT fit earlier mainshafts. All of these DRIVE gears work only with the "small diameter" driven gears of 18-22 tooth count. The 22 tooth gear of this series is a SILVER gear of GM #3987922. It is the highest tooth count gear that can be used with the above-referenced DRIVE gears. For 71+ with manual transmission, if more than 22 teeth on the driven gear is required for accurate speedometer readings, then an adapter has to be used in conjunction with one of the 18-22 tooth gears. For earlier Corvettes, the DRIVE gear can be changed to "small diameter" (GM #3708145 for 58-67 and GM #6261795 (discontinued) for 68-70). With these gears, the "large diameter" series of driven gears can be used which have 22 to 25 tooth counts.

[Joe L.]

Re: 72 rear spring re-installl + speedo gear quest

Gary-----

If the spring is COMPLETELY dearched on both sides (i.e. "FLAT"), then you can torque the 4 spring bolts to final torque and be done with it. However, personally, I would still check the bolt torque with the car sitting on the suspension. How to do this with the low ground clearance of a Corvette? Simply back the car onto a set of ramps. The black plastic Rhino ramps will work well for this.

[Joe L.]

Re: 72 rear spring re-installl + speedo gear quest

Gary-----

If the spring is COMPLETELY dearched on both sides (i.e. "FLAT"), then you can torque the 4 spring bolts to final torque and be done with it. However, personally, I would still check the bolt torque with the car sitting on the suspension. How to do this with the low ground clearance of a Corvette? Simply back the car onto a set of ramps. The black plastic Rhino ramps will work well for this.

[Gary S.]

Re: 72 speedo gear question

Joe, thanks again for the speedometer gear information. I copyed what you had stated earlier this month to my personal archive so I was aware of the single drive gear. Wayne helped me with his report that a single tooth increase/decrease on a driven gear will result in a 5% change in indicated speed.

Also, thanks for the information on the ramp. Short night last night - not enough sleep. I guess the simple solutions escaped me on torquing the spring.

Gary

[Gary S.]

Re: 72 speedo gear question

Joe, thanks again for the speedometer gear information. I copyed what you had stated earlier this month to my personal archive so I was aware of the single drive gear. Wayne helped me with his report that a single tooth increase/decrease on a driven gear will result in a 5% change in indicated speed.

Also, thanks for the information on the ramp. Short night last night - not enough sleep. I guess the simple solutions escaped me on torquing the spring.

Gary

[Patrick H.]

Re: 72 rear spring re-install

Joe,

Absolutely. Tighten the bolts a reasonable degree, then finish the job with the car on the ground. It is possible to get at them as well with the spare tire carrier removed. Or, of course, Gary is welcome to drive on over and I'll do it on the lift. It's only 8 hours to drive.

Patrick

[Patrick H.]

Re: 72 rear spring re-install

Joe,

Absolutely. Tighten the bolts a reasonable degree, then finish the job with the car on the ground. It is possible to get at them as well with the spare tire carrier removed. Or, of course, Gary is welcome to drive on over and I'll do it on the lift. It's only 8 hours to drive.

Patrick